Air duct leaks, condensation, flooding or wetting = contamination:

This article describes causes, health concerns, & repairs to correct wet or corroded heating & cooling ducts. Rusty metal ducts or corroded HVAC ducts are an indicator of improper and possibly unhealthy conditions in the building as they are almost always associated with unwanted moisture in the duct system.

Unwanted moisture in cooling ducts in particular, is an invitation to mold, bacterial, or other pathogens in the duct system and thus in the building air. We point out and include photographs of locations where you can spot unwanted HVAC or cooling system moisture and rust, cite some related health concerns, and discuss ways to avoid duct corrosion or rust. We also offer advice on what to do about rusty ductwork.

Causes & Cures for Wet, Iced, or Corroded Air Conditioning Duct Work

Article Contents

• DUCTWORK MOISTURE SOURCES
• DUCTWORK MOISTURE HAZARDS
• DUCTWORK MOISTURE RISK ASSESSMENT
• WET DUCTWORK REPAIRS

The photo at page top shows a very rusty air conditioning ductwork register, and a careful look into that system of metal duct work, found additional heavy rust in the duct system along with lots of debris.

The photographs shown here are of a less obviously rusty duct system and the rust stains on the vinyl floor suggest that the water in this duct system came from the floor level.

[Click to enlarge any image]

Causes of & Cures For Water or Moisture In HVAC Ducts & Air Handlers

Excessive indoor humidity and its related mold, dust mite, or bacterial hazards may be traced to either a cause or an effect of high moisture inside of an air conditioning system or even heating system duct work or air handler. Here are some common examples of sources of condensation or actual water leaks in HVAC duct systems:

• Missing HVAC duct insulation:
  
  Voids in the insulation or un-insulated supply ducts on an air conditioning ductwork system are likely to cause considerable condensation on the duct exterior wherever the ducts pass through un-conditioned space such as an attic floor where the ducts are exposed to warm moist air.
  
  Some brands of HVAC flex duct have been particularly prone to loss of both the exterior plastic duct covering and ultimately the duct insulation itself.
  
  See GOODMAN GRAY FLEXDUCT for examples of this problem (photo at left).
  
  External condensation on supply ducts is sufficient to show up as a ceiling leak or stain.
  
  Internal condensation may also occur in the duct system where insulation is missing. The accumulation of condensate on the duct interior also risks growth of bacterial, mold or other contaminant hazards.
• Convection air currents:
  
  When an HVAC system is off, depending on building temperatures, air may circulate up through ceiling ducts or other ducts by natural convection, carrying moisture that condenses in cooler duct sections (such as in a cold attic in winter) where the condensate in the ductwork may actually form frost or ice that later melts and shows up as a ceiling leak stain.
  
  See WATER & ICE IN DUCT WORK
• An over-sized central air conditioner:
  
  Even an over-sized room air conditioner may cool the building area so quickly that the A/C system does not run long enough to adequately dehumidify the building air. A result can include accumulation of condensate in the ductwork interior, especially if the ductwork insulation is incomplete. Details are
  
  at DEHUMIDIFICATION PROBLEMS.
• High indoor moisture moving through the building: 
  
  A wet basement or crawl space often sends moisture up through the entire building, overwhelming central air conditioning or local dehumidifier's ability to dehumidify the building air to an acceptable level. Ceiling-mounted air ducts may circulate moist indoor air into the duct system by convection even if the blower or air handler is not running. In winter moisture may condense, even freeze and accumulate in such duct systems.
  
  See WATER & ICE IN DUCT WORK
  and also
  
  see HUMIDITY CONTROL & TARGETS INDOORS
• Leaky HVAC ducts or duct connections:
  
  Leaks in return ducts, because they are under negative pressure, can bring warm moist unconditioned air into the duct system adding to both contamination and moisture condensation problems there.
  
  See LEAKY DUCT CONNECTIONS
  and
  
  see RETURN DUCT AIR LEAKS or
  
  see SUPPLY DUCT AIR LEAKS
• Leaks or actual flood-waters entering the HVAC duct work: 
  
  From a roof leak, plumbing leak, or from a humidifier that is improperly adjusted or improperly located can induce excessive moisture into building air as well as inviting mold contamination of the HVAC system. An extreme example of this problem occurs when a building has been actually flooded.
  
  See FLOOD DAMAGED DUCT WORK
  and
  
  see FLOOD DAMAGE HEATING EQUIP.
• In-slab HVAC ducts: 
  
  Are often subjected to leaks into the duct system or moisture and water collection from ground-water, from plumbing leaks, or from condensation;
  
  see TRANSITE PIPE AIR DUCT ASBESTOS RISKS an example
  or
  
  see SLAB DUCTWORK
• Condensate blow-off from the cooling coil:
  
   In an air handler will often wet the supply plenum and nearby ductwork in some very humid climates.
  
  Unless there is also a condensate drainage problem in the air conditioner we don't usually see extensive problem mold growth from this blow-off effect, but as the following reader comment (originally posted
  
  at HUMIDITY CONTROL & TARGETS INDOORS) explains, in high-velocity air conditioning systems condensate blow-off can be so extreme as to create a serious indoor moisture problem.
  
  Examples of condensate spill-over inside of an air handler blower compartment, a condition creating a risk of mold growth inside the HVAC system are
  
  at BLOWER LEAKS, RUST & MOLD
  
  See MOLDY Growth in Air Conditioning Blower Fans for an example of mold growth on the air handler blower that was probably encouraged by condensate.
• Sagging poorly-supported ductwork: 
  
  Encourages the collection of condensate in low portions of the ductwork. We show two sagging flex-duct examples just below.

See DUCT ROUTING & SUPPORT

Excessive indoor humidity traced to wet air ducts caused by high velocity air conditioning system coil condensation blow-off

Reader Comment: Mike / DFW

For years I've been trying to figure out the excessive humidity problem in my home too. I finally found it after working with foundation people, plumbers, a/c techs -- nobody could figure it out, but I finally did. When the cooling kicks on, the moisture level skyrockets. It has affected the inside of my home tremendously. We thought it was the a/c drain.

