HVAC ductwork in floor slabs: diagnosis & repair or abandonment FAQs.

Questions & answers about how to diagnose, repair, or abandon under-slab or in-slab heating or air conditioning ductwork.

This article series describes heating and air conditioning ducts that have been placed in or beneath concrete floor slabs.

FAQs about Abandoning or Troubleshooting & Fixing HVAC air ducts in concrete floor slabs

These questions & answers about diagnosing & fixing problems traced to in-slab air ducts were posted originally

at SLAB DUCTWORK - topic home. Be sure to see the in-slab ductwork description, problem identification and solutions described there.

Is it possible to waterproof in slab air conditioning duct work?

Is it possible to waterproof in slab air conditioning duct work?

I have water in my ducts and don't want to move the ducts overhead. On 2021-01-25 by Steve

Answer by (mod) - abandon, don't "waterproof" ducts in slabs

Steve

Please take a look at the home page for this topic SLAB DUCTWORK for which the page you're on is a Q&A collection.

You'll see that IMO an in-slab duct system is asking for trouble over the life of a building.

Yes there are procedures to clean and even to re-line such ducts but over a building's live I have found none of those to be reliable long-term.

There is a particular concern for any health and safety topic when a prior "repair" effort leads us to think, in error, that a problem has been taken care-of, causing us to ignore warning signs.

I appreciate that it is far easier for me to say "abandon those ducts" than to be the person going to the cost and trouble of doing so.

So sure, you can try having the in slab ducts cleaned and spray-lined with a sealant, but at the very least you'll want frequent duct inspections by camera to find where the coating didn't adhere, fell into the ducts, and there's now a pond of stagnant, contaminated water, pathogens, mold, etc.

What's the effective fix for horrible mold smell from moldy in-slab ducts?

I have a large ranch with a 3500 square foot finished walk out lower level. My home has 3 furnaces and 3 air conditioners. One unit runs the lower level.

The ductwork for that level is under the floor. We have been having a terrible mildew, musty smell for quite some time. In one room it is humid.

The smell is worse when the heat is on and is slightly lessened when the air is on. The smell permeates to the main level. We have had everyone from HVAC contractors, air quality inspectors, home inspectors, thermographers, etc to investigate to no avail. Recently a camera inspection found evidence water of infiltration in numerous areas of the ductwork and found a great deal of standing water/mud in the ductwork too.

Unfortunately, the ductwork for the other furnaces/air conditioners is in the ceiling of the lower level and therefore there is not any space to run new ductwork overhead with an up flow furnace/air conditioner to heat and cool the lower level. The only option that the professionals have come up with is to use a split unit such as Mitsubishi to heat and cool the level and filling underneath ductwork with concrete.

They believe they can sufficiently heat and cool the level with these type of units. It would require a heating/cooling apparatus on the wall in almost every room.

They are rather unattractive. Do you know if these are effective systems? Should I worry about this type of system decreasing the value of my home when I sell? Do you have any other ideas to fix the problem?

The water is allegedly coming from the ground. We already have a drainage system around the home and have been told that additional drains will not correct the problem. Any ideas or help would be appreciated. On 2020-09-16 by IM

Reply by (mod) - find and remove the moldy material

IM

You may get adequate heat and AC with the proposed split systems but it seems to me you'd still want to find and remove the moldy materials. If you smell mold, there is mold to be found and removed. Get rid of those wet moldy ducts.

 

On 2020-05-12 by raphael hirsch

The house I live in has under the slab ducts that need to be filled with concrete and isolated from the other ducts in the house.
They are already disconnected from the HVAC unit.

How do I find a contractor experienced enough to know how to fill the ducts?

I live in Atlanta

by (mod) -

Raphael

Finding a contractor to fill ducts in a concrete slab

Talk with some concrete placement contractors to ask them details about their familiarity with the job and how they'll proceed.

I'd be listening for someone willing to answer questions, and who is familiar with both concrete placement and also site and roof runoff control and the issue of leaks into slab ducts.

Talk with your neighbors to see if a similar house with in-slab ducts has been so treated, by whom, and with what satisfaction.

 

On 2022-10-18 by EJ Sorrell - fill in the plenum when abandoning transite ductwork?

Should I fill in the plenum when abandoning transite ductwork ? I am trying to figure out if I should just seal the registers or also the plenum

Should the area directly underneath the air handler/furnace (the deepest point where all the ducts meet ) and be filled with concrete as well as the openings to the supply registers?

On 2022-10-18 by InspectApedia (Editor) -

@EJ Sorrell,

I see all the individual registers and also the floor opening into an air handler which perhaps is what you are referring to as the plenum.

It's really worth reading the articles on this subject so that you can be more completely informed than just an off the cuff or excerpt type reply.

And sometimes it's best to completely fill in the end slab system so that you're eliminating water and other under slab concerns. At the very least you want to seal all the openings in the floor.

 

On 2022-08-25 by Greg - how thick and what is the diameter of a 16-inch PVC pipe used for in-slab return duct?

I have to run a 16in return under the slab about 12 feet. Was wondering how thick the pvc pipe has to be?16 inches round would

On 2022-08-25 by InspectApedia-911 (mod) - ducts in slabs tend to be a problem

@Greg,

You'll see in this article why ducts in slabs tend to be a problem - to be avoided.

A 16" PVC pipe, Schedule 40 weight, would have an outside dimension of 16" and an inside dimension of 14.94 inches.

On 2022-06-18 by Jewell - history of water in floor vents and ducts - what do I do?

I inherited my mom's slab house in toronto ontario, we get winters.

there is history of water in floor vents during snow melt especially, I'm concerned. Thinking of filling them in but wondering where new ducts would go?

There is no attic space and square footage is limited. Also wondering if I should be adding a Sump Pump?

Also there is definitely problems with grading, downspouts etc that need to be addressed, problem is I will be taking possession in September, I have no idea what to expect for costs and how to address these issues before laying down new floor and renovations, need to know what to budget for.

On 2022-06-18 by InspectApedia-911 (mod) - suggestions on filling in ducts as well as an understanding of the hazards

@Jewell,

Please do take a look at the article above - as it includes suggestions on filling in ducts as well as an understanding of the hazards.

Just where replacement heat distribution ducts (or something else) should run in a home with in-slab ductwork depends on the specifics of the building as well as on choice of heating system.

