Central Air Conditioning Frozen Up

by Dave on July 5, 2012

Is your air conditioning coils or line frozen? If so this could be either an easy fix or a more difficult one that would require a licensed hvac technician that has had the proper hvac training to diagnose. I am here to educate customers and help younger technicians learn the trade better, so this should benefit those that fit into that category.

Basically when you get a certain amount of suction pressure drop, the refrigerant is going to be at a temperature that will cause condensation to freeze up, and if it runs long enough like this, it can freeze up all the way from the air handler to the condensing unit. This can cause major havoc in the refrigeration cycle, and actually have liquid refrigerant slug back to the compressor, which most compressors don’t like, ultimately leading to further Hvac component damage.

If your air conditioning line is frozen, not only will this result in a block of ice covering your evaporator coils, preventing any air from passing through, it can really damage the heart of your system which is the compressor.

1. – Clogged Air Filter, Coils, or Obstruction in Front of Return Grille

Your air filter needs to be checked monthly, especially if you have a lot of people living in the home, have pets or have carpets. From my experience the more occupants that are in a home the faster that air filters get dirty. If you are having remodeling done in the home there is going to be a lot of dust in the air, and most of that dust is getting sucked into the central air conditioning system.

Air filters can get clogged enough to where little to no air can pass through, and when that happens suction pressure drops and your evaporator coil will freeze up.

Some people may just toss the old filter out and never replace it. If they do this (which I have seen plenty of times), the air will pass through like normal but then debris and other particles are going to build up on the evaporator coil, and in the future, the coil will be clogged, which is much harder to fix than just replacing the air filter, as you will have to have the coils professionally cleaned.

If you have your central air conditioning air handler located in a closet, don’t store stuff in there, especially plastic bags and paper, you know stuff that will be sucked up when the ac kicks on. Also avoid putting couches, laundry baskets, and other obstructions directly in front of the return grills.

2. – Low on Refrigerant

Another major cause that makes air conditioning units ice up. This unfortunately has to be fixed by a professional, and when I mean fixed, I don’t mean just add gas and go. Sure that may be the cheap way (for now) but R22 has recently tripled in price, and customers that have leaking systems are going to find that out quickly, that the yearly or monthly gas and go is not going to be in their best interest.

Your technician should perform a full system leak search and fix the problem accordingly, which in most cases is not going to be cheap.

3. – Bad TXV Valve

If you are an inexperienced HVAC technician and you suction pressure is low after thawing an iced up air conditioner, your first instinct would be to add gas and the problem will be fixed. But if you keep adding gas, and that suction pressure stays the same and the head pressure keeps rising, this means that you have a restricted TXV valve. Sometimes this restriction can be cleared and the system will operate again, but it could happen again, best bet is to offer the customer a new TXV valve. Restricted TXV valves in some situations cause an air conditioner to ice up also.

 

4. – Non Functioning Blower Motor

If the blower motor on the air handler is not operating, then you have refrigerant running through the lines not able to absorb any heat from the space that is supposed to be being cooled. There are several reasons why blower motors failed depending on whether they are variable speed, constant or communicating, so this again is up to a qualified air conditioning technician to determine.

5. – Low Ambient Temps

There is no set temperature at which coils will start to freeze up as different manufacturers design the coils to cool at a certain temperature. But a good rule of thumb is don’t try to cool the home below 72 degrees. If it’s below 70 degrees Fahrenheit outside, don’t even try to bump the thermostat down. Even though the unit may not freeze up, that coil is getting too cold and you risk sending liquid refrigerant back to the compressor, which is a bad thing.

6. – Pitted Contactor on Condensing Unit

A contactor is a mechanical device which opens and closes the circuit to high voltage, which is controlled by low voltage (the thermostat). When the contactor gets pitted it basically welds itself shut, and won’t unstick. So basically if you have your thermostat off, and the inside unit is not running , but your outside unit is constantly running and won’t shut off, then most likely the contactor is pitted and needs to be replaced. If you are a homeowner and this happens just shut the breaker to the outside air conditioning unit off and call for service.

So what should you do if you discover your air conditioning has frozen up?

First go to cause one and see if that solves your problem, but either way you are going to have to thaw out that frozen coil first!! Set your thermostat to the off position and turn the thermostat to ‘fan on’. Check to see if your outside unit is still running with the thermostat in the off position. Then call for service. The reason you want to turn the fan to the on position is that it will help thaw out that frozen coil faster (given the blower motor isn’t bad…). Because if the service technician shows up to your door and the coil is frozen, there is nothing he or she can do to troubleshoot a frozen air conditioning coil until it’s completely thawed, nothing.

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