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

hese 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.

On 2018-10-04 by (mod) - Taking return air at a fireplace could interfere with fireplace draft

Debbie

Often the HVAC installer/designer has to make compromises to fit the system to the house. And, with apologies, I can't know enough from a brief text to have a full, confident opinion.

In general:

Optimum return air for heating systems is near the floor where it picks up cooler air;

Better return air location is to keep it some distance from the supply air for both heating and cooling, to avoid a short circuit in the system.

Taking return air at a fireplace could interfere with fireplace draft, but, then, building a new home you ought to be providing outdoor air to the fireplace so as to avoid that issue in the first place; otherwise the fireplace just sucks heat out of the home.

And in an extreme or worst-case, drawing return air at a fireplace can be unsafe, drawing combustion products into the home or even (rare) contributing to a house fire.

Research on Fire Spread due to HVAC DUctwork

• Quintiere, James. "The spread of fire from a compartment—a review." In Design of Buildings for Fire Safety. ASTM International, 1979.
  Excerpt:
  
  The spread of fire from a compartment is considered as spread through a window, through a doorway, through openings associated with entry conduits, or ultimately through openings caused by deterioration of the structure shell. The literature is reviewed for each mode of spread.
  
  Emphasis is placed on the interaction of building geometry and building materials and their relationship to fire spread from a compartment. Examples are given in which mathematical design procedures or analysis could determine a fire safe design.
  
  Recommendations are presented for continued studies and practices in this area.
  

On 2018-10-04 by Debbie - air vents located close to fireplace

We are building a 1900 square foot one level home on a slab. No basement, no upstairs. The heat vents are all located in the ceilings. I noticed two cold air return vents in an area where we will have a built in cabinet.

So they raised the cold air vents to the ceiling. These air vents are also located within inches of our fireplace. Is this ok, or will it cause inefficiency in our heating/air conditioning.

On 2018-07-17 by (mod) -

Randal

Thank you for an interesting question. I too am cautious about assuming that water is not going to get into an under slab duct.

It's just my experience after many years of investigating buildings that assuming everything is going to go according to plan can be a little risky and it's better to design to avoid trouble.

Still if that's the only option to you or the only one that's appealing, it would be worth some more research into the specific methods proposed by your company.

If you can find out the name of a process that they're using I will research that further.

In my experience relying on sealing the interior surfaces of any duct is rather iffy.

You can't see it all, you can't monitor at all, and you don't know when the sealant has failed to adhere, been left out through some spot, or have simply fallen off later.

I still might try it if it were quite inexpensive, but I wouldn't bet on it as a long-term solution.

That's a confession of a starting opinion, as a way to resist that bias as we continue to review.

So let me know what products you can find and I'll do some research as well.

On 2018-07-17 by Randal

I am looking at a home in Albuquerque, New Mexico. It like many, have in slab ductwork. The home is 18 years old and already shows moderate to heavy rusting.

The heating is gas and the cooling is evaporative. I was told by a duct repair company that they clean out the duct, lightly brush it, fill the bottom with about 1 inch of concrete, and then spray the rest of the inside with a sealant that is about 1/16 to 1/4'' thick.

The duct repair specialist recommended that foundation drainage be addressed and in this opinion this would take care of matters

. I am concerned because this is a flat top home and there is no place to put new ductwork unless it comes from the top, exposed to the outside. I am also concerned that narrowing the diameter of the ductwork (round sheet metal) will cause a constriction

I calculated that cutting the diameter by 1/4 to 1/8 of an inch can reduce airflow by more than 10%

I cannot find long-range data on this repair system and have a feeling that I am stepping into a larger, long-range problem.

Thank you for your input.

Randal

On 2018-04-24 by (mod) -

Something about this doesn't sound right. you should be able to abandon or fill in an in floor slab duct without interfering with a sub slab drainage system.

On 2018-04-17 by Dave

I have the problems discussed here with specifics being the duct work installed in the slab of a basement. Further though, the duct work was run over top of a french drain type system that leads to a basement sump pump. I have a register a few feet from the sump pump.

The duct work has completely rotted away and is no longer even visible. As such, I can see to the bottom of the drainage and have been able to observe water from below the register into the near sump pump.

