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

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Repair procedures for cooling coil ice & frost formation in air conditioning or heat pump systems. This air conditioning repair article series discusses 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. If you don't see information you want, ask us for it using the comments box on this page. The air conditioning system evaporator coil and problems of frost build-up on the air conditioning coil are explained and diagnosed here

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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Repair Procedures s for Ice or Frost Build-up on an Air Conditioning Cooling Coil (the Evaporator Coil)?

Photograph of attic air conditioning air handler, condensate drips on floorTo repair an ice or frost problem on the evaporator or cooling coil in an air conditioning or heat pump system we fdirst need to know exactly why the ice is forming. Here we list causes and thus identify possible repairs that are needed to cure coil frosting.

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.

The following causes of cooling coil icing or refrigerant line icing are arranged roughly alphabetically, not in order of most-likely. In our experience the most common causes of A/C coil icing are blocked/stopped air flow or lost refrigerant charge.

  1. Control board failure: if the the HVAC/R system control board is defective it may not be sending the signal to the compressor to shut down when the thermostat has been satisfied. In that case at the indoor air handler / blower unit the blower fan has stopped (and on commercial equipment louvers in the duct system have closed) but the compressor keeps sending refrigerant to the cooling coil. The problem could be in the thermostat, thermostat wiring, or the control board itself may need replacement. Thanks to reader R. Hansen for this tip.

  2. Cooling coil fan (the blower in the air handler unit) has stopped working. A bad fan motor relay, a bad fan motor itself in the air handler unit (some call this the "evaporator fan"), lost (or someone turned off) electrical power to the air handler blower, or even a blocked fan blade or loose fan blade on motor shaft or a fan blade blocked by ice (rare in most residential air conditioning system designs), or a lost, or broken fan belt (if your motor is not a direct drive unit) can cause coil frost formation.

    The blower fan (air handler fan or evaporator coil fan) should run when your thermostat is calling for cooling. The motor could also be off on thermal overload or reset -
    see ELECTRIC MOTOR OVERLOAD RESET SWITCH. With electrical power to the blower unit off, see if the fan blade moves freely. If not the fan motor assembly needs repair or replacement.

  3. Damaged cooling coil fins can also lead to evaporator (cooling) coil freezing: when the coil cooling fins are bent and crushed sufficiently to block a significant portion of air flow across the coil, icing is likely.
    See DAMAGED COOLING COIL for details on how to recognize and fix this problem.

  4. 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.

    We discuss dirty evaporator coils in more detail
    at DIRTY COOLING COIL. Dirty or debris-blocked evaporator coils are caused by running the air conditioning system without an air filter in place.

    The coil will need to be cleaned to get the system working again. See DIRTY COIL CLEANING PROCEDURES.

  5. Defrost control: A malfunctioning auto-defrost control or bad defrost timer control (less common on residential air conditioning systems)

  6. Dirty air conditioning filter can block or reduce air flow across the cooling coil, leading to coil frost. This is the first component a homeowner should check since the fix: replace the air filter, is so easy. See AIR FILTERS for HVAC SYSTEMS

  7. Dirty blower fan blades or non-functioning blower fan assembly: an air handler blower unit that is not moving as much air as it should will be blowing too little air across the evaporator coil. This is a less likely but possible cause of frost build-up on the cooling coil.
    See DIRTY A/C BLOWERS for details. Don't forget to check for a dirty blower fan itself - dirt can significantly cut airflow produced by the fan.

  8. The refrigerant charge is too low. If there is a refrigerant leak, the first symptom may be coil icing; but later as refrigerant continues to be lost, all cooling may be lost and the coil will no longer be frosted or iced over. In our opinion it's better to find and fix the refrigerant leak.
    See articles beginning at REFRIGERANTS & PIPING.

  9. Refrigerant loss or expansion valve problems might lead to cooling coil ice-ups: an improper charge or amount of refrigerant in the system can cause frost build-up on the evaporator or cooling coil. Too-little refrigerant can cause temperature in the coil to be abnormally low, leading to icing. Really. We discuss the detection of air conditioning refrigerant leaks in detail at

    Watch out: air conditioning refrigerant leaks are not normal and should be found and fixed. it's better to find and fix a leak than to turn your leaky air conditioning system into a stop on your repairman's regular refrigerant delivery route.

  10. Thermostatic Expansion Valve malfunction: a bad TEV or capillary tube that is not metering refrigerant into the evaporator coil at the proper rate can cause frost build-up or icing on the evaporator coil or cooling coil. We discuss thermostatic expansion valves (some call them thermal expansion valves or TEVs) in more detail

  11. Wall thermostat not working properly: a thermostat that fails to stop calling for cooling can lead to coil icing. When the set-temperature on the thermostat has been reached in the room where the thermostat is mounted, the thermostat should stop calling for cooling (or its switch should "open".

    But wall thermostats are so simple that unless someone has damaged the thermostat or operated it in a very dirty environment we don't find that the thermostat problem is a defect in the unit itself. More often it's operator error: the thermostat is not set properly, or it is set to a low temperature that the cooling system simply can not reach.

    Watch out:Don't just try quickly and repeatedly turning the thermostat up and down. Some air conditioner compressors may have trouble re-starting against the head pressure of refrigerant in the condenser unit. So if you keep switching the A/C system on and off the system may stop on a thermal reset. If you suspect you've caused this just leave the air conditioner off for 15 to 30 minutes and then turn it back on.