They re-routed it, made sure it was draining well and clear. It is. Leak near or under the foundation. We checked everything-that wasn't it.

What's happening is the fan is actually sucking the moisture out of the evaporator coils before the condensation off the coils can drain away. The design of this Lennox horizontal system in the attic is such that the small space right above and right below the squirrel type fan, creates a venturi effect, increasing the velocity of the air being sucked into the fan.

The velocity of the air is so strong that the coils (about 12 in. away) have the moisture sucked right off of them and into the fan, which, of course is then blown into the ducting. I'm not sure if this is an engineering design problem, if the a/c co. wired the fan to a speed that's too high, or if perhaps a part is missing that is supposed to prevent this.

Regardless, the inside of the unit is now so covered with mildew and mold and the electrical connections on the heating elements as well as all the electrical connections on the inside of the unit, are so corroded and rusted, it's a wonder that love thing works at all. (This also probably explains why sometimes the heat works and sometimes it doesn't.

The a/c repair guys have never been able to figure out why. They always seem to think it's the thermostat It's not. It's new and has recently been completely re-wired when I moved it from an outside wall to an inside wall.)

Anyway, that's where MY moisture problem is coming from. Good luck! - Mike / DFW 12/13/2012

Reply:

Mike, gold star to you for good detective work. Like other areas in the Southern U.S., Dallas/Fort Worth experiences seasonal periods of high humidity that can result in an extraction rate of water from building air faster than the design of the air handler and condensate drain system can handle it.

More about dehumidification problems traced to central air conditioning systems can be read

at DEHUMIDIFICATION PROBLEMS.

Do you think that the root problem, then, is an improper duct or plenum size or design or a mismatch between blower fan capacity and the HVAC duct system?

Are Rusty, Previously Wet, Iced, or Dirty Air Conditioning Ducts a Health Concern?

Not necessarily:

To avoid inappropriate anxiety on the part of any readers we state up front that it is normal for some dust and debris to accumulate on the inside of heating or air conditioning ducts, and this material is not necessarily a hazard to building occupants.

But when ducts have also been wet, or when ducts are constructed of materials like fiberglass insulation that have been damaged or can't be cleaned, some potential health or respiratory issues may be present, as we discuss further here.

Our flooded air duct photo (above left) shows ductwork with a layer of mud - this home was flooded to a level that included ceiling-mounted basement air ducts. The ducts needed to be cleaned and sanitized.

Soil particles themselves may not be a serious health hazard but area flooding often brings pathogens (such as bacteria or viruses found in sewage) into buildings and can form a serious health hazard. Details are

at FLOOD DAMAGE in DUCT WORK.

Rust flakes

from rusty heating or air conditioning ducts themselves are unlikely to be much of a health hazard - these particles are pretty big, not easily airborne, and probably won't be found at high levels in indoor air except in unusual circumstances. But rust in ducts is a problem indicator, showing quite clearly that the duct system has been wet.

Dust & normal air duct debris:

The chief components of house dust, which will certainly collect within a duct system include fabric fibers and skin cells, often also including starch fragments and other organic debris.

Watch out:  The combination of organic debris within a duct system and water (indicated by rusty ducts or duct registers) indicates a possible risk of mold or bacterial hazards within the air conditioning or heating system. Since blowing air through the system can pick up and distribute these hazards to occupants of the building, wet or previously-wet duct work is a potential health hazard to building occupants.

Wet HVAC Ductwork Actual Risk Assessment

The actual health hazard level from wet or previously wet air conditioning or heating ducts in a building depends on these factors:

• The total area of duct or other surfaces that have been wet,
  
  including both duct system interior and the interior of the air handler or fan/blower unit and evaporator coil surface
• The level of organic debris 
  
  within the duct system. Our photo (left) illustrates a small wet area in a section of flexible air duct - a problem that should be corrected. But if this is the only wet area it does not itself appear to be a health hazard. Make sure that flexible ductwork is properly supported to avoid collecting condensate in low spots.
• Presence of visible mold contamination
  
   in the duct system or within the air handler itself. Small amounts of mold, often a species or two of Cladosporium form on blowers and in small areas of the air handler.
  
  While even some species of Cladosporium, the most common mold family, can be a health hazard, a greater risk is likely to be present if the system is contaminated with small, easily-airborne molds such as those in the Aspergillus and Penicillium families.
  
  These are less commonly found as are ductwork mold contaminants that originate not from growing mold colonies in the duct system or air handler but from other building mold reservoirs.
  
  See MOLD in AIR HANDLERS & DUCT WORK
  
  Since odors, including moldy smells, are easily transported from one building to another through its HVAC system, sorting out moldy smells or other odors and tracking them to their source can be tricky.
  
  See ODORS in AIR HANDLERS & DUCT WORK
• The materials used to insulate the duct interior:
  
  fiberglass and similar materials form a "sticky" surface that tends to hold debris and also to act as a sponge holding moisture, possibly increasing the risk of mold or bacterial hazards, and not easily cleaned.
• The frequency of wet conditions:
  
  recurrent wetting obviously increases the risk of pathogens in the duct work, though we've seen significant contamination as a result of a single building flooding event which wet the ducts
• The health vulnerability of individual occupants 
  
  in the building: elderly, immune-impaired, asthmatics, infants, people with allergies, people with COPD or other respiratory illness, are more likely to be bothered sooner by duct contaminants that are produced by wet conditions in the air conditioning or heating system.

