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Leak stains around an air conditioning ceiling register (C) Daniel Friedman Heating or Air Conditioning Duct Ice-up, Duct Flood Damage, Duct Water Leaks & Mold

  • WATER & ICE IN DUCT WORK - CONTENTS: Sources & causes of water or condensation inside or on the outside of air conditioning ducts. Frost or ice build-up in heating or air conditioning ducts is often mistaken for a roof leak. Central HVAC humidifiers may also leak into air handlers and ductwork. Central Air Conditioning Cooling Coil Condensate May also leak into air handlers and ductwork. Inadequate ceiling vapor barriers or ind adequate attic-run air conditioning ductwork. Building Floods May Include HVAC Ducts. Can I run heat or air conditioning to just dry out flooded HVAC ducts?
  • POST a QUESTION or READ FAQs about the detection, cause, effects, and cure of water or ice formation or flooding in HVAC ducts
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Water, condensate, or ice formation in air conditioning or heating ductwork:

Here we explain the causes and cures for problems of condensation, water, or ice formation in heating or air conditioning system duct work.

We describe and include photographs of various sources of water, leaks, or flooding in HVAC ducts and we describe the IAQ and health problems that these conditions can cause, along with recommended solutions.



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Causes of Condensation, Water or Ice Formation in Duct Work, What Happens, How to Prevent Condensate, Icing, Leaks in & From Ducts

Warm Indoor Air Convection Currents Entering Cool HVAC Ducts

Leak stains around an air conditioning ceiling register (C) Daniel FriedmanArticle Contents

Our page top photo, courtesy of Indoor Air Care, shows water inside of flexible ductwork. Are these due to roof leaks, condensate leaks into the duct system from the air handler, or condensate due to convection air currents entering the HVAC duct during the heating season?

In freezing climates such as New York where some homes route their top floor HVAC ducts along the attic floor, sometimes that ductwork is not well insulated and just as it gets too hot in summer (increasing the cost of air conditioning), in winter the same ducts become too cold, increasing heating costs.

But something else funny can happen in homes with attic ducts that are used only for air conditioning.

One of our clients called us to investigate a claim that had resulted in litigation against the company who had installed a new roof on their home.

The owner claimed that the roof was leaking. The roofer claimed that the roof was perfect. What was curious was that the roof "leaked" only at the end of winter, and at times when there had been no rain and when there was no melting snow on the rooftop.

Sources of Ice Formation in HVAC Ducts

We observed the following causes of ice in the air ducts that ultimately showed up as leak stains on ceilings:

The duct ice problem was occurring because warm moist air was circulating by convection during winter, rising up into both the supply and return registers, flowing through the duct work, and leaking out of an open air handler. As the warm moist air entered the attic, the ducts were absolutely freezing cold. Moisture first condensed, then formed ice inside the duct system.

Ice accumulated in the duct system throughout the winter a little at a time, until it was several inches thick.

When the weather warmed all that ice in the ducts melted and leaked back out into the upper floor in a stunning flood. The owners, who were not thinking particularly clearly about whether or not it was raining or whether or not there was melting snow on the roof, saw that it was "raining inside" out of their air conditioning ducts and through other ceiling locations (since the ducts were not water tight there was leakage out of the ducts at other areas besides just at the supply and return air registers.

The solution to this problem had these components:

  1. Close off all of the (no longer in use) ceiling mounted supply and return ducts at the end of the air conditioning season since they were not needed for heating. This stopped the flow by convection movement of warm house air into the un-used A/C duct system. (Warm air rises up into a cool space.)

    WARNING: DO NOT close off air duct registers if your ductwork is being used for warm air heating in cold weather.
  2. Identify the sources of building leaks, high building moisture levels, or ongoing water problems such as basement water entry and fix them so that the building is not abnormally damp. (Stop providing high levels of moisture un-wanted in house air.)
  3. Support flexible ductwork properly - be sure that flex-duct is not hanging or sagging in crawl spaces and attics - sags will easily collect water and are potential mold harbors as well.

The roofing contractor was happy with this solution and the building owner was relieved as well. Perhaps because their roof had previously been leaking, before it was replaced, when they saw water coming through their top floor ceilings they thought that it was still leaking. Of course the ice in ducts problem won't occur in homes which use the same duct system for winter heating, nor will it occur in climates where freezing weather is uncommon, though we still might see some surprising in-duct condensation in some cases.

