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Air Conditioning Cooling Coil or Evaporator Coil Ice-up AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Frost or ice build-up on evaporator coils and its effect on cool air flow and mold
• Frost or ice formation at air conditioning compressor/condenser units
• Ice and condensate problems in air conditioning duct work, why it forms, how bad it can get, how to prevent it
• Air conditioning air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement

This chapter of "How to Inspect the Central Air Conditioning or Cooling System" discusses the problems of ice formation in air conditioning system air handler units, blower units, or AHU's, duct work, or other air conditioning system components.

The air conditioning system evaporator coil and problems of frost build-up on the air conditioning coil are explained and diagnosed here. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. [Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante] © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner

Why Frost or Ice Forms on an Evaporator Coil

In normal operation an air conditioning system is cooling air by moving it across a refrigerant-cooled "evaporator coil" or "cooling coil" in the air handler. Cooling air also removes moisture from the air - a key factor in making indoor air comfortable in hot weather.

Normally the moisture that's removed from building air forms condensate on the surfaces of the cooling coil, runs down that surface to a collector pan, and is drained away. [CONDENSATE HANDLING discusses disposing of air conditioning condensate.] But if the air flow is too slow across the cooling coil or if the refrigerant is not being metered properly into the cooling coil, the condensate on the coil surface can form frost and may build up into a coil icing problem.

What Happens When an Evaporator Coil Ices Up?

When the cooling coil has a nice thick ice build-up on its surface there will be no cool air produced by the air conditioning system at all. The fan runs, outside compressor/condenser run, but little or no air moves through the duct system. The photograph shows icing on the refrigerant line at an outdoor compressor/condenser unit but you might see this same icing at or on refrigerant lines entering the air handler or at close to the evaporator coil even if you don't see the evaporator coil itself.

The cooling coil, or evaporator coil is visible if the air handler is opened on some air conditioner units. At other installations the cooling coil is completely covered and can't be seen at its location (say on a retrofit installation atop an existing hot air furnace) unless an inspection opening has been made, or unless there is an opening that was made previously to install a humidifier in the same plenum chamber.

When an air conditioning system with a frost-blocked coil is turned off and allowed to warm up the ice on the coil melts and spills into the internal condensate collector tray in the air handler. Then when the air conditioner is re-started it may for a while produce cool air before becoming ice blocked again. If an air conditioning system behaves in this way coil icing is a possible explanation.

Frost build-up indicates an air flow or refrigerant problem. A blocked coil (by dirt) or a blower fan which has lost its ability to move air (such as a dirty squirrel cage fan) will reduce air movement across the coil and lead to frost build up there. I suspect this is the more common cause of this defect. We discuss the problem of dirt on the cooling coil slowing air flow and leading to ice-build-up at DIRTY COOLING COIL.

[Photograph of ice formation at the suction line of an air conditioning compressor/condenser unit (and some odd insulation there) courtesy of Mark Cramer a past president of ASHI and a Florida home inspector.]

What Are the Common Causes Ice or Frost Build-up on an Air Conditioning Cooling (Evaporator) Coils?

As we introduced in the previous article, when the surface temperature of an air conditioning or refrigeration evaporator coil (cooling coil) drops below 32 degF or 0 degC, condensate forming on the coil surface begins to freeze, leading to sometimes some pretty weird behavior of the cooling system, none of it good.

Debris-blocked evaporator coils might lead to evaporator coil icing: When an air conditioning or refrigeration unit evaporator coil becomes sufficiently blocked with debris as to slow down the air flow enough, the coil may actually become so cold that the condensate forming on its surface freezes, completely blocking the coil. That's because the rate of release of refrigerant into the evaporator coil was designed with an assumption of a sufficient volume of air moving across the coil to keep it from becoming too cold.

Refrigerant loss or expansion valve problems might lead to cooling coil ice-ups: Improper metering of refrigerant into the coil or an improper charge or amount of refrigerant in the system can cause frost build-up on the evaporator or cooling coil.

An air conditioning system will not operate properly and will lose cooling capacity if the evaporator coil becomes blocked with frost or ice. Even though there is all that ice on the evaporator coil the cool air flow out of the system will be reduced as air flow across the coil becomes less and less as the ice area grows.

Other Causes of Ice Formation in Duct Work, What Happens, How to Prevent Icing

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:

• There were leak stains at most of the top floor air conditioning ceiling-mounted supply registers.
• 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 two components:

1. Close off all of the 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.)
2. Identify the sources of 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.

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

Technical Reviewers

• Thanks to Guy Benfante, Chesapeake, VA 8/26/07 for the photograph of an ice-blocked air conditioning system evaporator coil and for his suggestion that we provide an air conditioning system troubleshooting FAQ.
• Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
• Thanks to Jon Bolton, an ASHI, FABI, and otherwise certified Florida home inspector who provided photos of failing Goodman gray flex duct in a hot attic.
• Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and for providing a copy of Carson Dunlop Weldon & Associates Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Wikipedia provided background information about the definition of HEPA and airborne particle interception.

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