What To Do About Rusty HVAC Ducts, Wet or Previously Wet Fiberglass Ducts or Flex-Duct

• Clean the ductwork: 
  
  Metal ducts that have been wet but are not severely damaged can and should be professionally cleaned. Be sure that the interior of the air handler is also inspected and cleaned.
• Replace damaged ductwork:
  
  Where you observe metal duct system components which are quite corroded, badly corroded materials should be replaced. Depending on the location of the ductwork and its accessibility, this might not involve significant expense if only limited sections of readily-accessible duct work are involved.
• Replace fiberglass-lined ducts that have been wet:
  
  Fiberglass-lined ducts or flex-ducts which have been wet should be inspected thoroughly, and sections which were wet and/or are particularly soiled should be replaced. While it may be possible to remove-and re-line HVAC ducts that use an interior fiberglass board liner, if more than a small amount of such work is needed, replacement of the ducts may be less costly.

• Inspect for significant mold reservoirs in the ductwork:
  
  Leaks into ducts routed through inaccessible building areas or cavities, and leaks into duct work which has insulation, particularly fiberglass insulation, in its interior, (typically fiberglass board), may harbor a mold colony, leading to both significant cleanup costs and potential health concerns.

Use of Sprays, Sealants, Sanitizers in HVAC Duct Systems

Reader Question: considering buying a house with very corroded in-floor rusty ductwork - can we just spray-seal the duct interiors?

I am considering buying a 20 year old house which has underground ducting serving both refrigerated air and heat.

I had a duct inspection which revealed severely corroded and rusted ducts with some holes through the ducting.

I have heard of a product by the Duct Seal Technologies in which the inside of the ducts are sprayed with a coating of kevlar fibers and some bonding agent. Do you know anything about this process?

Thanks, Doug Spence - Spence 6/19/2011

Reply: in-slab HVAC ducts are asking for trouble from flooding, vermin, water, mold, etc.

Spence:

About buried or in-floor HVAC ducts: as we suggest in the article above, even if you could reliably reline your in-ground or under-slab air ducts (and how without a detailed inspection would we know that the re-coating or relining was complete, and that it remained intact?) there remains a concern with radon gas or water leaks into the duct system.

And we are doubtful about the reliability of this approach: spray-on coatings don't bond well to dirty, rusted, corroded surfaces and may fail to seal large penetrations or holes; And even a well-sealed spray-coated in-slab HVAC duct, if subjected to future flooding from ground water or plumbing leaks, or invasion by rodents or other pests, is simply going to have new contamination in its interior.

See DUCTWORK CONTAMINATION

A better, but usually much more costly and disruptive solution is to eliminate the in-slab ductwork by rerouting it through the building.

If you go ahead with the process I'd recommend first an inspection for evidence of a history of duct flooding, and an annual inspection of the duct condition until you are confident of its dryness and cleanliness.

Also take a look at TRANSITE PIPE AIR DUCT ASBESTOS RISKS for more examples of problems with heating or air conditioning ducts found in slabs.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2024-05-19 by Sarah - cause & risks of corrosion at HVAC duct supply registers?

We see corrosion at the ends of many of our HVAC ducts where they end at the registers. The amount of rust is not massive - more like drops/splotches. We ran our camera down each duct in question to see how far the rust went and it was only at the very end points where the registers are.

We assume something may have spilled over during construction. In any case, is there cause for concern for this level of corrosion? Of note, the ductwork in our unfinished basement is pristine. There is no evidence of any issues there.

Additional info: My husband just told me one of the ducts (laundry room) has corrosion that's more of a patch in terms of the area (vs small splothces) that's about 5" in diameter). And, in that duct, when he ran the camera down, although the rust didn't continue, he could see marks like staining for another 18" where some liquid had obviously run.

On 2024-05-20 by InspectApedia Publisher

@Sarah,

It may be that all you need to do is clean the areas of corrosion and spray a reasonably secure sealant on the rusted surface.

But it was certainly help me form an opinion if I could see some photos of the damage. You can post one photo per comment so use more comments if you have more photos and that will let me make further suggestions.

Daniel

On 2024-05-20 by Sarah

@InspectApedia Publisher, Thank you very much. I'm going to send two pics: one that's the lightest amount of corrosion in the stack head and one with the most. I'd also be interested to know of any product you could recommend we could use as a sealant over the rusted areas.

I'm very sensitive to VOC's so wouldn't want to introduce something that will smell nasty. If I didn't use a sealant, what would be the downside to leaving things as they are?

Here's the second photo, which is an example of the most impacted stack head.

On 2024-05-21 by InspectApedia Publisher

@Sarah,

First let's put an important safety question to bed.

Please confirm that we are talking about duct work, that is the metal large diameter tubing that delivers heated air to rooms in your home or returns cool air back to a furnace.

We need to be sure that we are not discussing a fluent connector that vents exhaust gases from combustion into a chimney or ultimately to outdoors.

Concern is that if you had a hole like that in a chimney or flue vent connector it's dangerous, risking carbon monoxide or other hazards that an extreme cases can even be fatal.

In contrast if the holes are simply induct work then what we are facing is an inefficiency from air leaks and possibly a concern for moisture induced problems elsewhere in the system..

Next I want to be sure that the location of these leaks, assuming they are induct work, is nowhere near the furnace or air handler itself. That's because leaks at the air handler could cause costly and dangerous damage to the heat exchanger in that device.

So tell me where these are in your home.

Finally, at least for the moment, I'm seeing more than simple superficial rust. I'm seeing rust perforation. That tells me this is an old and long-standing problem. It also means that nearby sections of the sheet metal that form your ductwork are probably fragile from corrosion even if they haven't yet become perforated. In a case like this the most effective repair, particularly if the duct work is accessible, it's just simply replace it with new sections of metal ductwork.

I don't think it would be useful to try to spray it or use another sealant.

A temporary measure, which I emphasize is only temporary, Voice spraying or painting anything at all and instead simply uses some metal tape to cover the perforated or damaged duct sections on its exterior, or if those aren't accessible possibly with a bit of preliminary cleaning you could get the tape to stick on the inside of the duct.