In some cases people opt to run hot water heating lines as pipes are smaller and easier to route to various building areas = less obtrusive; that's particularly appropriate where the old forced warm air heating furnace was old and at end of life.

I would not in any case continue to use in-slab ducts.

It would make sense to have a thorough home inspection done before hiring contractors to do anything.

That will help you set priorities of repair work and may avoid wasting money on unnecessary repairs or on something that has to be torn out soon afterwards to repair something else.

3 Priority categories for building repairs

• Dangerous
• Doesn't work
• Damaging the building in a costly and rapid way

Everything else: you're in control of your wallet

Call a sharp home inspector, maybe one of the inspectors from our friends Carson Dunlop Associates (you can find their contact info at the page-bottom Citations & References -

or my friend and an independent Toronto home inspector, Terry Carson, Toronto Ontario Home Inspector: 416-440-0062 4 or others in Toronto found at HOME INSPECTORS-CANADA

All of those people have contributed technical content to this website and are my friends; we have NO financial nor other business relationship with any products or services discussed at this website.

 

On 2022-01-26 by Travis B - what's the best way to fill in ducts in a slab with concrete?

I have in slab ductwork that we are abandoning. I want to fill with concrete. What is the best way to fill the ducts entirely? Should I just have a concrete contractor come pump them full or can I just pour five gallon buckets down at a time?

On 2022-01-26 by Inspectapedia Com Moderator - is it functionally necessary to fill in all of the in-slab ducts completely?

@Travis B,

It's unlikely that manually pouring buckets of concrete into your in-slab ducts will be sufficient to push it all the way through all of the ducts; to do that you'd probably need to hire a concrete company who can pump concrete into the duct system with a hose.

I'm not sure that it's functionally necessary to fill in all of the in-slab ducts completely.

If a camera inspection shows most of the duct runs are not leaking water such as to threaten the building, I might pour concrete into the ducts at each of the floor registers. Typically you could fill just a few feet of duct at the register using this method.

I would at least consider the complete fill-in IF a duct inspection camera showed damage, cracks, breaks, and leaks under the slab throughout the home.

On 2022-01-10 by Denny Brown - gravel and asphalt shingles forming in-slab ductwork?

My house is on a slab and the duct work is made of concrete. There is gravel and what appears to be asphalt shingles in the bottom with more gravel on top of that. Is this a normal application? I am also having water pouring into my ventilation system

On 2022-01-11 by Inspectapedia Com Moderator - collapsed ductwork

@Denny Brown,

NO that sounds like very strange construction or perhaps collapsed ductwork.

BE SURE to read the explanation of the problems with ducts in slabs given above on this page.

On 2021-12-15 by Steve - I appreciate all the info here - what a wealth of knowledge! about in-slab HVAC ducts

Great info here; it makes me feel much better to read "We've been the human sump pumps with our shop vac ..." after spending last night sucking water out of our duct after being completely surprised to find it.

I assume the water is a result of poor drainage from the house though I'm wondering if this problem has existed for a long time without us noticing or if it's new. It did rain a lot recently, though we have had larger storms before.

Question: We've sucked as much water out as we can and are running the heater to try to dry things out as much as possible otherwise. Is there anything else we should do in the short term?

For the long term, I can look into sealing that portion of the system, though it does make me nervous that if the water intruded there that we may make things WORSE elsewhere as mentioned in the article.

I'll also look into what I can do to get water away from the house - gutter extensions, etc. There definitely was pooling water near the house since the hardscape isn't sloped sufficiently to shed the water away from the house.

On 2021-12-16 by Inspectapedia Com Moderator - how to properly abandon and seal in-slab air ducts

@Steve,

If the only water that you have found was in ductwork under a floor slab, that is to say if no components of your house such as drywall or trim or walk have these have been wet then it's appropriate to be focused for now on the in slab ductwork.

A description of the health and functional problems that may be traced to air ducts that were routed in a concrete floor slab as well as our advice on how to properly abandon and seal in-slab air ducts are found

at DUCT in CONCRETE FLOOR.

In the meantime to speak more directly to your question, you might measure the humidity level in your house to be sure that it's in normal range. And of course inspect your home talked about and be sure you don't have other leaks that you had not previously noticed.

You might have your furnace blower inspected and cleaned and of course change your air filters, and inspect any above ground ductwork for cleanliness.

On 2021-12-31 by Steve

@Inspectapedia Com Moderator, Thank you for the response - I got into the crawl space and was able to see where water was entering the slab portion of the house. There is definitely water coming into the crawl space and then into the chase where HVAC ducting enters the slab; this seems to be the lowest point and a low resistance path for water to take.

I've started focusing my efforts on exterior drainage starting with a surface and downspout drainage system to move water far, far away from the house. As you said, no other components of the house appear to have gotten wet - so hopefully once the exterior drainage issue is solved and the chase / crawl space is thoroughly dried out the problem will be solved.

I'm amazed at the volume of water - though we have had multiple days of heavy rain, including a 200 year storm event and our high clay soil seems to just be fully saturated with water and hasn't had a break to dry out. Thank you again for your time and response.

On 2021-12-31 by Inspectapedia Com Moderator

@Steve,

Very often the primary source of building water entry is roof spillage around the foundation - gutters not working;

That concentrates a huge amount of water right at the foundation walls.

When grade doesn't slope away that traps that spillage against the building.

Even if there were footing drains intended to take that water away, they fail: they are likely to be overloaded and ultimately clogged by that overloading.\

 

On 2021-12-03 by Pye - just bought a house with ducts in slab - what do I do?

I recently purchased a house built in the 90s that had a furnace closet that had been converted to a closet closet (furnace moved to a utility closet in the garage).

I had the house painted shortly after purchase in the course of which we removed two ducts under this closet on the front wall in the hallway and on a side wall in the living room at floor level, to discover a lot of filth and a 15" PVC pipe that is all smashed up and about 3' deep. Pictures attached. I assume this had something to do with the original furnace. Ducts for the functioning furnace are in the ceiling. Bathrooms and kitchen were tiled by the original homeowner, there is carpet everywhere else.

I have seen no sign of in floor ducts but there may be some sign under the carpets which we have not yet pulled up. I had assumed the thing in the closet was for cold air return but now I have no idea.

This thing is at least 3' deep and it looks like it may be going off horizontally at the bottom, I can't really tell.

So the questions are, what the heck was this for (if you can guess) and what the heck do I do about it now?

