Why your AC coil keeps freezing up
Ice on an air conditioner when it is over 100 degrees outside sounds impossible. It is one of the more common calls we get in an El Paso summer.
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(915) 751-9803What causes an AC coil to freeze up?
An air conditioning coil freezes when it gets colder than it should, and that almost always comes down to one of two things: not enough warm air moving across the coil, or not enough refrigerant in the system. Restricted airflow from a dirty filter, blocked vents or a dirty coil is the most common cause. A refrigerant leak is the next. Either way the coil drops below freezing, moisture in the air condenses on it, and that moisture turns to ice.
It helps to know what the coil is actually doing. Your system does not make cold air so much as it moves heat out of it. Warm air from your house passes over the indoor coil, the refrigerant inside absorbs the heat, and the cooled air goes back to your rooms. That process depends on a steady stream of warm air crossing the coil. Slow the air down, or reduce the refrigerant that carries the heat away, and the coil keeps getting colder with nothing to warm it back up.
Below 32 degrees, the water that normally drips off the coil and out the drain line freezes in place instead. Once ice starts forming it blocks the airflow further, which makes the problem worse. That is why a small issue can turn into a solid block of ice over a day or two.
The five things we find most often
A clogged air filter
The single most common cause, and the one you can check yourself in two minutes. In El Paso, desert dust loads filters faster than the printed replacement interval assumes.
Closed or blocked supply vents
Furniture over a floor register, or vents closed in unused rooms. The system is sized for the whole house, and closing vents starves the coil of the air it needs.
A dirty indoor coil
Dust that gets past the filter settles on the coil itself and insulates it. It cannot transfer heat through a layer of grime, so it runs colder than it should.
Low refrigerant from a leak
Refrigerant is not consumed the way fuel is. If the level is low, it escaped somewhere, and topping it up without finding the leak just delays the same call.
A failing blower motor
If the fan is not moving air at the rate the system expects, the effect on the coil is the same as a blocked filter.
What to do when you find ice on the system
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Turn the cooling off, leave the fan on
Set the thermostat to off for cooling but switch the fan to ON. Room temperature air moving over the coil thaws it faster than leaving everything off, and it does it without heat or sharp tools.
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Let it thaw completely
This can take several hours for a heavy freeze. Running the system while ice is still on the coil risks sending liquid refrigerant back to the compressor, which is the most expensive part of the system.
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Check the filter and the vents
Replace the filter if it is grey or you cannot see light through it. Open every supply vent and clear anything sitting on top of a register.
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Watch the drain pan
All that ice becomes water. Make sure it is going down the drain line and not onto a ceiling or into the air handler cabinet.
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Run it again and see
If it cools normally and stays clear, it was airflow. If it freezes again within a day or two, the cause is inside the system and it needs a technician.
Do not chip at the ice. The coil and the lines around it are thin metal, and a screwdriver or a knife will puncture them. A puncture turns a service call into a refrigerant leak repair.
Why this happens more here
Two things about this climate make frozen coils more likely than the national advice suggests. The first is dust. Air here carries a lot of it, and filters load up faster than the interval printed on the box. A filter rated for three months can be restricting airflow well before then.
The second is run time. In July and August a system in El Paso may run most of the day. A minor airflow restriction that would go unnoticed in a milder climate has hours of continuous operation to turn into ice.
If your system is also cooling a house that was built for an evaporative cooler, the ductwork may be undersized for refrigerated air, which restricts airflow before anything is even dirty. That is worth checking if freezing keeps coming back after the obvious causes are ruled out. See our refrigerated air conversion page for how that work is done.
Questions we get about frozen coils
Can I just run the system with the ice on it?
No. While the coil is iced over, refrigerant that should have evaporated can return to the compressor as liquid. Compressors are built to compress gas, not liquid, and that is the failure that turns a repair into a replacement.
How long does a frozen coil take to thaw?
Anywhere from one hour for light frost to most of a day for a solid block. Running the blower with cooling switched off speeds it up considerably.
Does a frozen coil mean I need more refrigerant?
Not necessarily, and it is worth being careful here. Low refrigerant is one cause, but restricted airflow is more common. Refrigerant does not get used up, so if it is genuinely low there is a leak, and adding more without finding the leak means the same problem returns.
How often should I change the filter in El Paso?
Check it monthly during summer rather than following the interval on the packaging. Desert dust loads a filter faster than those intervals assume. If it looks grey and you cannot see light through it, change it.
Can a frozen coil damage the system?
It can. Beyond the risk to the compressor, melting ice can overflow the drain pan into the cabinet or the ceiling below. A system that freezes repeatedly is telling you something is wrong upstream of the ice.
When to call us
If you have changed the filter, opened the vents, let it thaw fully and it freezes again, the cause is inside the system. That usually means a refrigerant leak, a dirty indoor coil or a blower problem, and all three need gauges and access to diagnose properly.
Call (915) 751-9803 or request a visit. We will tell you what we found before we touch anything. If you would rather get ahead of it, a maintenance plan includes the coil and airflow checks that prevent most of these calls.