Photograph of attic air conditioning air handler, condensate drips on floor Cooling Coil or Evaporator Coil Ice-up or Frost Over
in Air Conditioners or Heat Pumps

  • FROST BUILD-UP on AIR CONDITIONER COILS - CONTENTS: what causes frost & ice build-up on the cooling coil or evaporator coil in an air conditioner or heat pump; knowing the cause of frost-blocked cooling coils can make the repair easier.
  • POST a QUESTION or READ FAQs about the causes, effects, and cures of icing or frost formation on the air conditioner or heat pump cooling coil

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Cooling coil ice & frost formation diagnosis & cure in the air conditioner or heat pump; refrigerant piping line frost causes & remedies.

This article series explains the causes & cures for evaporator coil icing: the problems of ice and frost formation in air conditioning system air handler units, blower units, or AHU's, duct work, or other air conditioning system components. A freezing or frosted A/C coil blocks air flow and leads to loss of cooling.

Our page top photograph of a thoroughly ice-blocked air conditioner evaporator coil was contributed by a reader who described: "I cleaned the coils & installed a new filter - obviously I have a low refrigerant problem. This is an 11 year old furnace/air handler with no history of other problems, but low on Freon."

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Causes of Ice or Frost Build-up on the Evaporator Coil or Refrigerant Suction Line on an Air Conditioner or Heat Pump System

Ice on the A/C refrigerant piping and thermal expansion valve (C) Bill Cauthen D Friedman

Discussed in this article: Frost or ice build-up on evaporator coils and its effect on cool air flow and mold: Freezing AC coils; 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 prevention.

Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante.

[Click to enlarge any image]

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.

Our photo (left) of ice formation on refrigerant piping and on the thermal expansion valve in an air conditioner air handler unit was contributed by reader Bill Cauthen.

Article Series Contents

Why Frost or Ice Forms on an Evaporator Coil

AC Coil ice up (C) Daniel Friedman Bill McNeillFrost line on the cooling coil: When liquid refrigerant enters the evaporator coil temperatures may be as low as 10 degF at that point - that is at the top of the coil at the point of refrigerant entry. In normal operation of a refrigeration system, air movement across the evaporator coil provides enough warmth that frost or ice do not form on the coil.

In fact, as one sees in a refrigeration class, releasing liquid refrigerant into a coil over which air is not being blown will quickly result in frost formation on the coil surfaces, beginning at the point of entry of refrigerant into the coil.

At the point on the cooling coil (with no air blowing across it) where no more frost forms on the coil, we know that there is no more liquid refrigerant in the coil. That is, at this point in its travel through the cooling coil all of the liquid refrigerant that has been introduced has boiled (evaporated) to a gas. Now as all vapor, the refrigerant begins to absorb sensible heat and its temperature will increase. There are pressure increases at this point in the coil too, but they are insignificant.

In a refrigeration class demonstration, we learn that one could, given no other data, determine the proper refrigerant charge or better, the proper adjustment of an adjustable refrigerant metering device (Thermostatic expansion valve) by adjusting the refrigerant flow rate into the coil so that the frost line stops just before the end of the coil.

Normal cooling of building air at the cooling 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.

Dehumidification at the cooling coil: Cooling air passing over the coil also removes moisture from that air - a key factor in making indoor air comfortable in hot weather. (Photo at left of an iced-up cooling coil courtesy of Bill McNeill.)

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.]

Why frost or ice forms on a cooling coil in an active or in-use air handler

  • The air flow is too slow or has completely halted across the cooling coil. The cause of this problem could be as simple as a dirty air filter or it could be crimped, disconnected ductwork or even improperly-sized ductwork.
  • The refrigerant is not being metered properly into the cooling coil, (too little is being released). A clogged capillary tube or a frozen, dirty, stuck thermostatic expansion valve can cause this trouble.

    Watch out: adding refrigerant to "fix" this problem by raising the compressor head pressure will indeed force more refrigerant through the system.

    But if/when that piece of crud blows out of the metering device too much refrigerant will flow back to the compressor, slugging it, perhaps destroying it when liquid refrigerant reaches the compressor internal parts.

Any or all of those conditions cause the level of refrigerant in the cooling coil to be too low; if there is some refrigerant but not enough the coil may become abnormally cold, freezing the condensate that forms on the cooling coil surface as moisture condenses out of air moving across the coil. This freezing condensate liquid can form frost and may build up into a coil icing problem or frost may appear on the cooling coil's refrigerant suction line.

When the surface of a cooling coil or suction line drops below 32 degF (say from too little refrigerant in the system or too little flow of warmer air across the cooling coil) frost formation is likely on that surface. Conversely, when the air conditioning system is working properly the surface temperatures on the cooling coil and on the refrigerant lines stay above 32 degF.

In some installations the evaporator coil tend want to drop below 32 F even in normal operation, but air movement across the coil keeps its temperature higher, and thus avoids freezing. On some commercial refrigeration or air conditioning systems where lower temperatures are common, a defrost cycle is designed into the equipment.

If an icing problem is occurring on commercial cooling systems, in addition to checking the refrigerant charge and air flow, the service technician will also check out the defrost cycle timer.


Continue reading at FROST / ICE IMPACT on A/C

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FROST BUILD-UP on AIR CONDITIONER COILS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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