Why Your AC Coil Freezes Up

Ice on an air conditioner in the middle of summer feels like it should be impossible. It is one of the more common calls we get.

The mechanism

The evaporator coil is the indoor coil. Refrigerant inside it is much colder than the room, so heat moves from your house air into the refrigerant. That is how cooling works.

The coil sits below the dew point of the air passing over it, which is why it also pulls moisture out and drips into a drain pan. Normal.

For the coil to stay above freezing, enough warm room air has to keep flowing across it. Take that airflow away, or drop the refrigerant pressure too low, and the coil surface goes below 32 degrees. Condensate freezes instead of draining. Ice forms, and ice blocks airflow, which makes it colder, which makes more ice. It runs away from there.

By the time you notice, there can be a solid block on the coil and ice creeping down the line set toward the outdoor unit.

Two root causes

Not enough airflow. In order of how often I find them:

• Dirty filter. By a wide margin the most common.
• Closed or blocked supply registers, or a return blocked by furniture
• Dirty evaporator coil itself, which happens when a system has run without a filter or with a bad one
• Failing blower motor, or a blower running at the wrong speed
• Collapsed or disconnected duct
• Undersized return duct, which is a design problem rather than a maintenance one

Low refrigerant charge. Less refrigerant means lower pressure, and lower pressure means a colder evaporator. Refrigerant is not a consumable. If the charge is low, it leaked, and the leak needs finding.

There is a third, smaller category: running the system when it is too cold outside. Standard residential air conditioning is not designed to run below roughly 60 degrees outdoor. People do this on a cool evening trying to dehumidify, and it will ice up. If you want dehumidification in mild weather, that is what a dehumidifier is for.

Thawing it safely

1. Turn cooling off at the thermostat. Set the mode to OFF.
2. Set the fan to ON. Room temperature air moving across the coil is the fastest safe way to melt it.
3. Wait. One to three hours typically, longer for a heavy block. Do not rush it.
4. Have towels ready. All that ice becomes water and the drain pan may not keep up.
5. Do not chip, scrape, or heat it. The coil is thin aluminum and copper. Puncture it and you have converted a service call into a coil replacement.

While it thaws, replace the filter and open every register in the house.

After it thaws

Turn cooling back on and watch it. If it cools normally and stays clear, the filter was your problem. Keep an eye on it for a few days.

If it ices up again within a day or two, do not keep thawing and running it. Repeated freezing is hard on the compressor, because liquid refrigerant can make it back to a compressor designed only for vapor. That is how a nuisance turns into a compressor replacement.

What we do on the call

Measure static pressure to find out whether airflow is actually adequate, check blower speed and amperage, inspect the coil for the kind of dirt a filter does not catch, and put gauges on to check the charge and the superheat and subcooling numbers.

If the charge is low, we look for the leak. Common spots are the evaporator coil itself, the line set connections, service valves, and on older systems the coil can be pinholed from formicary corrosion, which is a genuine thing and not a good outcome.

Adding refrigerant without finding the leak is a temporary fix that costs you again next season, and the newer refrigerants are not cheap. We will tell you honestly whether the leak is worth repairing or whether the coil is at the end of its life.

Prevention

Change the filter on a real schedule. Keep registers open. Have the system looked at each spring before you need it. Most of the freeze ups I get called for in July were preventable in April.