You'll find that the metal tape, which is basically an adhesive coated aluminum with a peel off layer, is very sticky and will adhere well to metal ductwork.

On 2024-05-21 by Sarah

@InspectApedia Publisher, Thank you so much for your prompt and thorough reply. To answer your questions: Yes, this is duct work. No, it's a fluent connector. The corrosion is not holes. The exterior of the duct work is intact and looks pristine.

The locations are not near the air handler. They're at the opposite end of the system. Nothing is perforated. I see in the second photo there's a black spot that looks like it is, but it isn't. Again, there is no corrosion on the exterior of any of the duct work at all. The corrosion appears to be superficial and located at the stack heads / end of each duct at the floor registers.

On 2024-05-21 by InspectApedia Publisher

@Sarah,

Ok, on that case just clean the surfaces. Simple damp wiping is fine.

On 2022-04-05 by David Robinson - risk of damage to flex duct left in ground contact

How long will it take mobile home flexible plastic under the home ductwork to rot if left lying on the ground?

On 2022-04-05 by Inspectapedia Com Moderator

@David Robinson,

I want to be helpful but I'm not sure what material we're discussing. Plastic flex-duct or a plastic vapor barrier under the mobile home on the ground, or plastic stapled up underneath a mobile home - stapled to its under-side ?

In any event plastic doesn't actually "rot " - as that's an organic process. Leaves and wood rot.

But plastic will deteriorate, become brittle, and begin to fall apart - with age, exposure to sunlight, and over a longer period if protected from the weather.

There are also some brands of flex duct that disintegrate because of defective product - discussed at this website.

Watch out: however for rodent damage to flex duct left unprotected on or close to ground, as well as for water entry and contamiantion through holes or tears in that ductwork.

On 2022-01-21 by Charlotte M - HVAC tech says rust on A/C & Heat ing ducts due to condensation - clean them?

I noticed rust on the ceiling AC/heat duct registers this summer. Most rust on the kitchen register. Less rust noticed on the 2 bathroom duct registers. This is a one story house. the bedrooms also have less rust. I keep the largest master bedroom register closed all the time.

I mentioned the problem to my HVAC service tech and he stated it was condensation, but failed to offer a solution. The office staff stated it was not anything to be worried about. However, I've lived in several homes over the years and I have never had rust spots on duct registers.

After reading your advice on several posts should I seek an inspection from a professional duct cleaning company and fire my HVAC servicing company?

On 2022-01-21 by Inspectapedia Com Moderator

@Charlotte M,

I agree with your HVAC service tech that the most-likely cause of rusty metal HVAC ducts is condensation, though on a few occasions we've found roof leaks and even plumbing leaks that sent water into the ductwork causing the same rust damage.

You don't give country and city of location: some areas such as warm humid coastal areas cause a much bigger load on your system's ability to dehumidify indoor air. An over-sized cooling unit may also fail to dehumidify properly.

You might check your home to be sure there are no sources of unusual indoor moisture and also ask your service tech if she agrees that your system is dehumidifying indoor air properly during humid weather.

On 2022-01-06 by Robin - found sand in supply registers - rodents: how to restore contaminated ducts to service?

I discovered that a bedroom vent on an outside wall which is buried in a slab was full of sand. I vacuumed it out and found that it was plugged for at least two feet. No heat is coming through the duct so it may be clogged further down.

My vacuum hose was only about six feet long on a 20 foot run that ends in a crawlspace where it connects to the rest of the duct-work leading to the furnace plenum. The pipe must be collapsed or rusted out somewhere along the run and sand has gotten in where it was possibly carried along by airflow until it clogged the pipe?

The other less desirable answer is that rodents such as chipmunks have gotten into the ducts through the hole and moved sand to the end to clog that vent? I say this because we have chipmunks under our slab (including the garage which is in front of the bedroom).

I have seen piles of sand on my sill plate in the crawlspace next to the slab where they are presumably excavating burrows and depositing it there (which is another major problem all on it's own).

So my questions is how to restore the duct to workable condition. It is a straight 20 foot run that I have access to from the crawlspace. It is five inch pipe so my thought was to sleeve it by feeding through a 4 inch pipe to the vent.

This will require me to first clean all of the sand out of the existing duct and get a better inspection camera through to see if this is possible. I was thinking of heavier gauge rolled galvanized such as the snap lock pipe from Nordfab or some other manufacturer or heavier gauge aluminum pipe that won't rust.

I could re-rout a new vent through the attic but that would require going from the plenum up through the wall of the garage and into the attic. A total run of about 60 feet or more. The other option would be to just fill the duct with foam and add a baseboard electric heater. Any thoughts or suggestions?

On 2022-01-06 by Inspectapedia Com Moderator

@Robin,

Please read our advice at

SLAB DUCTWORK

then don't hesitate to ask follow-up questions if needed.

In sum we suggest abandoning in-slab HVAC ducts as almost always problem prone; above grade any contaminated metal ducts that are not so damaged as to need replacement can simply be cleaned and sanitized by your duct cleaning company.

On 2021-09-15 by Jeannette - HVAC duct leaks stain dining room ceiling - can I just wrap the ducts?

My dining room ceiling started to show wet spots, after ruling out a pluming leak, we discovered the duct work was covered in condensation, dripping down causing the staining. After taking down more areas, its clear the problem has been there for a while.

We did replace our 20 year old a/c unit a few years back. How long would a problem like this take to show up. There is actually a hole where the duct is "rotted" out. Will wrapping the ductwork stop help? (the black tape is where the hole was on the left, and the seam was a little open on the right.

On 2021-09-16 by inspectapedia.com.moderator

@Jeannette,

I might "bandage" duct leak points as a temporary repair, but leaving water inside the ductwork, even if I stopped the leak for now, isn't going to stop further rust, nor future perforations and leaks, nor will it avoid a risk of mold contamination in the duct system.

