Ice on the AC line, in a Phoenix summer.

A coil freezes because it gets too cold, and it gets too cold when it cannot absorb the heat it was expecting. That is counterintuitive on a 115 degree afternoon, and it is still what is happening.

Refrigerant enters the indoor coil cold and boils off as it picks up heat from the air moving across it. Starve it of that air, or run it with less refrigerant than it should have, and the coil surface drops below freezing. Moisture condensing out of the air turns to frost instead of water, the frost closes off the fins, and less air gets through, which makes it colder still. It compounds until the coil is a solid block.

The two causes

Airflow. This is the common one and the cheap one.

• A filter that has not been changed since spring. If the house simply is not cooling and there is no ice, start here instead.
• A blower wheel coated in dust, which looks fine and moves half the air.
• Supply vents closed off in rooms nobody uses, or a return blocked by furniture.
• A crushed or disconnected flex duct in the attic.

Refrigerant charge. This is the other one and it is not cheap.

A sealed system does not consume refrigerant. If it is low, it leaked. A low charge means low pressure, and low pressure means a colder coil. You can top it up and get a few good weeks, and then you are back here with the same symptom and a second bill for the same gas.

Frost that starts at the outdoor service valve and works its way back usually points at the charge. Frost that starts on the indoor coil and spreads usually points at airflow. Neither is a certainty from the driveway.

What to do right now

1. Turn cooling off at the thermostat. Set the fan to ON. Room air moving across the coil is the fastest safe way to thaw it.
2. Change the filter while it melts.
3. Open the closed vents. All of them, including the guest room.
4. Put towels down under the air handler. A thawing coil releases more water at once than the drain pan was built for, and it will overflow a drain line that is perfectly clear.
5. Wait. Two to four hours, longer if it was solid.

Do not chip the ice off. The fins are soft aluminium and the tubing underneath is thin copper. Do not aim a heat source at it.

Why running it frozen is the costly choice

Refrigerant is supposed to arrive back at the compressor as a gas. When the coil is iced, it does not all boil off, and liquid gets carried back into a pump designed to compress vapour. That is how compressors die, and a compressor is an order of magnitude past anything on the price list here.

If the house is uncomfortable, it is still cheaper to sit at 85 for an afternoon than to replace a compressor in August.

What it costs to find out

If it has frozen once and a filthy filter explains it, you have fixed it for the price of a filter. Say so and keep your money.

If the filter was clean, or it freezes again after a thaw, that is a diagnostic: pressures, temperature split, airflow, and a look at the coil. It is on the price list like everything else, and you get the readings in writing either way.

Where the real answer is a refrigerant leak, the honest version is that finding and fixing the leak is the job, and some of those sit past what can be done here under the one thousand dollar ceiling. If that is where it lands, you will hear it plainly and get handed to a licensed Arizona contractor, at no fee and with no markup.