Can I just box around it - I am wallboarding over where the grills were mounted on the wall, it was all just open space and dirt back in there - or had I better do something more definitive about it (and if so, what)?

There was 30 years of dirt, dust, and dirty-dusty-cobwebs or something worse behind those grills and it seems dust and dusty odor is still hanging in the air since I tried to clean it out yesterday. Even with all the windows open.

Even over the paint smell from the paint job that was finished 2 days ago.

On 2021-12-03 by Inspectapedia Com Moderator

@Pye,

That doesn't look like PVC to me - I see what looks like a spiral assembly pattern and a papery-like material.

If the home had no in-slab ducts then what you set could have been an air return or a combustion air supply, or ... something else.

Why not fill it with stone and cement?

Or if you're really curious you could follow it using a sewer line camera first, before abandoning it.

On 2021-12-04 by Pye

@Inspectapedia Com Moderator,

I have read that entire article several times and have not found a passage or passages describing exactly how to do this.

What little I HAVE been able to glean from disparate bits and pieces and what you say above makes it sound like I have to pump concrete down that hole until it stops running and hope nothing collapses or shifts somewhere down the line and/or over time.

I don't have the equipment or the money to try to fill with concrete a 15" diameter hole that is over 3' deep and then joins with a horizontal duct of unknown length and diameter.

Probably that's why it was left the way it is, which is apparently the case for every single house in this subdivision.

I guess it will just have to go on as it is.

To be a little clearer: The instructions you DO give are to fill a heating vent in the floor to a depth of 2" with concrete. That is a much smaller area that would be nowhere near as deep as this ginormous hole into the earth under that closet @ 15" x >3' deep plus the horizontal run.

I would be comfortable with doing what is described in the article, but my situation is not only a much larger hole that goes way deeper and way further back but also is lined at the top with PVC down to the level of whatever that horizontal duct it runs into is, so at least 2' from what I can tell of slippery PVC that concrete won't bind with.

Concrete shrinks - admittedly not a whole lot but it does shrink and I am concerned that even if I were to fill that hole with rubble to within a foot of the top and then pour concrete, that concrete won't even remotely bind with the PVC liner.

Shrinkage is something a bit over a half inch per 100 feet which is tiny but combined with the smooth PVC walls I feel like chances are a plug like that in a PVC pipe would not be stable over time. Possibly that "time" is longer than I'm likely to live anyway and/or the instability I fear is either insignificant or nonexistant. But its not the same situation as described in that article.

At best I feel like I would somehow have to manage to chip out that PVC pipe down to at least 18", after of course totally tearing out the closet so I could even get at it. Also would still have to fill the bottom with rubble up to a foot below the top of the slab.

I fear it is too big a project all things considered, if I'm right to be concerned about that PVC liner, shrinkage, and slippage.

So I'll just have to leave it alone, I guess.

On 2021-12-04 by Inspectapedia Com Moderator - Problems to avoid by filling and sealing abandoned ducts in slab

@Pye

Thank you for the helpful discussion.

The reason I prefer to fill and seal abandoned ducts in slabs is to reduce the chances of a future problem with

• radon gas
• building water entry
• mold contamination sending harmful mold or MVOCS up into the occupied space
• avoid possible surprise drafts
• avoid pest havens or pest entry points especially for mice and occasionally bigger critters

The amount of trouble and expense for sealing a single opening is usually trivial;

For the 2-inch concrete seal example we give in this article, there was a small rectangular hot air supply duct in a concrete floor slab; the duct made a right-turn down near the bottom of the slab, less than 12" down.

We stuffed newspaper to form a backer and to reduce the amount of concrete needed, and we left about 2-inches from the floor surface or concrete slab surface down to our temporary stuffed-backer.

Then we filled concrete atop that to bring concrete in the abandoned duct opening up level with the existing floor slab.

You can certainly do the same with your PVC pipe.

While some contractors offer an expensive concrete pumping option to fill entire runs of abandoned under-floor-slab ductwork, usually that's not necessary nor cost justified. I would consider that more troublesome and expensive step in buildings subject to rising flood waters but otherwise I'd take the simpler approach I've outlined.

On 2021-12-04 by Pye

@Inspectapedia Com Moderator,

The top part is definitely PVC pipe. I can't say what is at the bottom. I'm a 63 yo disabled female so I'm looking for the easiest way for me to deal with this in the short term and while filling it in with rocks and topping off with cement is undoubtedly the RIGHT way to deal with it (and that is what I told my son should probably be done),

it would require ripping out most of the closet that currently sits on top of it plus a lot of physical labour that I am no longer really up to any more.

Also I have visions of the rock sliding down that horizontal whatever-it-is over time and the cement on top subsiding, possibly that's just little old woman paranoia.

There were actual pieces of PVC broken off in that cubby hole under the bottom of the "closet" so definitely the top part is PVC.

For the last 25 years (since whenever the original furnace was replaced) or so that has all been behind vertical grates where I assume cold air return ducts were - the ducting itself, if it ever existed, is long gone.

There may be signs of in-floor ducts under the carpet, the tile obscures everything where it has been laid (and it was not original to the house, I would guess given the time of construction that was originally sheet vinyled).

I guess it doesn't matter what it was originally IF it is OK for me to just wallboard over the mess. It is at least 3' deep and I am concerned about future moisture issues because it is so freaking deep and I don't know what's down there.

With the grates installed (I will attach pictures again), any moisture back there would have dissipated but if I close it off with wallboard it COULD build up. This is in Lubbock TX and its well under the house - doesn't rain a lot here, so again, that may be old lady paranoia speaking.

I guess my other option is to just put the grates back and continue to ignore it but given the amount of filth that built up back there over the years, that doesn't seem like a great option either. BTW it was just the duct GRATES I took down when painting, any ducting was itself long gone.

I suspect the small duct you see at the top left of the first photo is for the dryer, the laundry backs on this mess (the dryer is vented into the attic, yes, that's what I said, into the attic which is allowed by code here and yes I will be fixing that).

So those are the three solutions I can see - #1 do it right and fill it in; #2 leave it as is but cover with wallboard; #3 put the grates back up and just continue to ignore it the same way it has been ignored for decades.

#1 would be my preference if I were 20 years younger but given that's not happening, #2 is my first choice, but if #2 is a horrible idea then I'll most likely go with #3 I guess unless you tell me that's also a horrible horrible idea.