So we need to diagnose the cause.

If the ducts are rusted through they should be replaced.

When ducts are wet and corroded the cause is usually one of those listed on the page above - such as missing insulation, or condensate being splashed into the duct system from the cooling unit's fan or clogged overflowing condensate pan and drain at the air handler.

Details are above on this page.

On 2021-07-09 by Heidi Smith - water in ductwork risks mold contamination, collapsed ceilings, other hazards

Here's the most recent chaos at my house(I'm renting) but I'm trying not to be too pushy with my landlord because he lost his job at the start of the Pandemic and he's been struggling too...

But what concerns me most, is the widespread areas of dark-green and blackened (once pink) insulation... (my husband and I have a 3 year old daughter living in this home - though she's been with grandma since the ceiling came down....

On 2021-07-09 by danjoefriedman (mod)

@Heidi Smith,

I'm re-posting your photo with what I think are the right annotations identifying HVAC supply-air ducts, return plenum and duct,
and I can but comment on what we can see in this photo:

Observations:

1. The return duct is severely rusted, risking mold contamination in the air handler and ductwork or other pathogens and IAQ hazards (bacteria, allergens);

2. Your comment and the photo suggest that enough water collected in the duct system to wet nearby building materials: insulation, drywall or other ceiling coverings, wood framing; this is the likely cause a ceiling collapse; indeed we see what look like leak stains ob framing below that rusty return duct opening.

3. Proper repair will require at least the following steps:

- identify and fix the moisture source in the HVAC system or at some other building leak(s) if that was the source of water in the duct system

- inspect the air handler and ductwork for debris, other water damage, contamination, and clean ducts that can be cleaned; some of the duct should probably be replaced; and some types of duct such as fiberglass-insulating-board or insulation-lined would be replaced as those cannot be effectively cleaned

The air handler may need to professionally cleaned as well;

- remove all building insulation and drywall and other ceiling materials that were wet as those risk ongoing mold growth and they cannot be cleaned;

- inspect remaining ceiling to assure that it is un-damaged and not at risk of further collapse: a ceiling collapse not only makes a mess, it can injure someone

- If wall-to-wall carpet below this ceiling collapse was wet it probably needs to be replaced along with its padding

- Follow the water! If water also entered the walls then those wall cavities need to be inspected for mold

- clean all exposed surfaces and be sure all demolition is complete and all leaks fixed and all ductwork repair complete before restoring the ceilings;

- Don't do a half-way job or the risk is doubling the cost by having to do the entire job over again later if the original cleanup was insufficient as leaving wet or moldy drywall in a building leaves a growing mold contamination area.

- if inspection finds more than 30 sq.ft. of contiguous moldy material, then professional mold cleaning help would be needed; smaller moldy areas are normally handled by a handyman or HVAC contractor

4. Some insulation stains are caused by air bypass leaks and do not represent a hazard: see INSULATION STAINS - AIR BYPASS LEAKS

Some insulation is naturally dark green - depending on brand and in that case would not, in and of itself, be a hazard; we could not make out "green insulation" in your photo;

See FIBERGLASS INSULATION IDENTIFICATION & PROPERTIES - inspectapedia.com/insulation/Fiberglass_Insulation_Identification.php

Watch out: you don't give your country and city nor climate so we don't know what use you're making - or not - of this HVAC system and its ductwork, but if the system is mold- or other-contaminated it could be a health hazard to building occupants, particularly those who are elderly, infant, immune-compromised, asthmatic, allergic, etc.

Keep us posted, and post additional photos, comments, questions as needed.

Question: what is causing fan motor and shaft rust in my Unico Mini Duct System

JL 2021/07/01

I have Unico mini-duct system in my attic. Every winter, the fan motor and shaft accumulate rust. I live in a very cold climate in winter (Vermont). Last year, I had the motor replaced and this spring, there was rust on the motor again (no visible rust last Fall when I closed the unit up for the winter). I had the New England Unico rep at my house when we closed up the unit for winter to ensure it was properly winterized.

Specifically, the plugs were put into all the outlets and a piece of insulation board wrapped inside a plastic garbage bag was placed tightly into the air return. In addition, I had an additional 18 inches of cellulose blown into the attic last fall and the attic entrance / jacob’s ladder was sealed up properly and a new roof put on (both last Fall).

The condensate drain used to drain into the sewer vent but that was moved last summer to instead drain to the outside of the house directly via a PVC drain pipe. Note there is just a small v-shaped trap on that PVC drain but my contractor thinks that is fine given it drains to the outside.

So, the current theory is the rust is occurring due to moisture in the attic in the winter, but there are no other signs of moisture in the attic. The laundry room is almost directly below the attic A/C unit and the dryer vents to that outside wall.

A friend suggested that dryer vent could be blowing warm air that rises up into the soffit vents and finds its way into the attic, but I think this is not likely the problem given I have no other signs of moisture in the attic plus the fact that the A/C unit is pretty tightly sealed up itself to avoid leaking air when it is operating.

(This Q&A were posted originally at AIR HANDLER / BLOWER UNITS)

Moderator reply: track down source of moisture, rust, corrosion in air handler

From your description of near-heroic measures to try to close off your attic duct system in winter one might guess that you've had reason to be concerned for moisture or ice accumulation in ductwork in the past.

What you've been doing sounds reasonable to me.

But I've also seen that the natural wish of warm air to rise into cool spaces means that even the smallest air leak into the duct system from anywhere will invite moist air and condensation and thus moisture collection during cold weather.

Because the ductwork is out of use there's no air circulation through the A/C system so no moisture removal; so any moisture accumluates, and can be corrosive.

I've even seen so much moisture accumulate in attic ducts as to form thick ice that later melts and shows up as a "leak" that has been mistaken for roof leaks.

If air is leaking directly into the duct system from anywhere in the warmed building during winter, that could explain why you see duct or attic AHU moisture signs but not other moisture or condensation signs in the rest of the attic.

