An article by David Smith.

What do you do if you have a Frozen Air Conditioner?

I received a call from a client that I had designed and installed a Heating, Ventilation, and Air Conditioning (HVAC) system in his home. He informed me that he had no Air Conditioning (AC) and was getting ice on the larger insulated pipe going to the outdoor unit. I was not available. He could not wait for me, so he called another HVAC expert. A week later, the client called me again, informing me that his system was making strange grinding sounds, and it was laboring more than usual. I had an opening and told him I would stop by late in the afternoon. 

Frozen Air Conditioner, What To Do?

When I arrived, The client informed me that the original technician had checked everything and concluded that the unit was low on refrigerant. I turned the thermostat down to ensure that the system would run. When I approached the outdoor unit (AKA condenser) I could hear that the outdoor unit was louder than a normal functioning condenser should be. It was emitting a laboring sound that was not right. I measured the high and low pressures along with suction (the larger insulated pipe)and discharge (the small uninsulated pipe) temperatures to determine the units present superheat and subcooling

I suspect that the original technician either did not determine superheat and subcooling and came to the hasty conclusion that the unit was low on refrigerant charge based on a lower than expected suction pressure. This misdiagnosis is a common mistake as many get used to seeing the pressures on a warmer day with a warmer inside temperature. Because of the Pressure-Temperature (PT) relationship the lower the temperature of the refrigerant the lower the systems operating high and low pressures.

Before assuming low refrigerant, you should calculate the superheat on the low or suction pressure and subcooling on the high or discharge side. You determine this by measuring actual pressure and actual temperature. Then look up the pressure on a PT table for the appropriate refrigerant. In my case, I measured 68 Pounds per Square Inch (PSI) with a temperature of 46°F on the suction line. If one looks this up on a PT chart, you would see that 68 PSI has a temperature of approximately 40°F as per National Refrigerants, Inc. (NRI) source 1. If you subtract the PT chart temperature of 40°F from the actual measured temperature of 46°F, you end up with a superheat of 6°F. The difference between the saturated suction temperature and the measured suction line temperature is the suction superheat. Most systems have a superheat of two to three times that amount. To determine subcooling, you would determine the difference between the measured liquid or discharge line temperature and the saturated condensing temperature. Most manufactures have a suggested superheat and subcooling for their equipment, you should check their website for this information or contact their technical support for these values.

If I had been to look at this unit on the original service call that the other technician addressed, I would have diagnosed the problem properly in the following manner :-

The unit’s suction line would have been all iced up. The first thing to check is airflow over the evaporator (the indoor coil), ensuring that the fan motor and filter are good. If the airflow is good, one would need to de-ice the unit by operating fan only, with no AC operating, so that the ice could melt. Once the ice is gone, you can put the AC unit into cooling mode to check for the cause of the issue.

Some common causes of an AC unit icing up are as follows.

  • Insufficient airflow, typically a dirty air filter, indoor blower issue blocked, or restricted ductwork, can cause the evaporator to freeze.
  • An AC unit, when operating below design ambient outside air temperature or when the inside air temperature is below what it is designed for (usually in the low seventies), can cause the evaporator to freeze.
  • AC system with a low refrigerant charge can cause the evaporator to freeze.

All of the issues outlined above will lower the refrigerant pressure/temperature point as it exits the expansion device below 32°F. The surface of the evaporator coil will then start to freeze any moisture in the air as it condenses into a liquid form. In this case, the system is R22, so if your look at a PT chart for R22, that would be approximately 58 PSI. What this means is that when a refrigeration system operates at a suction pressure of less than 58 PSI, one runs the chance of evaporator icing up.

Now that we know the cause of the icing issue, the question is how best to deal with it. The easiest thing to do would be not to operate the system in the conditions that caused the icing up of the evaporator coil. Not operating the AC system below design temperatures is always not practical nor desirable. Adding a control device to elevate the condenser pressure by shutting off or modulating the outdoor fan, thus raising the operating pressure, these are called head pressure regulating control or something along those lines. Some systems employ a capacity control of some sort that either unloads cylinders or reduces capacity in some form. Another popular choice is a hot gas bypass controller. All of these are effective but expensive options that unless familiar with them should consult someone who is familiar with implementing them. I will expand upon this subject in an upcoming article.

There is one cost-effective choice that I have used repeatedly, and that is to monitor the suction temperature with the use of a temperature control like a Honeywell T6031A1029 or equal source 2. You mount the controller near the low voltage wiring on the outdoor unit install the remote bulb onto the suction line (the larger insulated pipe that is cold). The remote bulb requires to be in solid contact with the pipe and also to be well insulated. You intercept the cooling wire AKA Y wire before it goes into the condenser control compartment landing the wires on the R and W terminals of the T6031A1029 controller. You set it at approximately twenty-nine degrees Fahrenheit and adjust the differential to its maximum of sixteen degrees Fahrenheit (important setting and a must) this will shut off the compressor long enough to defrost the evaporator coil.

Source 1 – Retrieved from the National Refrigerants, Inc. (NRI) WEB site.

Source 2 – Retrieved from the Honeywell WEB site.

Smith, David (2020), HVACTechBlog 

David Smith is an Application/Design Engineer for HVAC/R and Building Automation Controls & Mechanical Consultant. Certified Niagara AX & N4 located in Marlboro, New York.

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