Photograph of rusty air conditioning duct register Wet or Corroded Heating or Cooling Ducts
Causes, Health Concerns, Repairs for wet air ducts

  • WET CORRODED DUCT WORK - CONTENTS: Flooded, Wet, or Iced Air Ducts: Corroded Air Conditioning Duct Work Cause, Effects, & Prevention. Common Sources of Water or Moisture In HVAC Ducts & Air Handlers. Health Risks from Wet Air Conditioning or Heating Ducts - Flooded air ducts or leaks into air ducts: health hazards & HVAC system damage risks. What To Do About Rusty HVAC Ducts, Wet or Previously Wet Fiberglass Ducts or Flex-Duct
  • POST a QUESTION or READ FAQs about the cause, detection, effects-of and cures for wet, flooded, ice or water contaminated ductwork

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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

This article describes Wet or Corroded Heating & Cooling Ducts: Cause, Health Concerns,& Repairs to correct this condition. 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.

Green links show where you are. © Copyright 2017, All Rights Reserved.

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

Photograph of rusty air conditioning duct register

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 just above 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.

Photograph of rusty air conditioning duct register

[Click to enlarge any image]

Article Contents

Common & Un-common Sources of Water or Moisture In HVAC Ducts & Air Handlers

Photograph of  heat-deterioration of Goodman Gray Flex Duct in an attic

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:

Sagging air duct (C) Daniel Friedman


Sagging air duct (C) Daniel Friedman

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


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

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?

Wet rusted spiral ducts (C) D Friedman

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

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:

Wet spot in flex duct (C) D Friedman

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

Wet rusted spiral ducts (C) D Friedman

Use of Sprays, Sealants, Sanitizers in HVAC Duct Systems

rusted metal ductwork (C) Daniel Friedman

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.


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.

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.

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


Water in flex duct (C) Daniel Friedman

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

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.


Look for

1. missing duct insulation

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


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

Or see TRANSITE PIPE AIR DUCT ASBESTOS RISKS where we describe flooding of HVAC ductwork in slabs



Suggested citation for this web page

WET CORRODED DUCT WORK at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman