Common reasons your AC runs but doesn’t cool
An AC that runs without cooling is usually a fixable problem, not a death sentence for the system. These are the usual causes.
1. Low or leaking refrigerant
Refrigerant absorbs heat from your home’s air and carries it outside. When it’s low, almost always because of a slow leak, the system can’t remove heat no matter how long it runs. Signs: air that’s slightly cool but never cold, or ice on the refrigerant lines or indoor coil. Refrigerant doesn’t get used up; if it’s low, the leak needs to be found and repaired by a certified technician before a recharge will last.
2. A frozen evaporator coil
When airflow across the indoor coil is restricted, or refrigerant is low, moisture can freeze on the coil and insulate it so it can’t cool. Signs: the system runs but barely cools, there’s ice on the copper lines near the air handler, or water pools around the unit after it shuts off. Turn the system off and let it thaw completely (this can take several hours) before calling; running it frozen can damage the compressor.
3. A dirty air filter
A clogged filter is the most preventable cause of AC cooling problems, and one of the most common our technicians find. It chokes off airflow, reduces cooling, and over time leads to coil freezing and compressor strain. In the Treasure Valley’s dusty climate, filters can clog well before 90 days, especially with pets. Check it first and replace it if it’s gray or blocked.
4. A thermostat set wrong or failing
A thermostat set to “Fan On” instead of “Auto” runs the fan all the time, blowing air that isn’t cooled whenever the compressor is off. A failing thermostat can also keep the compressor from starting. If yours is old, unresponsive or showing odd readings, it may be the culprit.
5. A failing compressor
The compressor drives the whole cooling cycle. When it’s failing, the fan may still blow while little or no cooling happens. It’s one of the more serious AC repairs; caught early it can sometimes be repaired, but on an older system, replacing the unit may be the better value.
6. A dirty or blocked outdoor unit
The outdoor condenser releases the heat taken from your home. Coils caked with dirt, or plants crowding the unit, keep it from shedding heat, and the system struggles to cool. Boise’s dry, dusty summers coat outdoor units quickly; a yearly cleaning prevents it.
What you can check yourself
If all of these check out and your AC still isn’t cooling, it’s time to call. The remaining causes (refrigerant leaks, frozen coils, compressor problems) aren’t safe or practical to diagnose yourself.
- The thermostat is set to “Cool” with the fan on “Auto”
- The air filter is clean
- No breaker has tripped (the outdoor unit can lose power while the indoor fan keeps running)
- The outdoor unit is running and clear of debris
- Supply and return vents around the house are open and unblocked
When it’s time to call
Call a technician right away if:
- Your AC has run for more than an hour with no drop in indoor temperature
- You see ice on any part of the system
- You hear unusual sounds from the indoor or outdoor unit
- You’ve replaced the filter and checked the thermostat with no change
- It’s above 95°F outside and the house is warming up quickly
Preventing it next summer
An AC that won’t cool is one of the most common problems we fix. A yearly AC tune-up keeps coils clean and catches refrigerant leaks early.