If #1 is really necessary any pointers on exactly how to do that would be great. EG rock size and layering of fill material. There are no quarries anywhere near here so laying hands on rock and gravel is a problem.

On 2021-12-04 by Inspectapedia Com Moderator

@Pye,

I wouldn't just add wallboard over an opening like that, I would first fill with concrete to at least a depth of a foot.

We describe that in our article on abandoning ducts in slabs.

It's actually quite easy to fill the upper part of a deep in-duct slab at the registers by stuffing in newspaper tightly enough to hold the concrete, then pouring concrete atop the paper "cork".

On 2021-10-29 by Jay - does this cement duct contain asbestos?

Wondering if this in cement duct looks like it contains asbestos?

On 2021-12-03 by Inspectapedia Com Moderator - cement duct looks like it contains asbestos

@Jay,

If that's transite ductwork from before 1986 in the U.S., ... yes.

But what we THINK we see is a terrible HVAC duct that was probably a flex duct product or paper-wrapped something that looks disintegrated by rust.
If that's right, it's not transite - which is an asbestos-cement product, it's metal duct that's shot.

It looks to as if some poured concrete oozed into the duct and the duct interior is blocked by duct fragments.

If this is a duct in slab it's one to abandon.

More asbestos-duct photos are

at TRANSITE PIPE AIR DUCT ASBESTOS RISKS

On 2020-10-07 by Anon - in slab duct problems: cracks, gravel, rodents, bugs, smells - covered by insurance?

Re-posting from private email:

I have a home in Indianapolis built in the mid 50's, the in slab ducts have become quite a problem. I can look in my ducts with my phone and see very large cracks, showing pea gravel where foundation was poured.

I have rodent problems in my ducts, many insects and a musty smell. Was curious if my home owners insurance would cover any of these issues? - Anonymous 2020/1-0/05

Moderator reply;

Thing to do would be to ask them. But I am doubtful - ordinary homeowners insurance will not usually cover original defects in construction of a building.

However **IF** an expert testified that the damage to your in-slab duct system was caused by an event that is covered by your HO policy, such as fire, flood, or earthquake (if your policy included those endorsements) THEN you might have basis for a claim.

On 2020-02-13 by Mickey - ants in around the HVAC supply vents in my home

I had been noticing ants in around the vents in my home so I removed them to vacuum them as I assumes they were dirty.

This is what I found.

The ducts are rusted away completely and there is nothing but dirt from under the slab.

This is a rental home and I’m seriously concerned that this may be an issue that the owner won’t want to fix

This Q&A were posted originally

at WET CORRODED DUCT WORK

 

On 2020-02-13 by mod) - metal duct register rusted away to dirt/soil below slab - health hazard

I suspect you are right on all counts.

That looks like a health concern to me.

All of the building's heated or cooled air is blowing simply over dirt below a slab.

Any leakage into that area or invasion by pests or chemicals to treat pests all add up to health risks.

See details at SLAB DUCTWORK

On 2020-06-19 by Sylvia - unpleasant smells coming out of ducts when the heat

I own a ranch style home that is in a concrete slab, and I get unpleasant smells coming out when the heat is on and I believe there are mice in the ducts.

How can I check the condition and detect problems in the duct work in the slab?

On 2020-06-19 - by (mod) -

Sylvia

You can have a duct company or plumber who has one, run a remote camera through the system to examine it.

But as you'll read in the article above on this page, if your ductwork is in the slab we recommend that it be abandoned and sealed for several reasons you can read in the article.

On 2020-06-14 - by Anonymous - sewage leak into heating ducts. Flies breeding in our ductwork!

Re-Posting

Anonymous said:

Need help figuring out possible slow sewage leak into heating ducts. I have lots of photos and videos. Just somebody please help. We have a house on a concrete slab.

No crawl spaces.

In order to see into the vent we had to rip out the whole furnace (which we did already see added photo.)

We got a new furnace installed.

The last 3 months, we have been trying to figure out why water has been pooling in a specific vent

. Fruit flies or sewage flies have been breeding down there as well.

This Q&A were posted originally

at SEWER SEPTIC ODORS in HVAC DUCTS

Moderator reply:

Anon:

That in-slab duct is unsanitary and not entirely functional. I'd abandon it.

See notes on in-slab ductwork hazards & solutions beginning above on this page.

On 2020-03-31 by Marcella - Termites in slab HVAC ductwork

Termites in slab HVAC ductwork in guest bedroom and main bath. 3 years straight. We closed off vent in bedroom and think that's what moved the termites to bathroom. Terminix pulls back metal at opening, sprays a foam inside.

Gets rid of termites for few days/hours, then back again. I can always tell when termites are back because of the smell.

Terminix had also dug a "trench" around the area where the rooms are but that has not helped either. Terminix has been out 5 times since February when termites first appeared. What can be done to get rid the termites?

On 2020-03-31 by (mod) - significance of Termites in slab HVAC ductwork

Marcella

It sounds as if you do need a more-thorough inspection and treatment, plus, as you'll read in the article above, some serious thought to abandoning the in-slab ductwork.

Above we explain why we abandon slab ductwork: leaks, rot, attack path for insect, mold, odors;

Watch out: I'd add that I'd be rather nervous about breathing air in a home whose conditioned air is blowing through ductwork treated with a termiticide.

Termites rely on wood-soil contact. Your inspection needs to include a survey of all of those points.

 

On 2020-02-19 by Aaron Burnham - Great information on duct work in concrete - what can be done with respect to air-conditioning in the summer if I abandon in slab ducts?

Great information on duct work in concrete, and I'm thinking about going with the heated water baseboard option in my 70 year old ranch/slab in Cleveland Ohio.

At the risk of sounding uninformed, what can be done with respect to air-conditioning in the summer?

On 2020-02-19 - by (mod) -

I prefer either using split systems or of course ducts that do not run in the slab but rather in walls and ceilings.

Never worry about sounding uninformed. I've been uninformed and have spent a liftime asking questions. What's more embarrassing is not having the nerve to ask and then suffering the consequences.

On 2019-10-06 by Jim E - 4 to 6 inches of water in my underground cement air return.

Ths 4 to 6 inches, not 40 to 6.:-)

I currently have 4 to 6 inches of water in my underground cement air return. I have no clue how the water got there.

