Why is my AC freezing up?

An AC freezing up almost always comes down to one of two things: not enough warm air is moving across the indoor coil, or the system is low on refrigerant. A dirty air filter, blocked return vents, a dirty evaporator coil, or a refrigerant leak causes most cases. The fix starts the same way every time: turn the cooling off, run the fan, and let the ice melt before you do anything else.

What’s happening when your AC freezes up

The evaporator coil is the indoor coil, paired with your furnace or inside your air handler, as Trane describes it. Its job is to pull heat out of the air that the blower pushes across it. That only works if enough warm air keeps flowing over the coil. Carrier explains that when warm air can’t flow over the evaporator coil, the coil’s temperature drops too low and condensation on it freezes. A refrigerant leak or low charge can do the same thing by dropping the pressure in the coil.

Trane lists the warning signs: the system runs but doesn’t cool well, the air coming out is warm, ice shows up on the refrigerant lines, or water pools near the indoor unit and drain pan. Carrier adds weak airflow from the vents. If warm air is your main symptom, see why an AC blows warm air.

Common causes of an AC freezing up

Carrier and Trane list the same short set of causes:

• Dirty air filter. A clogged filter restricts airflow through the whole system. The U.S. Department of Energy says dirty filters “significantly reduce a system’s efficiency.”
• Blocked return vents. Furniture, rugs, or closed vents cut off the air the coil needs.
• Dirty evaporator coil. Dust acts as an insulator on the coil, Trane says, and restricts the air circulation the coil depends on.
• Low refrigerant from a leak. The Department of Energy notes that a low charge means either the system was undercharged at installation or it leaks, and that simply adding refrigerant is not a solution.
• Fan or thermostat problems. A failing blower fan can freeze the coil, according to Carrier. Trane says a coil that is frozen only in the morning may point to the thermostat.

Phoenix adds its own pressure. Air conditioners here run for months at a stretch, which makes regular filter checks more important. ENERGY STAR recommends checking filters once a month, and the Department of Energy recommends cleaning or replacing them every month or two during the cooling season. Monsoon season, which runs June 15 to September 30 according to the City of Phoenix, also brings dust storms, so check the filter after a big one. See how often to change your AC filter in Phoenix.

What to do when you find ice

Don’t keep running the system. Trane warns that running an AC with frozen coils may cause irreparable harm, and Carrier says it puts heavy strain on the compressor.

1. Switch the thermostat from Cool to Off.
2. Set the fan to On. Carrier says this pushes warm air across the coil to melt the ice.
3. Check the filter. If it’s dirty, replace it.
4. Make sure return and supply vents are open and clear.
5. Watch for water. Melting ice drains through the condensate line. If water backs up or overflows, turn the system off at the thermostat and call for service.
6. Wait. Carrier says light frost may thaw in about 1 to 3 hours, and heavy ice can take 4 to 8 hours or longer.

Don’t chip at the ice; Trane says it can damage the coil and cause a refrigerant leak. Carrier also warns against using heat sources such as space heaters, which can crack components.

When to call a pro

A single freeze caused by a dirty filter may never come back once the filter is replaced. Carrier’s rule is simple: if the coil freezes again, call a professional.

Call for service if:

• The coil refreezes after you’ve replaced the filter and cleared the vents
• The blower doesn’t run when the fan is set to On
• Water keeps overflowing from the indoor unit or drain pan

Refrigerant leaks, coil cleaning, and fan motor problems are jobs for a licensed technician. The Department of Energy says a trained technician should fix any leak, test the repair, and then charge the system to the correct level. Don’t try to add refrigerant yourself, and don’t open electrical panels on the equipment.

Timing depends on the heat. Trane suggests an emergency call in the middle of a heat wave, which in Phoenix covers most of the summer; waiting a day or two only makes sense when the weather is mild, and the system should stay off until it’s inspected. In a Phoenix summer, if the house is getting too hot to stay in safely, find a cooler place while you wait.