AC Freezing Up in Phoenix: Why There's Ice on Your Unit in 110° Heat

The indoor evaporator coil is supposed to run cold, around 40°F, with a steady stream of warm house air blowing across it. That airflow keeps the coil above freezing. Cut the airflow with a clogged filter, a dirty coil, closed vents, or a weak blower, and the coil temperature falls below 32°F and ice forms.

Low refrigerant does the same thing from the other side. With less refrigerant, the pressure in the coil drops and it runs colder than designed. A system that's been slowly leaking will freeze up more and more often as the charge falls.

Phoenix dust makes the airflow problem worse. A filter can load up in weeks in spring and monsoon season, and evaporator coils in older Valley homes are often coated after years of fine dust getting past cheap filters.

Set the thermostat to OFF for cooling and FAN to ON. The blower will move warm air across the coil and melt the ice, usually within one to three hours. Don't chip at ice on the coil or lines; you can puncture the copper.

While it thaws, replace the filter and open every supply vent in the house. Closed vents in unused rooms are a common cause of freeze-ups in Valley homes with undersized ductwork.

Check for water. As the ice melts, the condensate drain has to handle a lot of water at once. If the pan overflows or water appears around the air handler, the drain line is likely clogged too, which is common after a freeze.

If the system freezes again within a day of thawing with a clean filter and open vents, stop running it. Repeated freezing points to a refrigerant leak, a failing blower motor, a dirty evaporator coil, or a stuck expansion valve, and each of those needs diagnosis with gauges.

Running a system that keeps freezing is expensive. Liquid refrigerant can flood back to the compressor and destroy it, which turns a few-hundred-dollar leak repair into a replacement.

PhoenixCoolQuotes can match you with licensed Valley technicians who handle freeze-up diagnosis routinely. We're a free matching service, not the contractor, and you're never obligated to hire.