While I have already abandoned heat to the basement and no longer have the furnace running, I am unclear as to my options at this point given that I don't believe I can fill in the register openings as the same would then potentially block the drainage system leading to the sump pump. Any suggestions or solutions would be greatly appreciated. Thank you!

On 2018-03-19 by (mod) -

Amanda

If possible I'd have the under-floor ducts pumped dry and sanitized, then filled-in with concrete. Your furnace will probably also need to be cleaned and probably modified to handle connection to above-slab ductwork.

On 2018-03-19 by Amanda

I have a 1 level concrete slab house. We found standing water in the vents & there was a god awful smell (it smelled like died) & it got worse when the heat ran. What is the best way to fix this & if we fill in the vents & duct work in the floor with concrete should we have the vents & duct work under the floor cleaned 1st?

On 2018-01-07 by (mod) -

Doug

The best solution for your home is not one I can make-up by e-text for a home we've never seen. I would not assume you need to put a new furnace in the attic, though that might be the least-costly solution given we want to avoid the disruption of routing ducts through finished space in the home.

Your heating company should have a duct designer who can look at the home, suggest the most-effective and most-economical routing of ductwork and connection of it to the existing or to a new furnace.

On 2018-01-07 by Doug

I have a slap on grade home built in 1961 with hvac duct in slab. What type of new system can I install? Must i install the new furnace in the attic?

On 2017-12-29 by (mod) -

Ann,

It might make sense to install insulated ductwork to deliver the warm air to the areas where it's wanted. But I agree that there are freeze risks if the crawl space is too cold. Current best-design is to convert a crawl space to an enclosed, INSULATED, dry, conditioned space.

So your crawl space may need wall and rim joist insulation as well as steps to assure its dry, as well as a heat source. One duct may include one or more registers that allow some but not all of the heated air into the crawl area. That's a design question for your onsite HVAC company who can benefit from seeing the home and from knowing its climate.

On 2017-12-29 by Ann

my old one story home has a forced air furnace that blows into the crawl space with no duct work..so the rooms close to the furnace are warm as well as the flooring, but the rest of the house farther from the furnace is cold..is it possible to put some type of duct work in to help push the air farther through the house more evenly? also my plumbing is under there, so how would I keep that from freezing if duct work is installed?

On 2017-12-28 by Marcia Thompson

Of all the choices for in slab duct work would the AQCs Blue Duct be worth considering or given the choice and all the issues cited in your extensive article, would you still recommend avoiding an slab duct system?

On 2017-12-27 by (mod) - Hercules HDPE plastic ductwork

AQC's Blue-Duct is an underground HVAC duct system using HDPE plastic duct components rated for "direct burial". HDPE plastic resists corrosion and air leaks.

An example photo, below, is clipped from an AQC-Herculese Corp. training video on the installation of this system.

The product is produced by CDC Enterprises, Inc. in the U.S.

On 2017-12-27 by (mod) - AK Blue Duct Pipe manufactured by Hercules Industries.

RE-posting

Marcia Thompson said:

Have you heard of AK Blue Duct Pipe manufactured by Hercules Industries.

They claim it is " the only proven direct burial underground duct system made from Ultra durable Advanced high density polyethylene designed to protect against corrosion, mold, mildew, radon, rust, water and air leaks.

The Blue Duck systems have been installed in Thousand buildings for over a decade with trouble-free long-term performance.

On 2017-11-28 by Marcia Thompson

Thank you so much for taking the time and providing such good references. We will probably avoid taking the risk. Much appreciation and this is a great site. Keep up the good work.

On 2017-11-28 by (mod) -

Adding:

NFPA 90A covers construction, installation, operation, and maintenance of air conditioning and ventilating systems, including filters, ducts, and related equipment, to protect life and property from fire, smoke, and gases resulting from fire or conditions having manifestations similar to fire. - available at http://www.nfpa.org/

Bottom line: in my OPINION I would always avoid putting HVAC ducts into a slab.

On 2017-11-28 by (mod) -

Advantages and disadvantages of using Sch 40 PVC or Sch 80 as in-slab HVAC air ducts:

I take it you mean that the contractor is using large-diameter PVC pipe as HVAC air ducts? If that's the case I would still be concerned about ultimate leaks into and contamination in the in-slab ductwork, though the contractor and I might agree that PVC is probably less-likely to leak from corrosion than the older traditional metal ductwork.