Any suggestions?
I live in a very dry and arid location in Scottsdale Arizona so I know it's not because of rain or watering the lawns.

On 2019-10-06 - by (mod) -

Jim

Sorry but from etext alone I can't say for sure why your underground return air duct is flooded;

Considering that you're in a dry area and have had no rain and have not run a watering system that could have flooded the duct from outside, I'd be looking for a plumbing source such as condensate drainage or a leak supply or drain line.

For example readers have on occasion found a sewer line that leaked into the duct system. Or an A/C condensate system that leaks into the ducttwork

Combine looking for those potential sources with pumping out the duct and inspecting it using a sewer line camera. LEt me know what you're told.

On 2016-07-10 by Barry

Thank you for your response. You've said everything I was thinking but still not sure how such a shallow slope would allow the concrete to flow all the way to fill in everything, unless it was almost like a milkshake consistency. Wouldn't that just be useless for any kind of strength? Is there a way to actually pump a better strength material? I can't think of any company that would have something that could send the concrete down a vent, turn almost 90 degrees and still send the concrete to the end of the run. Any ideas there?

On 2016-07-10 - by (mod) -

I agree with the concern. You could use a concrete pumping service who insert their pumping hose in one register while you watch for the concrete to appear at the next one in the line. Or the liquid can make the bends you describe. It's an interesting question to ask your installer.

IMO the critical feature is to seal off all openings between the in-slab ducts and the building.

How to clean then fill with concrete our in-slab Sonotube air ducts

What a terrific article with so much sharing of issues and helps. Thank you. I have under floor ducts as well and have seen that they were poured with Sonotube concrete forms that have badly deteriorated from water. I have a couple of questions that have been very hard to find answers to.

First, is there a way to clean the ducts out so when the concrete is poured there will be a much better bonding between the floors and new concrete?

Secondly should the sump pump that was added because of the water issues in the ducts still be left in place once the ducts are filled?

Also, probably the hardest question to find answers to, how do you force concrete through the ducting so they completely fill the ducts? On 2016-07-08 by Barry

Answer by (mod) - Sonotube ducts are at risk of crushing; pump in concrete

Thanks Barry, we work hard to provider accurate, useful information and are really pleased when a reader finds it so.

Sonotube is particularly vulnerable to crushing as it's basically a paper product. If you're filling in the system with concrete that should be sufficient, without trying cleaning. (You could hire a duct cleaning company but frankly unless there is some unusual situation it's probably not cost-justified.)

Concrete can be poured into the duct system at each floor supply register.

Properly-mixed (not too stiff) it should flow to fill the in-slab duct system adequately, particulary if the pumping hose can be shoved into the air duct runs; if a few small voids are left in the system they're not highly likely to be a problem as they should amount to small sealed pockets that don't communicate with the building.

If there is water under your floor slab, keeping the sump pump working makes sense to me;

On 2016-11-19 by (mod) Does the insurance companies cover any remediation costs for sealing up the vents and relocating them to another area? attic or walls ?

JodiAnn

OPINION: I would doubt if many insurance companies will cover the cost of any building "repair" that is essentially correcting an original design, nor do most policies cover the cost to repair problems traced to wear and tear, lack of maintenance, nor in many cases leaks.

I agree that the cost to both close off in-slab ductwork and to provide alternative heat can be very costly.

But while I in-slab ducts are a common problem source, it would not be correct to assume that every home with in-slab ductwork has a serious problem that needs to be addressed.

It would be reasonable for a home inspector to inform a buyer or owner (whoever has hired the inspector, usually the buyer) that in-slab ducts present certain risks of air quality and health concerns, particularly if there has been water or flooding into the in-slab ducts

OR

if the area where the home is built is known to often have actionable levels of radon. In such cases it would make sense to plan to do a bit of exploring in the duct system, inspecting for evidence of collapse, flooding, or if appropriate, radon testing.

Some ducts that are in good condition and are below grade are re-sealed (using a spray) in place and continue in use, but I'm nervous about that approach as I'm not confident there won't be a future water and contamination problem. You wouldn't want to be the one "guaranteeing" that such a job will be in the future trouble-free.

I am not sure that it's appropriate either to assume that a new furnace is needed just because of in-slab ducts. If the up-flow furnace is relatively new and in good condition I'd probably consider modifying it to work with new in-wall above-slab ducts on the return side.

You are welcome to give your clients or homeowners a printout of this page, and any such readers are welcome to contact us by on-page comment (or in emergency we may be able to reply by email). We welcome questions or content suggestions.

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We are dedicated to making our information as accurate, complete, useful, and unbiased as possible: we very much welcome critique,questions, or content suggestions for our web articles. Working together and exchanging information makes us better informed than any individual can be working alone.

On 2016-11-20 by Anonymous

Thank you for your response. I appreciate your insights.

I didn't know that it was possible to convert a down spouted furnace to a side or up. I was told by a tech that it would need to be replaced, but I guess that is not the case.

The spray was something I thought would be the cheaper way to go. If you have a home owner looking to sell. They more then likely will want to take the cheaper route :)

Thanks again & have a good night
JodiAnn

On 2016-11-19 by JodiAnn

Hello, I have been reading through all your information and recommendations and have found it very helpful

However,I have a couple of additional questions. As a realtor in Monmouth County NJ I am often asked about this and to be honest I am afraid to answer. I don't want to open myself to being suited by a past client. So here are my questions

Does the insurance companies cover any remediation costs for sealing up the vents and relocating them to another area? attic or walls

This is quite costly. Most of the time it requires additional work to be done not just filling the slab with concrete.

A new furnace with an up-spout. Running all the ducks and possibile replacement of the AC. It can cost thousands of dollars so I've been told. $15,000 to $25,000 as an estimate is this true?

Most importantly does homeowners insurance cover this? ANy type of rider that can be added if needed?

I have also heard of a possible solution called Areo seal. Where they claim to spray a sealant into the ducks designed to prevent water coming in.

There are many homes in the area where I live with this type of duck work. I'd really like to be able to help make suggestions?

Any help or answer are greatly appreciated

On 2015-11-07 by (mod) - are some HVAC systems noisier than others?

Yes some HVAC systems are normally-noisier than others.