Just let the cooling coil ice melt? Watch out: advice you may find in some air conditioner repair articles such as "turn off the system and let the ice melt" are only partly correct.

Turning off the air conditioning system for a sufficient length of time will indeed let the ice melt. But icing will simply return when the system is turned back on if you have not also found and fixed the cause of ice and frost formation in the system.

Why Frost or Ice May Appear on an Air Conditioning Refrigerant Suction Line

Photograph of ice formation on the suction line of an
air conditioner compressor/condenser unit in Florida -- Mark Cramer The ice formed here is at the low pressure inlet to an air conditioning compressor condenser unit. Similar ice may form at the evaporator coil (also called the cooling coil) or at the refrigerant suction line on the cooling coil at other end of the air conditioning system, as you can see in our iced-up air conditioning cooling coil photograph at the top of this page. [More photographs wanted].

Frost and ice can even form inside air conditioning duct work, leading to troublesome leaks into the building. This article explains locations and causes of condensate, frost or ice formation in air conditioning systems, air handlers, compressor/condensers, refrigerant lines, and in air ducts.

Several reasons can cause frost or ice formation not only on the cooling coil, but on the refrigerant suction lines at the equipment as well:

  • Blocked air flow across the cooling coil, for example from a dirty air filter, collapsed duct insulation, crimped flex-duct, or similar problem.
  • Refrigerant charge level: Improper refrigerant charge (too low a charge of refrigerant in the A/C system can, for a while, lead to too-low temperatures in the coil which will then cause frost or ice build-up on the suction line.

    Ultimately however, when there is simply little or no refrigerant left in the cooling system, temperature at the cooling coil will climb back up, the frost will disappear, and you'll no longer have any cooling at all. In air conditioning service schools the instructor may demonstrate this effect by dynamically adjusting the amount of refrigerant in the cooling system as students watch the frost line extend down the suction line, then crawl back up to near the end of the cooling coil as the proper refrigerant charge amount is reached.

    Alternatively, on some cooling systems too much refrigerant can cause liquid refrigerant to flow past the cooling coil into the suction line,also causing icing.
  • TEV/Cap Tube: a malfunctioning refrigerant metering device like a bad thermal expansion valve (TEV). Conversely, a bad capillary tube (a more rudimentary refrigerant metering device found on refrigerators, dehumidifiers, and window air conditioners) won't fail by passing too much refrigerant but it might fail to pass any refrigerant at all if it becomes blocked by debris or by a slug of oil in the system.
  • Defrost control: A malfunctioning auto-defrost control or bad defrost timer control (less common on residential air conditioning systems)

Reader Question: why does a low refrigerant charge cause frost to show up on the coil ?

7/1/2014 Jonathan said:

Why does too low a charge of refrigerant in the A/C system can, for a while, lead to too-low temperatures in the coil which will then cause frost or ice build-up on the coil? Please excuse me for lack of knowledge. I'm a novice at HVAC.

Reply: low refrigerant pressure drops the in-coil refrigerant evaporation temperature to below the freezing point of water

Jonathan it's a good question and I've wondered if I needed to offer more explanation - thanks.

The problem is this: if the total refrigerant charge is low but not exhausted, then insufficient refrigerant is avaliable on the liquid side of the refrigerant metering device to adequately feed the cooling coil with liquid refrigerant. IN that condition the vaporization temperature of the refrigerant inside the cooling coil is around the same as the freezing temperature of water - which makes condensate freeze on the coil surface.

If on the other hand there were adequate refrigerant, then there is more liquid refrigerant inside the cooling coil and heat is being removed (the coil is being cooled and heat in air blowing across the coil transfers into the coil) at a higher temperature - above the freezing point of water.

The result is abnormally low pressure on the low side of the refrigeration system (inside the coil) which causes very rapid refrigerant evaporation of the small amount that *is* present, resulting in abnormally low coil temperature.

Later when still more refrigerant has been lost, there is insufficient refrigerant to cool the coil at all - so at that point there is no more frosting or icing but rather the cooling coil simply remains warm.

You can observe the key data by looking at a chart of the properties of a specific refrigerant to see its behaviour at different pressures and temperatures.

At refrigeration school we learn that ONE way to determine if we've got the right amount of refrigerant in the system is to see where the frost line is on the system. If the frost line stops in the top third of the coil then we don't have enough refrigerant - we are running out of liquid refrigerant inside the coil too far from the end of the coil.

Technical Note on Refrigerant Piping, HVAC Design and Heat Exchange Between the Low Pressure & High Pressure Refrigerant Lines: an HVAC economizer detail using refrigerant line brazing or soldering together

In some air conditioners or heat pumps at the point where the low-side suction line enters the compressor condenser unit the low-temperature (heat laden) vapor line (suction line) is soldered or brazed right next to and touching the high-temperature, high-pressure liquid refrigerant line. The purpose of this refrigerant piping detail is to act as a heat exchanger, to reduce the temperature of the liquid refrigerant that is going to enter the metering device (TEV or cap tube), gaining some benefit to system operation.

Details about soldering or brazing the two refrigerant lines in direct contact are discussed at REFRIGERANT PIPING INSTALLATION - at Technical Note on Refrigerant Piping: HVAC economizer detail


Continue reading at REFRIGERANT LINE FROST or select a topic from the More Reading links shown below.

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