I'd want to review also the specific PVC piping used and to check its specifications for tolerance of the temperatures produced by the heating system (probably ok) and for its rating and listing as approved for in-slab ductwork.

An advantage of PVC ducting is that it's less temperature-conductive than metal.

But still I would want to know how the in-slab ducts are being insulated against un-wanted heat transfer or heat loss into the slab. If the slab temperature is ever going to drop below say 55 degF the risk of condensation in the in-slab ducts may be significant.

Any in-slab duct system can and probably will accumulate condensate inside the ducts even if ground water is not leaking into the system.

That in turn invites development of mold or growth of bacterial or other pathogenic hazards. Going from metal to PVC does not eliminate that risk. But better insulation and less heat loss might reduce it.

PVC is not designed to withstand temperatures over 140degF.

PVC ducting for air movement may require special "site-made" adapters to connect it to the metal components of the HVAC system plenums and air handler - a place to watch for jury-rigged snafus.

Here is an excerpt from one of the model codes, the

International Mechanical Code or IMC 603.8.3 Plastic Ducts and Fittings

Plastic ducts shall be constructed of PVC having a minimum pipe stiffness of 8 psi at 5 percent deflection when tested in accordance with ASTM D 2412.

Plastic duct fittings shall be constructed of either PVC or high density polyethylene.

Plastic ducts and fittings shall be utilized in underground installations only.

The maximum design temperature for systems utilizing plastic duct and fittings shall be 150 degrees F.

Also see NFPA 90A that does NOT list PVC among permitted HVAC duct materials and review this fire safety question with your local code enforcement officials.

NFPA 4.3.1 Air Ducts:

NFPA 4.2.1.1 Air Ducts shall be constructed of iron, steel, aluminum, copper, concrete, masonry, or clay tile, except as otherwise permitted in 4.3.1.2 or 4.3.1.3

NFPA 4.3.1.2 Class 0 or Class 1 rigid or flexible air ducts tested in accordance with ANSI/UL 181 Standard for Safety Factory-Made Air Ducts and Air Connectors, and installed in conformance with the conditions of listing shall be permitted to be used for ducts where air temperature in the ducts does not exceed 121°C (250°F) or where used as vertical ducts serving not more than two adjacent stories in height.

On 2017-11-28 Marcia Thompson

Thank for your response. Just to clarify, I wasn't referring to a radiant heating ststem in the floor. We recently visited a home renovation project in which the HVAC contractor installed some type of PVC pipe instead of metal ductwork beneath the concrete slab. The owners wanted to eliminate the metal ductwork in the ceiling and therefore had it installed under the slab. We are considering the same system but we have radon and a radon mitigation system.

Sounds like the issue with radon leakage is caused by cracks in the slab not the material used for the ductwork. Correct? Do you know of any other problems with using PVC pipe instead of metal ductwork under the concreate slab?

What would you recommend is the safest material to use for ductwork under a concrete slab? And, if it were advising us, would you even consider putting the ductwork under the concreate slab. Thanks

On 2017-11-27 by (mod) -

No, Marcia,

IN-slab heating using hot water from a boiler, also referred to as radiant floor heat, is a closed system. You would not expect ground water to leak into the piping distribution system.

The piping or routing of slab heat itself does not affect radon leakage into a building through the floor slab as long as the slab is sound - i.e. not cracked.

Any building heating will create a stack effect that causes warm air to rise in the structure; that pressure differences increases the movement of soil gases up into the building if there are openings (such as slab cracks or porous concrete block) through which it can pass.

On 2017-11-27 by Marcia Thompson

Would installing PVC pipe for in slab heating system pose the same issues mentioned in your article? We also have radon and have a mitigation system in place but do not want to increase the radon problem.

On 2017-11-13 by (mod) -

Sorry, Joanne, but I don't have a specific recommendation for you. However if you tell me exactly what a contractor is proposing to do, I may be able to suggest some follow-up questions to ask.

On 2017-11-13 by Joanne

Can you recommend a company that would be able to fill in underground duct work in the Pheonix area ' Also need a company that would be able to install a sump pump.