See SOUND CONTROL for AIR DUCTS, HVAC https://inspectapedia.com/BestPractices/Duct_Noise_Control.php as a place to start;

Also see

HEATING SYSTEM NOISE DIAGNOSIS https://inspectapedia.com/heat/Heating_Noise_Diagnosis.php

On 2015-11-07 by Dave

Our furnace seems louder than our a/c unit. The hvac system is really close to our family room and seems really loud at times. Is there some systems that are quieter than others? Or is there any way sound proofing the closet?

On 2015-10-20 by (mod) seal the vents with foam and concrete the top portion

Denny that can in my OPINION be a reasonable approach to duct slab abandonment; I've done something similar. The risk is that water and pathogens collect in the un-filled under-slab duct sections.

My view is that IF there are no cracks, holes, pipe or wire penetrations or other openings that would allow contaminants to pass through the slab surface into the building, then sealing at the registers may be adequate. Your final "top portion" seal needs to be fail-safe.

On 2015-10-19 by Denny

I'm about to enclose slab routed ductwork in a commercial site. My thought was to seal the vents with foam and concrete the top portion. What do you feel about that.

On 2015-09-16 by Anonymous

Joey, you'll want to call 3 contractors in your area to ask for bids on the new HVAC work and you may need to find a separate masonry contractor to pump the appropriate mix into and thus fill and seal the slab cucts.

On 2015-09-16 by Joeyz

Ok. I have the problem and want to get rid of it. I have a 1377 sf ranch on a slab. Can anyone give me a ballpark estimate on How much it would cost and who to call? I want to seal off everything, run ductwork into attic where I will need to have it insulated and buy a new furnace and ac unit.

Question: so much debris from our ducts & it's making us sick

Dec 5, 2012) TA

HELP! so much debris from our ducts & it's making us sick; burning eyes, nose, throats & when it comes in contact with our skin it burns which is hard because it is everywhere.

The debris is significant the dust is covering everything in the condo some days its 1/4 inch thick from floor then onto everything up to the ceilings. Our family is getting very sick & neighbors in their units are getting sick too.

We were told we could even get silica poisioning. The association claims it's not their problem; how do we fix these serious & life altering issues. We heard some of our neighbors that have been complaining about the dust actually died from lung cancer & never smoked other cancers too. We've also had raw sewage leaks into some of the duct work.

They refuse to fix it correctly. We are still walking on steam cleaned kah-kah ffrom the 2 raw sewage episodes. PLEASE HELP US our child is having sever health issues & docs are telling us to see toxicologists. How do we fix this? Built in 1985 with beer cans in the attics & duct work & we are ill & annoyed in ILLinois

Question:

(Jan 24, 2013) ls

If you abandon the ductwork and forced air system of heating, what is an efficent and affordable way to heat your home?

Question: is HVAC duct sealant silicone free?

(Feb 14, 2013) Anonymous

Is this sealant is oil and silicon free

Reply:

Not necessarily; check the MSDS for the sealant that you are considering, or tell us the brand and product identification and we'll help research that data.

Question: could my deteriorated in-slab ductwork be contributing to indoor condensation?

(Feb 28, 2014) Incognito

I know the in-slab ductwork in my house has detiorated. Could this also be causing the severe condensation problems I'm having on the inside of my windows and on some of the walls? This only happens in the winter.

Reply:

Yes Incog. If there is water in the slab ductwork that moisture may be transported into the rest of the building.

Question:

(Mar 9, 2014) John

Hi Dan,
Great article because I'm worried.

I have 2 registers in my living room Ohio home. One works fine (had furnace recently checked out ok) and the other has no air coming out. The orange clay pipe duct collapsed and what is strange is there is no duct symbol on the design print where that register is located.

Anyways, the living room has heated up fine for me for 20yrs. Ive just always kept the register closed. There is no water inside the register duct, only what looks like tons of pea gravel underneath that I can fell with my hand reaching into the duct and remnants of the broken clay pipe duct.

My question is can I just legally close of this registerduct by filling with concrete or leave it as is? I want to sell my home this Spring and my conscience won't allow me not to disclose this on my property disclosure form, but I'm worried that this will make it hard to sell my home or impact the price greatly. I cant afford to put new heating system in and run all new ducts in the attic.
Help (wish I could talk to you).
thank you, John

Other details:
My house is on a slab.
The clay pipe ducting is in or under the slab.
Built in 1971

Reply:

Opinion: I would fill in the abandoned duct with concrete. The code or sale issue would perhaps turn on adequacy of heat delivered to the space. If you have other supplynregister(s) into the area that may suffice. If there is no active heat source into that room you may run into an appraisal or code compliance issue. If there is a supply,duct into a room on a opposing wall that might be extended to the unseated room as well.

Follow-up

Thanku Dan,

I plan to fill the register vent/duct with pea gravel and top with concrete at buyers request and get a local certified hvac techs approval. There is a 2nd register vent in the living room that works fine (95+F air).

thank you sir

John

Reply:

Thanks for the clarification John; having a second working register in the same room may get past the "no heat source" concern at appraisal time. Niether home appraisers nor home inspectors will normally perform a heat loss calculation nor heating adequacy assessment unless there is a visually apparent defect.

Question: rusty cement smell in the cooling system

(Mar 10, 2014) Wendy

I can smell musty cement in the evaporative cooling system in the unit I have just rented. Should I be concerned about this ?

Reply:

Wendy, the system may need to be cleaned; I don't know what you're smelling, but as an evaporative cooler cools by blowing air across water, if the water source is contaminated with bacteria it could be unhealthy.

Question:

(June 6, 2014) Anonymous

Thanks for this informative article. We are considering purchasing a house (split ranch) with duct under concrete in the lower level of house. I am guessing we can pour concrete to the ducts, close the vents and lay new ducts through the walls . Is this a realistic approach ? Any idea how much cost we are looking at ?

Question:

10/10/2014 Bryan

My home is approximately 6 years old and about 1 year ago I noticed noise coming from the under slab register openings as when heat/cooling system was blowing. I exposed the air supply and register vents and found standing water.

The builder used what appears to be flexible stainless conduit under the approximately 45ft span from mechanical room to below windows (2) on far side of family room wall. The room is a walkout basement room with open section to the main levels of the house for full air circulation.

Upon inspection again today I noted same problems following heavy rains and noted what could be mold and algae.

The conduit was pumped as much as possible and it is noted that holes near the air supply end exist in one line. Digital pics taken from each end show crushing of the line and obvious signs that water had been in line for substantial periods. What are my best options for the situation. I currently have sealed the line with foil insulation wrap and plugged air supply lines on main duct.