Your friend's idea about the dryer vent is interesting and helpful as a model: look for sources of warm air leakage into the duct system from anywhere in the building.

Two articles you'll want to review are

WATER & ICE IN DUCT WORK

WET CORRODED DUCT WORK -- I will copy our discussion to that article and will welcome your further comments, questions, suggestions here or there.

Reader follow-up:

Thanks for the reply.

I have had some condensation in past years around the air outlets (I.e. on the ceiling around the outlets) during very cold spells. That is one reason I had more insulation put in the attic last Fall when the roof was being replaced. Now most of the ducts in the attic are buried in insulation. No issues like that this past winter after adding the insulation.

The most likely place I can think of where air from the heated part of the house would be the return. Can you think of any special material the return should be made of or method of sealing (beyond what I’m doing)?

Regarding the condensate drain, do you think I need a bigger drain with antifreeze in it during the winter to keep air from getting into the unit in winter? Keep in mind it is currently just a small V-shaped PVC trap that drains outside.

One last question: is it possible the corrosion actually occurs during the summer/fall when the unit is operating but doesn’t turn into visible rust over time (e.g. winter)?

Thanks again! - JL

Moderator reply: special material the return should be made of

RE your question:

Can you think of any special material the return should be made of or method of sealing (beyond what I’m doing)?

-- If you are sealing the return duct at its inlet grille then that is normally sufficient; but you might want to inspect the duct system to be sure there are no obvious air leaks into it from elsewhere.

-- Something sounds wrong about all of this: when we close off duct inlets and outlets we should be closing off any significant air drafts into the ductwork during the off-season; So I keep thinking we're missing a significant airflow passage.

Regarding the condensate drain, do you think I need a bigger drain with antifreeze in it during the winter to keep air from getting into the unit in winter? Keep in mind it is currently just a small V-shaped PVC trap that drains outside.

-- The condensate drain trap may indeed dry out over winter; you could try mineral oil in the trap when the system is shut off - less of a contaminant than antifreeze;

One last question: is it possible the corrosion actually occurs during the summer/fall when the unit is operating but doesn’t turn into visible rust over time (e.g. winter)?

-- It is certainly possible that moisture is accumulating in the duct system during the in-use season and lying there corroding-away over the winter; It's common for water to blow off of the cooling coil into the supply ducts from that point onwards.

-- Sags in ductwork also pool condensate;

Reader follow-up:

Thanks for all the info. Your last point made me ponder something everyone has said should not be an issue but let me run it by you: I have 2 outlets over my 2-story foyer that require an extension ladder to plug / unplug so I stopped opening them several years ago.

Do you think it is possible that because the air isn't moving through those lines (remember this is a Unico high velocity mini-duct system) that there could be condensation sitting in those lines that could find its way back to the blower fan in the winter? FYI: I typically run the blower fan (i.e. not calling for cooling) for at least 24 hours before closing up the system for winter.

This is usually in October when the nights are cool and I'm no longer using the system daily for cooling. Could that be leading to warm moist air in the ducts that later causes the rusting on the blower fan once I close the system for winter?

My technician is supposed to come to replace the blower fan and motor today so I'll run some of these ideas by him as well. - JL

Moderator reply:

I'll be very interested in your report of what your on-site service tech has to say, as nobody can see as much by text and web posts;

But in general, for an outdoor receptacle to be responsible for rust and moisture in an HVAC duct system you'd have to have a pretty unusual circumstance in which sufficient volume of humid outdoor air is somehow conducted from an electrical box or the cutout opening for the box into the duct system. There should be no direct path through which that's possible. Visual inspection still is sensible.

Reader follow-up:

My technician agreed to problem cannot be caused by the dryer venting back up into the attic. The Unico system is pretty air tight due to it being a high velocity system.

He also did not think the two outlets that I never open would be the source of condensation for the winter months. Nor did he think running the fan in October just before I close it up could cause excess moisture to accumulate since the air is pretty dry in Vermont (low humidity) in October. So we still have a mystery. He said the unit was completely dry inside today despite the fact it was hot and very humid here yesterday. - JL

So we still have a mystery. Thanks for your suggestions and comments though. If we ever figure this out, I’ll let you know.

Moderator reply:

Thanks for the follow-up, do keep us posted, as what you find will certainly help other people too.

In investigating, start at the point of most-concern such as rust or corrosion, and follow the water or water indicators, stains, signs, rust patterns, etc.

On 2020-08-26 by Anna - corrosion spots in new HVAC duct air registers

I just posted about rust in a vent with plastic lined duct, but wanted to share other images for context

I am purchasing a condo and see that there is corrosion in the first floor vents. One is worse than the rest (photos attached- 2 of the bad one and one of another). I've been reading this thread and see the recommendation to investigate any rust or corrosion.

When I look at what these vent attaches to, I see what looks like something lined with black plastic. I would like to know 1) What kind of duct is this and is there still a concern for rust in the vents? 2) would you still recommend a scope of the vents, and 3) does this vent need to be treated in anyway?

The hardwood floors were recently redone and there was concern that product spill in the vents caused the corrosion, but the flooring company says that is impossible. Could this be caused by that?

On 2020-08-3 by (mod) - evaluate rust in metal HVAC ducts & registersRust-

Anna

Your original photo (further below) shows rust in the heating or AC duct register boxes that in turn are connected to plastic flex duct.

The second photo shows trivial rust; the first image you posted shows more rust and may be a concern - look for a spill or a leak source.

Those stains suggest there has been water in the ductwork - risking hidden mold or bacterial health concerns.

On 2019-12-08 y pantherprincess555 - white film and mites in my home coming from ducts?

All my air returns look like this and rotted sheetrock behind them. White film and mites all over my home.

