Heating or Air Conditioning Duct Ice-up & Water Leaks InspectAPedia® -
Frost or ice build-up in heating or air conditioning ducts is often mistaken for a roof leak
What causes water or frost or ice formation in HVAC ductwork
Ice and condensate problems in air conditioning duct work, why it forms, how bad it can get, how to prevent it
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Here we discusses the
problems of condensation, water, or ice formation in heating or air conditioning system duct work. Readers should also see WET CORRODED DUCT WORK and see FROST BUILD-UP where we discuss build-up of ice on the cooling coil in air conditioning air handler units. Also see Transite Pipe Air Ducts where we discuss flooding of HVAC ductwork in slabs. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
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
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:
There were leak stains at most of the top floor air conditioning ceiling-mounted supply registers. (Photo at left)
There were leak stains at the top floor ceiling mounted return air register
The home had hot water baseboard heating and used its ductwork only for air conditioning during the summer
The home had a problem of chronic basement water entry and was in general pretty damp, even in winter
Inspecting the home's attic and the duct work in the attic I was astonished to find that in the dead of winter the ducts had about 2" of solid ice in the bottom of the ductwork! Ice was thickest closest to the supply or return registers and was thinnest in the middle of the duct runs
[Have you guessed yet?]
Inspecting back out of the attic and at the supply registers on the second floor, all of them were open - which is pretty common - few people think to close off un-used air conditioning supply registers in winter.
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:
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.
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.)
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.
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.
Building Floods May Include HVAC Ducts
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.
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Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Thanks to Indoor Air Care Corp., for their photograph of flooded flex-duct. The Southhampton NY company can be reached at 866 580-MOLD
Thanks to Joe Hartoebben,
Hartoebben Integrated Services, for suggesting possible sources of condensation or moisture found on overhead ceiling registers in an air conditioning ductwork system.
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