Reply:

I'd like to see your photos of the slab duct conditions - use the email found at our CONTACT link (seen at page top or bottom).

Because the leaks into sub-slab ducts are and have been recurrent and are likely to recur, the best approach is to abandon and fill in those ducts and reroute the ductwork. In some houses where the cost of rerouting ducts is heinous owners opt for adding alternative heat, even a few electric baseboards.

Question: noise from ducts under the slab - water, collapse: what are my options?

(Oct 9, 2014) Bryan

My home is approximately 6 years old and about 1 year ago I noticed noise coming from the under slab register openings as when heat/cooling system was blowing. I exposed the air supply and register vents and found standing water.

The builder used what appears to be flexible stainless conduit under the approximately 45ft span from mechanical room to below windows (2) on far side of family room wall. The room is a walkout basement room with open section to the main levels of the house for full air circulation.

Upon inspection again today I noted same problems following heavy rains and noted what could be mold and algae. The conduit was pumped as much as possible and it is noted that holes near the air supply end exist in one line. Digital pics taken from each end show crushing of the line and obvious signs that water had been in line for substantial periods.

What are my best options for the situation. I currently have sealed the line with foil insulation wrap and plugged air supply lines on main duct.

Reply:

Bryan

I'd like to see your photos of the slab duct conditions - use the email found at our CONTACT link (seen at page top or bottom).

Because the leaks into sub-slab ducts are and have been recurrent and are likely to recur, the best approach is to abandon and fill in those ducts and reroute the ductwork.

In some houses where the cost of rerouting ducts is heinous owners opt for adding alternative heat, even a few electric baseboards.

Question: high radon levels in house with ductwork in the slab

(Nov 1, 2014) Sam said:
I have a 3 level house. Level 1 and 2 have slab floors. Level 3 is above level 1 and has wood floors. A sub floor set of ducts carries hot or cool air through the house from two furnaces located on level 1.
The house has a high radon reading. The radon cycles daily with lower readings in the day time and highest readings at around 1 am at night.

I installed a radon mitigation system on level 1 but was told thats its effectiveness could not be guarenteed becasue of the presences of in slab ducting.
I want to resolve the radon issue.

After reading a lot I felt it would be reasonable to consider the following as mitigation.

1 isolate the inslad system system from the ducts that connect to level 3 with wooden floors
2 seal the exits of all the in slab ducts
3 Find a way to put negetive pressure on the in floor ducts snd exhaust

I thought I would replace the current 2 zone ducted system with heat pump cooling and heating. Each room having its own unit allowing individual control
2 find a way to apply suction to the remaining ins

Reply:

Sam,

Rather than the cost and trouble of trying to establish negative air pressure in the in-floor or in-slab ducts - which is impossible if the ducts are being used to supply conditioned air to parts of the building - a more effective approach is to abandon the in-floor ducts by filling them with concrete (best) or filling and sealing at least the ducts at and around each supply and return register using concrete.

A heat pump does not of itself eliminate the need for ductwork.

But if you refer to using ductless split systems for heating and cooling that could work. Seeinspectapedia.com/aircond/Split_System_AC.php

Question: in-slab ductwork in a Minnesota home

(Jan 10, 2015) Stu said:
We are considering buying a home in MN that was constructed in 1997. It would be our perfect house in many ways. It is a high quality custom built home by a very reputable builder in this area.

However, when touring the home, I noticed that the walkout (finished) lower floor has in-slab duct work.

Based on what I am reading here, none of it sounds good. Are there ways to complete this type of construction that would mitigate the concerns?

Or, in your opinion, do we need to walk away? Thank you for your time.

Reply:

Stu:

If it were possible to guarantee that in-slab ducts were as leak-proof against water entry or condensation accumulation problems, rodents, etc. as above-ground duct system I'd be encouraged.

If I were buying such a home I'd feel safer assuming that at some point I'm going to want to convert the duct work.

Question: where is water coming from when it leaks into our in-slab HVAC ducts?

29 Jan 2015 Fred said:
We have water getting in a few areas of the ductwork in a 3 unit condo building the concrete slab was poured at one time and we have footer drains along the perimeter of the entire building were is the water getting in at ?

Reply:

Fred

I'd like to help but from a one-line e-text I can't know enough about your building to diagnose where water entry problems are originating.

Usual problem sources are mis-handling of roof runoff, in-slope grade draining towards the foundation, or missing, faulty, clogged footing drains. In all events, ducts in slabs are likely to give recurrent trouble as the article above explains.

Please take another look at that information as it might change the direction of your thinking.

Question: heat from the furnace(s) is pumped into metal troughs/channels

(Apr 13, 2015) Amanda Dunc

Have you ever seen a slab ductwork system where instead of traditional vents, heat from the furnace(s) is pumped into metal troughs/channels that extend around the exterior perimeter of the rooms? (Ranch built in 1966 with finished lower level.)

We just moved into a house with this setup. There are odor issues in the basement which appear to be from foundation cracks, but I've been trying to figure out the heating system as we may have to get into this air distribution system during foundation work.

Reply:

Yes indeed, Amanda. Beginning in the 1960s some builders considered long, narrow floor heating registers as a modern wat to better deliver more uniform heat to areas otherwise difficult to address, especially in the floor in front of sliding glass doors.

Start by identifying your type of heat: furnace and forced air, or boiler and hot water baseboards or floor convectors. If your system uses warm air then you may be looking for leaks into In slab ductwork as discussed in the article above.

Question: inspector commented in-slab duct work is in excellent condition and is not presently deteriorating or disturbed

(June 2, 2015) Jodi

The inspection of the home we are in the process of purchasing has revealed that the distribution ducts for the heating system are made of transite. Our inspector has commented that the duct work is in excellent condition and is not presently deteriorating or disturbed.

Much of the information that I am finding is regarding sealing or replacing the ductwork. However, it is my understanding that if the ductwork is not showing any signs of hazard that it does not pose a threat. Can you verify that this is indeed the case?

It is also my understanding that there is no way to test for asbestos without disturbing the material, thus creating a problem that would then need to be remedied. Is there any way to test for the presence of asbestos without compromising the current state of the ductwork?

Thanks in advance for any information you may be able to provide!