Water or Condensate Leaks into HVAC Ductwork

Water leaks into rooftop commercial ducts are common if the ducts were not properly installed and sealed against the weather. Especially when duct insulation is located inside of the HVAC ducts water leaking into the system invites mold and pathogenic growth in the HVAC system.

Water leak stains on HVAC duct interior (C) Daniel Friedman Water leak sources on rooftop ducts (C) Daniel Friedman

Our photos show water stains on duct interior insulation (above left) and ponding on a tar-coated rooftop commercial HVAC duct system (above right).

Central HVAC humidifiers may also leak into air handlers and ductwork

If you are using a central humidifier, typically turned on during dry winter months, be sure that it is not leaking onto your furnace heat exchanger - a source of costly damage and potentially a dangerous rust hole and heat exchanger leak of carbon monoxide into building air.

An improperly located or mal-adjusted central humidifier can blow excessive moisture into the HVAC duct system, a potential source of duct damage or mold growth in the air handling system.

Central Air Conditioning Cooling Coil Condensate May also leak into air handlers and ductwork

It's common, especially in very humid areas, for condensate to blow off of the cooling coil and into the HVAC duct system. See BLOWER LEAKS, RUST & MOLD for details.

As reader Joe Hartoebben pointed out, this problem may be more serious if the air handler's condensate drain pan and drain piping are not working properly as the combination of high humidity (typical for example in Florida) and inadequate condensate handling will invite the blower to send condensate water droplets blowing down the duct system.

Inadequate ceiling vapor barriers or ind adequate attic-run air conditioning ductwork

Loose blower assembly pulley or belt reduces airflow Carson Dunlop Associates

The sketch at left explains what happens if attic air conditioning ductwork is not properly insulated or if insulation has been damaged, torn, lost, or simply omitted on air conditioning duct sections.

The illustration also explains why the vapor barrier on air conditioning ducts is located on the exterior or "outside" of the duct.

Sketch at left courtesy Carson Dunlop Associates.

Building Floods May Include HVAC Ducts

Mud in a building air duct after building flooding (C) Daniel Friedman

Building floods may wet HVAC duct interiors, especially ducts located on lower building floors or in basements and crawl spaces.

Our photo (left) shows mud in the metal duct work in a residential basement of a home that had been flooded.

Following flood exposure metal ductwork should be inspected, cleaned, and sanitized.

Flex-duct that has been wet for any reason should be replaced.

Other Causes of Ice Formation in Duct Work, What Happens, How to Stop and Prevent Air Conditioner System Ice Formation

In freezing climates such as New York where some homes route their top floor HVAC ducts along the attic floor, sometimes that ductwork is not well insulated and just as it gets too hot in summer (increasing the cost of air conditioning), in winter the same ducts become too cold, increasing heating costs. But something else funny can happen in homes with attic ducts that are used only for air conditioning.

One of our clients called us to investigate a claim that had resulted in litigation against the company who had installed a new roof on their home. The owner claimed that the roof was leaking. The roofer claimed that the roof was perfect. What was curious was that the roof "leaked" only at the end of winter, and at times when there had been no rain and when there was no melting snow on the rooftop.

What we observed was the following causes of ice in the air ducts:

The duct ice problem was occurring because warm moist air was circulating by convection during winter, rising up into both the supply and return registers, flowing through the duct work, and leaking out of an open air handler. As the warm moist air entered the attic, the ducts were absolutely freezing cold. Moisture first condensed, then formed ice inside the duct system.

Ice accumulated in the duct system throughout the winter a little at a time, until it was several inches thick.

When the weather warmed all that ice in the ducts melted and leaked back out into the upper floor in a stunning flood. The owners, who were not thinking particularly clearly about whether or not it was raining or whether or not there was melting snow on the roof, saw that it was "raining inside" out of their air conditioning ducts and through other ceiling locations (since the ducts were not water tight there was leakage out of the ducts at other areas besides just at the supply and return air registers.

The solution to this problem had two components:

  1. Close off all of the ceiling mounted supply and return air ducts (at the registers) at the end of the air conditioning season since they were not needed for heating. This stopped the flow by convection movement of warm house air into the un-used A/C duct system. (Warm air rises up into a cool space.)
  2. Identify the sources of high indoor moisture (such as basement water entry) and fix them so that the house is not abnormally damp. (Stop providing high levels of moisture un-wanted in house air.)

The roofing contractor was happy with this solution and the building owner was relieved as well. Perhaps because their roof had previously been leaking, before it was replaced, when they saw water coming through their top floor ceilings they thought that it was still leaking. Of course the ice in ducts problem won't occur in homes which use the same duct system for winter heating, nor will it occur in climates where freezing weather is uncommon, though we still might see some surprising in-duct condensation in some cases.

OK to Just Use the A/C or Heat to Dry Out Flooded HVAC Ducts?

Reader Question: Hurricane Sandy flooded fiberglass heating ducts in our crawl space - can't we just run the heat to dry them out?

Flooded metal ductwork (C) Daniel FriedmanDuring hurricane Sandy the sump pump in my crawl space failed and my fiberglass heating ducts were partially submerged for two days before the water was pumped out. I replaced the sump pump and got an estimate of $7600 to replace the ducts. This may not be covered by insurance. Is it essential to replace the ducts or will the heating air dry them out sufficiently to avoid mold? - I.E.

Reply: Running heat to dry out flooded ductwork is a risky proposition and may be unsafe, as we explain here

A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with water entry, ductwork or an HVAC system. That said, here are some things to consider in deciding how to handle flooded ducts after Hurricane Sandy or similar events:

What are the Risks when Duct Systems are Flooded?

The risk of flooded ductwork, assuming that the air handler, furnace, and controls were not themselves flood-damaged as well, includes more than potential mold growth inside the HVAC system. Floodwaters usually contain sewage and sewage pathogens that can also be a source of serious infections or illnesses.

Even when materials wet by sewage-contaminated waters have dried, dust from the dried materials is likely to be unsanitary and to contain pathogens that can in turn cause illness.

When contaminated dust blows through the duct system it first contaminates the rest of the HVAC system - blower fans, air handler, plenum chambers, heat exchanger, filter systems, other ductwork, supply and return registers. And when such duct reaches room air and can be breathed by occupants it presents a health hazard.

Those same health and HVAC contamination hazards are repeated as a mold risk. Duct insulation that is actually mold contaminated may or may not be *visibly*contaminated, as you can read in our article about fiberglass insulation mold at FIBERGLASS INSULATION MOLD

Metal ductwork that is accessible can often be cleaned (photo above left) but any external duct insulation that was wet or contaminated will need to be replaced. Other types of ductwork that cannot be reliably or safely cleaned after flooding are described below.

Other Problems Occur due to Ductwork Flooding, including

Water damaged fiberglass lined ductwork (C) Daniel FriedmanIncomplete duct cleaning and replacement: we saw this water-soaked fiberglass-lined HVAC duct at an office building where the owners had been assured that all previously wet ductwork had been replaced and that there were no leaks into the duct system.

Insulation that has become wet and soggy can sag, collapse, block the ductwork completely, or partially block ductwork increasing air conditioning or heating costs and reducing system effectiveness.

Loose fragments of duct insulation, if drawn into the blower assembly, can jam the blower fan, leading to a fire.

The problems with wet HVAC ducts due to flooding include more than water, sewage pathogens, and mold contamination risk. There is also the problem of mud and silt that enter flooded duct systems. I've found inches of fine muddy silt on the bottom of ductwork in flooded buildings I've inspected.

Dirt and debris blowing through the duct system clogs the blades of the squirrel cage fan type blower fan in the air handler, reducing air flow, causing air conditioning or heating system malfunctions as a result, and of course also reducing system effectiveness and increasing system operating cost. Details are at DIRTY A/C BLOWERS

That same dirt and debris in the duct system is likely to block the cooling coil if the system includes air conditioning, causing icing and malfunction described at DIRTY COOLING COIL / EVAPORATOR COIL

Advice for Salvaging Flooded Ductwork - Avoid Running the HVAC System

Fiberglass insulation in an air handler (C) Daniel FriedmanJust what actions are needed regarding your flooded air ducts depend on the following:

Did anyone run the heating or air conditioning system after it was flooded and before it was inspected and cleaned?

If so the entire system is a candidate for professional cleaning and sanitizing for the reasons we explained above. It's best not to run heating or air conditioning, even if it's operational, if that system has been flooded, or if the building has known mold contamination, until those problems have been addressed, as doing so contaminates the system and spreads airborne pathogens increasing the risk to building occupants.

As you can see in our photo (left) fiberglass insulation in this air handler is quite vulnerable to wetting and contamination by floodwaters.

We might make an exception to that advice if experts on site determine that the benefit from running the heating or air conditioning system to speed building dryout are worth the costs in damage to the system itself, and of course provided that appropriate cleaning or replacement steps are going to follow. This situation might arise following widespread flooding or disaster when there are simply not enough free-standing air movement, fans, dehumidifiers and building dryout equipment to service all of the buildings in need.

What sections of ductwork were actually flooded - certainly all sections that were wet by floodwaters need to be addressed?

Important in Deciding how to Treat Flooded Ductwork is the Question: of what are the HVAC ducts constructed and where are they located?

Wet rusted spiral ducts (C) D Friedman

Metal hvac ducts that have external fiberglass insulation may be salvageable by removing flood-wet external insulation, professionally cleaning and sanitizing the ductwork, and installing new external insulation.

Incidentally, in our opinion ducts routed in or below building first floor slabs (photo at left) should be abandoned and replaced regardless of their construction materials.

Metal ductwork that used internal (usually fiberglass) insulation is probably not cost-feasible to salvage given the labor of removing and re-installing internal duct insulation liners.

Fiberglass insulation in an air handler (C) Daniel Friedman

HVAC ducts made entirely of fiberglass and that have been flooded should be replaced.

Even though the exterior of of all-fiberglass air ducts is usually coated with aluminum foil and a binder coats on the interior surface, those materials are not designed to prevent absorption of floodwaters.

And those materials cannot be mechanically cleaned without causing further damage to the ducts (and release of fiberglass fragments into building air).

Fiberglass insulation in an air handler (C) Daniel Friedman

Flex duct that has been flooded should be replaced.

Impact of HVAC Duct Routing & Location on Cleaning & Restoration Costs

Vertical fiberglass lined ducts (C) Daniel FriedmanVertical ductwork run through walls means extra costs to clean or replace.

Ductwork that runs through accessible areas is of course less costly to inspect, clean, or replace. HVAC ductwork running through closed building cavities such as ceilings or walls also needs to be addressed.

In a flooded building whose floors and walls were wet, demolition required to open and clean wall cavities will also expose wall or ceiling duct work for assessment and cleaning or replacement.

Vertical, fiberglass-lined ducts such as those shown in our photo (left) cannot be mechanically cleaned, and because they run through finished building wall cavities, unfortunately the cost to remove and replace the material will be significant.

Ductwork that runs through inaccessible walls, floors, ceilings that were not flooded should nevertheless be considered in restoring the building to safe habitable operation.

IF the HVAC system was not run, thus not contaminating those duct systems, and if they were not wet by storm or flood waters, they are probably usable.

Fiberglass insulation in an air handler (C) Daniel Friedman

HVAC return air ducts run horizontally under floors or ceilings are particularly likely to have accumulated mud or floodwater debris (as you saw in our muddy flooded duct photo above).

In older buildings often return ducting was constructed using the space between floor joists, nailing sheet metal over the bottom edge of joists. Often we find these return ducts loaded with decades of dust, debris, mouse droppings, even squirrell nests.

Soaking these materials is inviting production of more pathogens, viral or bacterial hazards.

Fiberglass insulation in an air handler (C) Daniel Friedman

HVAC return air ducts run through areas above suspended or "drop" ceilings that have been contaminated by floodwaters should be addressed by removing the suspended ceilings and complete cleaning of return registers as part of post-flood building restoration.

As our photo illustrates, in some buildings, particularly older offices, there may have already been an accumulation of unsafe (lead paint chips, asbestos) fragments and debris along with other hazards in these areas.

Soaking these materials is inviting production of more pathogens, viral or bacterial hazards.

What about Just Spraying a Sanitizer or a Coating Inside the Flooded Ductwork?

What about just spraying a sanitizer or a coating that seals the internal surfaces of fiberglass ductwork? This approach is worth considering for ducts that were not actually flooded but are suspect of contamination and not easily cleaned or replaced.

However there are some warnings to keep in mind, including the difficulty of assuring that absolutely all surfaces are coated and sealed and difficulty of assuring that the sealed surfaces remain intact throughout the remaining life of the building.

We have inspected sealed ductwork in which it was quite obvious that the spray procedure did not find and cover all of the contaminated surfaces and we have found other sealed ductwork in which later "sealed" sections fell away. Both of these defects exposed building occupants to air contaminated by mold or sewage pathogens or other pathogens that can occur in some duct systems (such as due to bird or rodent invasions).

More examples of damage caused by water and flooding to HVAC ducts are at WET CORRODED DUCT WORK where you will see photos of more rusted or deteriorated ducts.

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