Icing of Evaporator Coil

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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 restarting 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.
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