On 2019-12-09 by (mod)

Panther

I can't say from the photo whether that's normal dirt in older HVAC returns and ducts or rather water damage and mold; if the drywall is damaged look carefully to see if it is from water damage, mechanical damage, or simply the effects of rough cuts made during installation.

Gypsum board doesn't literally "rot" (as might wood) but it can be water damaged and become soft.
In your photo I don't see water stains. That would be a useful diagnostic clue.

On 2019-08-24 by Linda - ducts going in to the furnace was dripping and sweating - AC tech said ductwork needs to be vented

The ducts going in to the furnace was dripping and sweating an air conditioner tech said it was because there should be a vent in the duct work that is coming out of the furnace that is running across the ceiling in the same room.

So he put a hole in the duct, which did stop it from dripping. Should there be a hold in it? Is this correct? My furnace is very old.

On 2019-08-27 by (mod) - venting to fix a ductwork condensation problem? Really?

Linda

I want to help but am totally baffled by the question and by what the service tech told you. I have not idea why there should be a "vent" cut into ductwork - it sounds very odd to me and is by no means a common, standard, or recommended practice.

Ducts "going in to a furnace", would mean that these are the air return ducts bringing building air into the air handler to be heated (in heat mode) or cooled in air conditioning mode.

Sources of water in ductwork are discussed in the article above; some examples (not an exhaustive list) of causes of water or condensate in or on air conditioning or heating ductwork include:

If there is water inside the duct work it may be due to improper condensate drainage from the air conditioner.

Condensation might also appear on air conditioning SUPPLY ducts (leaving the air handler after the air conditioning cooling coil has cooled air therein) if the ducts are not properly insulated.

On occasion condensate may blow off of the cooling coil and into the supply air plenum or ductwork.

Rarely there are also external sources of water inside ductwork such as a plumbing leak that drips onto and into ductwork or

finally, ducts that run in floor slabs below grade, into which ground water leaks.

Reader Question: Mold on ceiling traced to condensation in the ductwork?

I have a condo that the garage is below the living space. My tenant noticed there is mold on the garage ceiling. One of the neighbors said they had the same problem and she had it fixed. She was told it was condensation in the ductwork. Water was damaging the drywall and leaking downward.

Of course, mine is worse. What is causing this? Could this be a problem in the construction? The units may be about 10 years old. - Carol 6/20/12

Reply:

Carol from the very limited information your note alone I can't be confident in a "guess" at what the problem is in your home and can only answer in general. An A/C duct that is not insulated is more likely to produce condensation inside the ductwork than otherwise, especially in humid weather.

Such condensate, if it collects in the ductwork where it does not belong, instead of in the air handler where it is drained away, can wet duct insulation (if there is some), and can leak onto or into building surfaces.

Moldy drywall is best removed and replaced, allowing you also to check to be sure the wall cavity was not also wet (and moldy).

Ask your HVAC service company to inspect the ductwork, fix the cause of condensation, help you decide if the interior of the ductwork needs cleaning AND IF it can be cleaned at all (fiberglass duct interiors don't tolerate cleaning). WHen that problem is cured, then address the rest of the building.

Watch out: mold can grow to problematic levels in fiberglass insulation in or on wet or previously wet HVAC ducts even when the ducts look clean to the naked eye. An explanation of this problem is

at FIBERGLASS INSULATION MOLD

Reader Question: dripping from the HVAC ducts

(Sept 29, 2015) Jyotika Bahree said:
Hello, we just had our evaporator coil replaced yesterday and the technicians had to alter the ductwork to put the coil in.

After the work was done, the technicians confirmed that the AC system was cooling properly. After they left, I went to our basement where our HVAC unit is located and found that there was air leaking from the duct joints where they had put the new coil in.

Thinking that the leakage did not seem normal and might compromise the cooling, I called him to ask about this. He said that they had deliberately not sealed it completely to prevent the system from "sweating" too much. Not knowing any better, I had to accept what he told me. I would like to get your opinion on this issue.

Please do let me know if this will compromise the system efficiency or if this will truly prevent the system from sweating and therefore we should do nothing.

Reply:

Look for

1. missing duct insulation

2. condensate blowing off of the cooling coil into ducts near the air handler

Question: does use of bleach or scented candles corrode our air ducts?

I was told products such as clorox, scented candles and sprays cause corrosion to air and heating lines. Is this true? - June Walker 10/2/11

Reply:

June, it's true that in areas where high levels of chlorine gas are present, such as around indoor swimming pools, corrosion of metal HVAC ducts and even evaporator coils can be a serious problem.

Scented candles? I am very doubtful that in normal use you could put enough corrosive material into the air to affect the ductwork. But indeed use of scented candles can be an indoor air quality problem source for some people such as asthmatics, and the soot deposition on indoor surfaces can be a cosmetic problem as well.

See THERMAL TRACKING & HEAT LOSS.

Question: is it ok for a window A/C unit to run continuously for 30 minutes or longer?

Is it ok for a window unit to stay on cooling for 30+ minutes or will this break the unit, see its above 96 degrees all the time outside so its hot hot and since I cleaned the evaporator coils & moved thermostat accidentally

I swear it doesn't cut off & on like before but I just don't want damage to unit and also coils were dirty before so maybe that's why it cycled quicker then anyway please help - Christie 7/25/12

Reply:

Christie,

Under normal conditions (that is for example no abnormal electrical voltage levels, proper wiring and installation) a window air conditioner could run continuously, 24-hours a day, without damage to the equipment itself. Of course if the air filter becomes debris-clogged and is not cleaned the system efficiency and effectiveness will decrease, and of course your electric bill will reflect that continued operation.

See WINDOW / WALL AIR CONDITIONERS for details.

Reader Questions about Wet Corroded Air Ducts

On 2019-07-15 by Shirley - There's mold, rust,lead and possibly asbestos thru out even the tile floor and the air feels humid,

My evaporative cooler or condenser and coils need cleaning from debris from gas powered leaf blowers next door twice a week.

