Why your air conditioner freezes up in July
Ice on the line set is a symptom, not a fault. Here are the four things that cause it and which of them you can fix yourself.
A frozen air conditioner is one of the few HVAC faults that looks dramatic and is usually cheap. The ice is never the problem. It is what happens when the evaporator coil gets colder than it should, and that only occurs for four reasons.
First, turn the compressor off
Set the thermostat to off and the fan to on. That runs warm indoor air across the coil and melts the ice without putting the compressor under load. It takes between one and three hours. Running the system while it is iced is the one thing that turns a cheap visit into an expensive one, because liquid refrigerant returning to the compressor is what kills compressors.
Cause one: airflow
Not enough warm air across the coil and the coil drops below freezing. This is the cause about half the time and it is the one you can fix yourself.
- A filter that has not been changed since spring
- Closed or blocked supply registers, often in rooms nobody uses
- A collapsed or disconnected flexible duct in the attic
- A blower wheel caked in dust, which needs a technician
Cause two: low refrigerant
A system low on charge runs at a lower pressure, and lower pressure means a colder coil. Refrigerant is not consumed, so low charge always means a leak. Anyone who tops it up without finding the leak is selling you the same repair next summer.
Cause three: running it when it is cold outside
Below roughly 62 degrees, a standard air conditioner will ice up because the head pressure drops too far. In Fresno this matters on cool spring nights when people leave the system on.
Cause four: a failing blower or control board
Less common, more expensive. If the blower runs slow or intermittently, the airflow problem is mechanical rather than something you can clear.
What we do on the call
We measure the temperature split between return and supply, check static pressure, then measure superheat and subcooling to establish whether the charge is actually low before touching a gauge to the system. If it is low we find the leak. The diagnostic is waived if you go ahead with the repair.
