There is ice on the outdoor unit.
Winter frost on a heat pump is routine. Summer ice outside is not, and it points somewhere specific.
Ice on an air conditioner in a Phoenix summer always means the same general thing: something is making part of the system colder than it was designed to be, and moisture is freezing on it.
Where the ice is tells you where to look.
Where it is matters
On the big insulated copper line running into the outdoor unit, and at the service valve. That is the suction line carrying cold gas back from the house. Frost starting here and working back usually points at the refrigerant charge.
On the indoor coil, spreading out. That usually points at airflow, and the frozen coil guide covers it in full.
All over the outdoor coil in July. Less common and worth taking seriously, because it suggests the system is running in a way it should not be.
What to do right now
- Turn cooling off at the thermostat. Leave the fan on ON. Moving room air over the indoor coil thaws it fastest and safest.
- Change the filter while it melts.
- Open every supply vent.
- Put towels under the air handler. A thawing coil dumps more water at once than the pan expects, which is how a clear drain still overflows.
- Wait. Two to four hours. Do not chip at it and do not aim a heat gun at a coil.
What it points at
Low refrigerant. Low pressure means a colder coil than the design intended. A sealed system does not consume refrigerant, so low means a leak, and the leak is the repair.
Restricted airflow across the indoor coil: filter, blower wheel, closed vents, crushed duct.
A TXV stuck mostly closed, starving the coil. This one looks exactly like low charge from the driveway, which is why it gets misdiagnosed and why superheat and subcooling readings together are what separate them.
A failing blower motor moving less air than it should.
Winter is different
If you have a heat pump, frost on the outdoor coil in winter is routine. The unit is pulling heat out of cold air, the coil drops below freezing, and the system runs a defrost cycle to clear it. You may see what looks like steam and hear the unit change note. That is the system working.
A heat pump that frosts in winter and never clears has a defrost problem, which is a real fault but a seasonal one.
Why not to just run it
Refrigerant is supposed to come back to the compressor as a gas. When a coil is iced it does not all boil off, and liquid gets pulled into a pump built for vapour. That is the expensive failure, and it is reached by running a system that is visibly iced.
Sitting at 85 for an afternoon is the cheaper choice.
Questions that come with this one.
My heat pump ices up in winter. Is that the same thing?
No, and that one is normal. A heat pump pulling heat out of cold outdoor air will frost its outdoor coil, and it runs a defrost cycle to clear it. You may see steam and hear the unit change note. Summer ice is a different situation entirely.
Can I pour hot water on it?
Warm water, not hot, and only if you must. Thermal shock on a coil is not kind, and you are not fixing anything, just removing the symptom. Turning cooling off and leaving the fan running does the same job without risk.
How long before I can run it again?
Until there is no ice anywhere, including inside where you cannot see. Two to four hours with the fan running, longer if it was solid. Running a system with ice still on it is how compressors get damaged.
Other things that go wrong.
Downstairs is fine. Upstairs is not.
Heat rises, the roof is the hottest thing you own, and one system for two floors is a hard ask here.
Read itThe bill doubled and nothing feels different.
Usually the AC, occasionally the rate plan, and the AC version is often cheap to fix.
Read itThe power came back and the AC did not.
Some of this is the system protecting itself. Some of it is real damage from the way the power came back.
Read it