Why your AC freezes up in a Florida summer

A frozen air conditioner in the middle of a Florida August looks like a contradiction. It is not. Ice forms because the evaporator coil has dropped below freezing, and that happens when the coil cannot pick up enough heat from the air moving across it. There are only really two reasons for that: not enough air is moving, or there is not enough refrigerant to carry the heat away.

Turn the cooling off and leave the fan running. That thaws the coil using your own blower instead of melting ice over a live compressor, which is the part you do not want to damage.

Cause one: restricted airflow

This is the majority of them, and the cheapest to fix. Anything that chokes the air crossing the coil will freeze it:

• A filter that has gone from dirty to blocked. In Florida, filters load faster than the box on the packet suggests.
• Supply vents closed or blocked by furniture, which people often do deliberately to "save" on unused rooms.
• A dirty evaporator coil — the filter's job is to stop this, and a filter left too long stops doing it.
• A failing blower motor, or a fan speed set wrong after a previous repair.
• Crushed or disconnected flexible duct in the attic, which is more common than it sounds.

Cause two: low refrigerant

Refrigerant is not consumed. A system does not use it up the way a car uses oil, so a low charge means it left somewhere. Low charge drops the pressure in the coil, the coil gets colder than it should, and it ices.

This matters because topping a system up without finding the leak is treating the symptom. The refrigerant will leave again, and in the meantime the compressor is running in conditions it was not designed for. A proper repair finds the leak first.

What the ice is costing you while it is there

A frozen coil moves almost no air, so the house stops cooling while the system keeps drawing power. Worse, ice can push liquid refrigerant back to the compressor, and compressors are built to compress vapour, not liquid. That is how a cheap airflow problem turns into an expensive one.

When to call someone

If it freezes once after a filter has plainly been left too long, change the filter, thaw it, and watch it. If it freezes again after that, stop running it. A system that refreezes with a clean filter and open vents is telling you something that a thermostat cannot fix.