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This air conditioning repair article 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. Readers concerned with ice or water leaks into or out of HVAC ductwork should also see  WET CORRODED DUCT WORK and see WATER & ICE IN DUCT WORK. 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."

If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. see How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building. 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.

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FROST BUILD-UP -Ice or Frost Build-up on the Evaporator Coil or Refrigerant Suction Line on an Air Conditioner

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.

Why Frost or Ice Forms on an Evaporator Coil

Frost 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 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. Don't forget to check for a dirty blower fan itself - dirt can significantly cut airflow produced by the fan. See BLOWER FAN OPERATION & TESTING
• 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. See REFRIGERANT METERING DEVICES TEVs and REFRIGERANT METERING CAPILLARY TUBES
• 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.

Below in this article you will see What Are the Common Causes & Repairs for Ice or Frost Build-up on an Air Conditioning Cooling Coil (the Evaporator Coil)? for our complete diagnostic list of causes and effects of cooling coil ice and frost blockage.

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.

What Happens to the Ability of the Cooling System to Cool the Air 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 page top photograph shows icing on the cooling coil and refrigerant lines exiting the coil inside air handler close to the evaporator coil even.

You might see ice formation on the suction line just outside of your air handler even though you cannot see the evaporator coil itself - on most residential air conditioning systems, the surfaces of the cooling coil are not readily accessible by the homeowner. But if you don't see ice on the suction line, ice could still be present on and blocking air flow through the cooling coil.

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 (by cutting the steel and installing an access panel cover), 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. We 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 & Repairs for Ice or Frost Build-up on an Air Conditioning Cooling Coil (the Evaporator Coil)?

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.

1. 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
   
   
2. 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.
   
   
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. 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.
   
   
5. 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.
   
   
6. 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 A/C REFRIGERANT LEAK DETECTION.
   
   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.
   
   
7. 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 at THERMOSTATIC EXPANSION VALVES.
   
   
8. 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. See HARD STARTING COMPRESSOR MOTORS.
   
   
9. 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.

CONTACT us to suggest additions or corrections to this diagnostic list.

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

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

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 & DISTANCES - at Technical Note on Refrigerant Piping: HVAC economizer detail

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

Details about water and ice in ductwork are found at WATER & ICE IN DUCT WORK.

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.
• 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 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.

Questions & Answers regarding this article

Questions & answers about the causes, effects, and cures of icing or frost formation on the air conditioner or heat pump cooling coil

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  FROST BUILD-UP on AIR CONDITIONER COILS

• Thanks to reader Tom Balla for suggesting clarification on ice formation on the air conditioning system suction line.
• Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
• Thanks to inspector Bill McNeill for providing a photograph of an ice-blocked evaporator coil or cooling coil in an air conditioning air handler unit. Mr. McNeill can be reached at wem007@hotmail.com
• Thanks to reader [Anonymous] for providing a photo of a totally blocked cooling coil on an 11-year-old system that was low on refrigerant. August 2010.

Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair

• Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
• Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

• Modern Refrigeration and Air Conditioning, A. D. Althouse, C.H. Turnquist, A. Bracciano, Goodheart-Willcox Co., 1982
• Principles of Refrigeration, R. Warren Marsh, C. Thomas Olivo, Delmar Publishers, 1979
• "Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
• Refrigeration and Air Conditioning Technology, 5th Ed., William C. Whitman, William M. Johnson, John Tomczyk, Cengage Learning, 2005, ISBN 1401837654, 9781401837655 1324 pages
• Carson Dunlop, Associates, Toronto, have provided us with (and we recommend) Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
• Air Conditioning Inspection, Diagnosis, Repair, Efficiency all the basics for home owners, inspectors, new repairmen
• Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard
• Asbestos HVAC Ducts and Flues field identification photos and guide
• Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
• ...