I should get my rental payments waived till the slumlord faces her responsibility, she just don't want to pay professional costs.

These apartments should be condemned built in 1949, the slum Lord told us not to lick the paint, it's full of lead, rusty water pipes, mold I need the Environmental Protection Agency to test these apartments, they are hazard to our health, and inhabitable to live a healthy living conditions

There's mold, rust,lead and possibly asbestos thru out even the tile floor and the air feels humid, the AC is potentially not giving out cool air as time goes by we have a slum Lord that refuses to hire a professional bonded and licenced, they use exfelons to do the handiwork, and refuses to exterminate by professionals but uses the cheapest extermination using claydirt making your apartment dirty around the base iarda and window sills

My apartment below the ceiling is cracking and buckling from the ceiling to the base board

On 2019-07-15 by (mod) -

Shirley

If your apartment is unsafe / unhealthy and the management is not addressing the situation you may need to ask for help from your health department.

On 2019-04-21 by Anonymous - What are the effects of a corroded return air duct in an air conditioning system?

I was going through an article on your site on '' Return Air HVAC improvement Guide.''

But my question now is; What are the effects of a corroded return air duct in an air conditioning system?

Moderator reply:

To ask about the effects of corroded return air ductwork in an A/C system is, I agree, a reasonable question in concept but unfortunately one that nevertheless cannot be answered usefully when we have not a shred of information about the air conditioning system, ducts, condtion, contamination, amount of corrosion or rust, its cause, possible pathogens in the moisture that caused it, air movement, type of building, other building conditions that impact the effects, and quite a few other questions.

To give a general ' "it's ok" or "it's dangerous" answer would have no useful reliability.

For example, sewage water leaking from an overhead sewer line into an air conditioning duct system is unsafe regardless of whether or not the duct is visibly rusty.

In sum, superficial or small amounts of corrosion or rust in an A/C or heating system air supply OR return duct may be of itself inconsequential though even then the cause of the corrosion should be considered and probably fixed - such as condensate blow-off or missing duct insulation.

But extensive corrosion inside HVAC ducts might indicate not only that the system is itself damaged and leaky (increasing operating costs and decreasing building comfort) but it might also mean that pathogens such as bacteria or mold are being blown through the system and onto occupants.

On 2018-10-03 by Fay

How long does it take for rust to form in ac ducks and is it hazardous to my health. I'm immune compromised. My complex is 30 years old I can see the rust on the ac grates in the apartment. What should I do?

On 2018-10-03 by (mod) - How long does it take for rust to form in ac ducts

Days or longer, depending on the materials involved and site conditions.

It's less the rust, and more mold, bacterua, or viruses in damp ir wet conditions, that I'd want to investigate.

On 2017-07-13 by Anonymous - My ducts are leaking water also, how do I clean air handler?

Mine are leaking water also, how do I clean air handler?

On 2017-07-13 by (mod)

Anon

In general water leaking out of HVAC air ducts is coming from

- condensate blow-off of the cooling coil, possibly requiring fan adjustments, a replacement of a baffle that a tech left out near the squirrel cage fan - or perhaps a baffle that was installed in the wrong location

- condensate accumulating downstream in ductwork perhaps because of missing insulation or air leaks into return ducting

Anon:

Cleaning an air handler might be considered at more than one level of work:

1. change dirty air filters regularly, usually monthly

2. remove any obvious clods of debris you can reach without disassembly - be sure power is OFF or you could be injured

3. hire a pro who removes the blower assembly, cleans it, lubricates it, cleans out the entire return air compartment, then rebuilds the system
Details are at BLOWER FAN ASSEMBLY CLEANING https://inspectapedia.com/aircond/Air-Handler-Blower-Cleaning.php

4. Cleaning a dirt-blocked cooling coil in the air handler is described at DIRTY COIL CLEANING PROCEDURES https://inspectapedia.com/aircond/Cooling_Coil_Cleaning.php

On 2016-09-14 by pt

Metal ducts that have been wet but are not severely damaged can and should be professionally cleaned. Be sure that the interior of the air handler is also inspected and cleaned - what do you mean by air handler?

On 2016-09-14 by (mod) -

PT

The blower unit - the machine that includes a fan and controls to blow conditioned air through the duct system.

On 2016-08-06 by dave - UV lights disinfect but don't dry ductwork

when I turn on my central a/c for the first time in the spring ...theres brown residue that comes out of the ducts...the a/c company claims its rust from the ducts in the humid hot attic space they want to install a uv light claiming this will take care of the situation ....im not buying it I feel like I'm getting scamed...any input would be appreciated

On 2016-08-06 by (mod)

A UV light will not do the least thing about drying out wet or rusty metal ductwork; IF and I empahsize IF the light were strong enough and the airflow slow enough, a UV light might kill some microorganisms that may be found in indoor air or in wet or damp ductwork. It sounds to me that the vendor is proposing to use a tool that they know and sell rather than that the problem is being properly addressed.

I suggest a duct inspection to find and fix the cause of the moisture problem, and perhaps duct replacement of rusted sections.

...

Continue reading at WATER & ICE IN DUCT WORK or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• ALUMINUM BUILDING MATERIAL ODORS
• BLOWER LEAKS, RUST & MOLD
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• FLOOD DAMAGE REPAIR PRIORITIES
• FLOOD DAMAGE CLEANUP & REPAIR GUIDE - home
• HEATING EQUIPMENT, FLOOD DAMAGE REPAIR
• TRANSITE PIPE AIR DUCT ASBESTOS RISKS where we describe flooding of HVAC ductwork in slabs
• PVC HVAC DUCTS
• SLAB DUCTWORK
• WATER & ICE IN DUCT WORK
• WET CORRODED DUCT WORK

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INDEX to RELATED ARTICLES: ARTICLE INDEX to HVAC DUCT SYSTEMS

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