Reply:

Jodi, how can we verify if this is the case without viewing all the lines, did the inspector scope all the lines? was there air quality tests done in the home? Did he test the material? Sounds like you are concerned? Ask yourself how the ducts get cleaned, and what kind of work is involved in that? Can that potentially disturb the material?

Bottom line... air quality is extremely important, and when there are risks associated you should try and reduce as much risk as possible.

I think most people would agree with that statement. Unfortunately money needs to be spent and the most important thing in life is a persons health. Get professionals involved to provide you with the right information.

Was your inspector working for you or was s/he working to please a real estate agent or seller?

(June 13, 2015) Bill commented on the above and warned about conlicts of interest.

I agree with Bill. Frankly it sounds to me as if your inspector was suffering from a conflict of interest: wanting to avoid upsetting the real estate agent. I claim this because even if the ducts were in good condition - which Bill points out the inspector could not possibly know without direct examination of the entire duct runs - ducts in slabs and transite air ducts ultimately give trouble in buildings, as you'll read in the article above.

If I were buying the home you describe and there were in-slab ducts of any kind, my financial plan would provide for duct replacement and re-routing even if that improvement or correction were to be deferred. Its urgency will only be known when the duct system has been properly inspected.

Thanks for the comments Bill.

Question: heat loss through ducts in slab

(June 17, 2015) Anonymous

is there any heat loss in a in concrete slab duct system in a home where to ductwork located in the slab
runs around the perimeter of the home?

Reply:

Sure, if the ducts are not insulated, in addition to condensation and contamination concerns both heating and cooling air will be affected by the temperatures of the surrounding concrete and soil.

Question: IAQ problem at our house we can't figure out. Terra cotta slab ductwork

June 23, 2015) Alison said:
We have an IAQ problem at our house we can't figure out. There is a strange odor and we get sore throats. We've had all the tests. It's not mold, radon, etc etc. The odor stays on clothes after we leave the house until they are washed.

The house has a partial basement and a living room is on a slab. I think the ducts under the slab are terra cotta. I can see a crack. I think this might be the source of our IAQ problem. I don't know what else it can be. I just don't know who to call to diagnose the problem so I know for sure.

I've called a couple HVAC companies and they weren't any help. Who should I call?

Reply:

Terra-cotta under-slab ductwork would be a bit unusual, including fragility, too-small diameter, difficulty sealing leaks between sections. More likely the material you have is cement-asbestos.

See ASBESTOS TRANSITE DUCTWORK

Look for an HVAC company or inspection company who can provide a camera to scope the ductwork to observe its condition - before you abandon it as you should.

Question: odor from vents and spaces around pipes coming through slab

(July 29, 2015) Anonymous

My husband and I recently bought a 1950's ranch on a slab foundation. We spent a few months painting the entire interior and putting in new flooring before we moved in. Now after living in the home for a few months, we are noticing an odor coming from the vents and from open spaces around piping under the bathroom sink cabinets.

We've contacted three companies, from HVAC to duct cleaning, for help and none of them were able to do anything for us.

Today we are moving to our fourth option of having Roter Rooter use their cameras to scope through our ducts.

We were told there is a board lining the duct and it looks like moisture may be a problem.

I'm worried about the odors and gases my family is being exposed to and need some help as to where to turn for finding someone experienced enough to be able to help us. We live in Ohio. Any suggestions? Thanks so much!

Reply:

It would make sense to check for sewer gas leaks and to track down a broken sewer line under the slab: use a sewer line camera scoping service company to inspect the system. Infrared or thermography can also help find leaks in sewer piping under a slab.

Question: ducts in concrete slab of my condominium: floor too hot, can't add zones.

(Aug 5, 2015) JillZ

I just bought a 30 year old condo and now that I am having the floors redone, we have discovered that every one of the 3 floors is concrete and all of the ducts on the 2nd and 3rd floors are in the concrete floor. The lowest level, a walk out basement, has ducts in the ceiling. It is built like a hotel!. There is also only about 14 inches of attic space.

My problem is that the top floor is extremely uncomfortably hot. I close all of the registers in the rest of the house and run the ceiling fans 24/7 but it doesn't really help. I had an hvac guy come look today and he says that he can not put in a 2nd zone because he doesn't have access to the ducts.

He thought he could put a ac unit on the roof and just the ducts in the attic, but this will require HOA approval and will have to run an electrical conduit on the outside of the building; probably not going to be approved by HOA. I am at my wits end. I can not live in this condo with the upper floors at 80+ degrees all summer. Thank you for any help you can provide.

Reply:

Your AC guy may be dead right but I'd get another opinion.

Usually we can locate the trunk or main air feeder between building areas and install thermostatically-operated vent dampers in those locations. Maybe not, depending on your duct layout. But I'd want to make darn sure before leaping to a much more expensive solution.

Question: concrete used for filling in in-slab ductwork

12 sept 2015 Rachelle said:
What type of concrete do you suggest when filling in a duct system? From reading over the different types, it looks like self compacting/flowing concrete should be used. Any thoughts would be greatly appreciated. Thanks!

Reply:

I agree that you want the concrete to flow into the duct system and that complete duct fill-in would be best;

Some sources we have studied (JLC online forum cited above) suggested using a slurry of concrete mixed with crushed limestone that can be more easily pumped into the in-slab ductwork to fill it completely.

...

Continue reading  at SLAB DUCTWORK - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see SLAB DUCTWORK FAQs-2 - more-recent questions and answers about in-slab air ducts.

Or see these

Recommended Articles

• ASBESTOS TRANSITE DUCTWORK
• DUCT & AIR HANDLER ODORS
• PVC HVAC DUCTS
• SLAB DUCTWORK - home
  • HOW to RECOGNIZE IN-SLAB HVAC DUCTS
  • WHY IN-SLAB HVAC DUCTS are TROUBLE
  • FUNCTIONAL PROBLEMS in IN-SLAB HVAC DUCTWORK
  • ENVIRONMENTAL PROBLEMS in IN-SLAB DUCTS
  • AIR QUALITY (IAQ) PROBLEMS in IN-SLAB HVAC DUCTS
  • IN-SLAB DUCTWORK INSTALLATION ERRORS
  • ABANDON or SEAL IN-SLAB DUCTWORK
• WATER & ICE IN DUCT WORK
• WET CORRODED DUCT WORK

Suggested citation for this web page

SLAB DUCTWORK FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC DUCT SYSTEMS

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