Wet or Corroded Heating or Cooling Ducts

Causes, Health Concerns, Repairs for wet air ducts

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

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.