On 2017-07-24 by (mod) -

Scott,

I forgot to add that

... where you find vines in an in-slab duct you can be certain that at times water is or has also in the ductwork. I would abandon such ductwork.

On 2017-07-23 by (mod) - troubleshooting high indoor moisture possibly due to in-slab ducts: vines in the ducts?

Scott

In many in-slab duct systems, over the life of the building, ducts leak somewhere. I would if necessary hire a plumber who has a sufficiently-long sewer camera system - some duct companies also have these - to inspect the whole system.

Rust may be from

- water entry from below or outside the building

- condensation (ducts in cool slab not insulated)

- leaks or floods in the building that leaked into the duct system

Where you see gaps but no obvious leaks it may be that the ducts are surrounded by concrete and no dirt or vines or rodents are entering there.

But where you find vines in an in-slab duct you can be certain that at times water is or has also in the ductwork. I would abandon such ductwork.

IN general if you are seeing a lot of indoor moisture and you become confident it's not a duct in slab problem, we need to go looking for another moisture source.

I'd start at - MOISTURE PROBLEMS: CAUSE & CURE - https://inspectapedia.com/Energy/Moisture_Problems.php

On 2017-07-23 by Scott

Thanks for your quick reply. During home inspection, they looked in a couple of the ducts and didn't find problems past the boots. However, I recently bought a borescope and decided this should be the day to look myself.

My flex rod is only about 15 feet long so I can only push the camera in about 15 feet. I looked into every duct. Some seemed ok except the normal settlement and dust debris. A few however had some definite rust spots where obvious water had been.

One even has vines inside it. The vines don't appear to be actively growing, but they obviously came through a hole in the duct and had been active for a while. There are a couple poorly constructed joints where I can see major gaps (1/2 - 1") between the metal material. I don't see rust or dirt in the gaps though.

No standing water though. I have a bit of a hard time believing the gaps are where my humidity issues are coming though. The ground here is bone dry as it hasn't rained in 2months and there's no obvious point of active water entry.

Regardless, I'm going to have a duct company come out and give me a better inspection and estimate. As for the HVAC system, both of the companies I had out said my system was sized correctly.

I would have thought at least one of them would be interested in selling me something if it were an obvious solution. I don't think duct work along the exterior of the house is a good idea, as this house will never sell if I ever choose to do so. Too many questions from potential buyers, I'd think. That means I can't seal the ducts in. I need to find a solution with what i have.

Thanks again for your feedback and thoughts. I'm sure there are other solutions that I've not even considered.

On 2017-07-22 by (mod) use a remote camera to inspect in-slab ducts for water, breaks, rodents, mold

If you have not had all of the in slab ducts in the specs for signs of leaks or periodic water entry I would do that.

A remote camera works perfectly for that job. If the ends lab ducts are seem to be absolutely dry then I would look next at the size of your air conditioning system to be sure that it is correct since a two large cooling system will not dehumidifier. Tell me what you find with those two steps and then we can proceed further.

On 2017-07-22 by Scott: tracing high indoor moisture to slab ductwork

Thank you for this great article. I'm trying to figure out my homes problems and decide on a course of action. I'm in a two story house. My problem is very high humidity. I have two dehumidifiers running, pulling out about 4 gallons of water every day. This only happens in the summer. The house is about 1800 sq ft downstairs and additional 1800 upstairs. The upstairs has nice crisp, cool air.

The downstairs has cool, but moist air and often feels like a sauna, despite the air temp being cool. The lower level has in-slab ducts with a single air return. I have increased the size of the return, as multiple HVAC contractors said it was too small for the size of the space. They also all confirmed that my HVAC unit is sized correctly for the size of the space. I do not see signs of duct failure, though during our inspection, the register boots were corroded or rusty.

We had them install new boots prior to closing. A separate issue with the house is that I've discovered the Sheetrock that was installed between the exterior siding and the 2x4's had cracked and been removed near the bottom of the structure.

It appears the previous owners just nailed the siding into the 2x4's to remove the gap that was left. I'm concerned that has created inadequate insulation so now we have humidity being sucked in through the exterior walls. Basically, no vapor barrier.

What is more likely causing my issues? Wall insulation or in-slab ducts? I can't afford to do everything so need to pick most likely culprit. Thanks for the feedback.

On 2017-02-15 by (mod) - meaning of standing water in ductwork

MG

Standing water in in-slab ducts now or in the past would tell me

1. I'm going to abandon use of that duct system, as it invites mold, other pathogens in my indoor air, improper heating and cooling operation, maybe even unsafe operation. Abandoning usually means filling-in as much as possible, sealing openings into the building, and installing an alternative heat distribution system - above-ground ducts or as was done in my house, conversion to forced hot water heat.

2. There are leaks or water sources (perhaps roof drainage) sending water under the slab, asking for an indoor moisture, water, or mold problem that need to be diagnosed and fixed

3. an alternative duct system is needed, above the slab.

On 2017-02-13 by MG

Hello! Thank you for all the useful information so far! We purchased a 1968 house in which the heat/cool supply runs through the slab. Upon moving in, we noticed standing water in the the ducts. At that point, we installed an interior french drain and sump pump.

I don't believe we have had any standing water (at least noticeable) since the french drain installation, now at 6 years. A couple years back, we hired a duct cleaning service to clean as I noticed the vents appeared dirty (rusty and full of bugs).

The duct cleaning company said they could not clean our ducts because they believed them to be asbestos lined. Fast forward, we are currently remodeling and think it is a good time to address the duct situation.

Do you believe that we should first line the ducts with protective coating and then fill with concrete? Or do you think we should just fill with concrete to abandon? As you know, this is a costly problem which also includes re-routing several supply lines for heat to the basement areas. Thank you, M.G.

On 2017-01-18 by (mod) recent research on HVAC ductwork in slabs focusing on Canadian buildings or Canadian research

In the 1940's and 1950's even into the 60's it was common to build homes with HVAC ducts in slabs in North America, including both the U.S. and Canada.

I haven't found a census of slab ductwork in Canada, but by 2000 many of those older in-slab duct systems (in both countries) had been abandoned, largely because of the problems discussed in this article series. But new in-slab duct designs are still being built as you'll read in the research I cite below:

Here is some more recent research on HVAC ductwork in slabs focusing on Canadian buildings or Canadian research

• Friedman, Avi, and Vince Cammalleri. "The impact of R‐2000 building technology on Canadian housing: The impact of implementing the R‐2000 principles is evaluated in the context of narrow‐front housing in Canada." Building research and information 24, no. 1 (1996): 5-13.
• Genest, Frédéric, and Roland Charneux. "Creating synergies for sustainable design." ASHRAE journal 47, no. 3 (2005): 16.
• Ibrahim, H., R. Boukhanouf, M. Kanzari, A. Choorapulakkal, and A. Alharbi. "Approach for Integrating Indirect Evaporative Cooling System into Contemporary Architecture." J Fundam Renewable Energy Appl 4 (2014): 131.
• Passa, Joseph, and David Rompf. "Energy Efficient Sustainable Schools in Canada South." Journal of Green Building 2, no. 2 (2007): 14-30.
• Petty, Stephen E. "Indoor Environmental Quality." Forensic Engineering: Damage Assessments for Residential and Commercial Structures (2013): 421. [discusses issues with water in slab ductwork]

On 2017-01-18 by MP How common is in slab ductwork in Canada?

How common is in slab ductwork in Canada?

On 2017-01-09 by (mod) - covering openings in slabs

Opening

If the opening you describe is the system's return air, closing it off will cause your heating system to work at about half of its capacity or less and at double its operating cost or more. You'd need to provide an alternative source of return air.

On 2016-12-16 16:52:48.292215 by I have a an opening in my slab. my umit sits on top of it can i cover it

Have an opening in my slab. Unit sits on top.
Can I cover it? Cold air coming up

...

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

Or see SLAB DUCTWORK FAQs-3 - more-recent questions and answers about contaminated in-slab HVAC ductwork

Or see these

Recommended Articles

• ASBESTOS TRANSITE DUCTWORK
• DUCT & AIR HANDLER ODORS
• PVC HVAC DUCTS
• SLAB DUCTWORK
  • 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

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SLAB DUCTWORK FAQs-2 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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