An AC that runs but does not cool the house usually falls into one of two groups. Either the equipment is producing less cooling than it should (dirty coils, low refrigerant, a weak compressor), or it produces enough and loses it on the way, through leaky ducts, a starved return or a house that gains heat faster than the system was sized for. The fix for one group does nothing for the other, so the first job is to tell them apart.
A fifteen-minute test you can do
You need one household thermometer; a kitchen probe or a small digital one works.
- Set the thermostat to Cool, a few degrees below room temperature, fan on Auto. Let it run for fifteen minutes.
- Hold the thermometer at the return grille, where air goes into the system, and note the reading.
- Hold it in the airflow at the supply register closest to the indoor unit.
- Subtract the second number from the first.
A drop of roughly 15 to 20 degrees means the equipment is making cold air; if rooms still stay warm, look at the ducts, at airflow to particular rooms and at how much sun the house takes in. A drop well under that means the system itself is short of capacity. If the air at the register is no cooler than the room at all, start with AC blowing warm air instead.
Nine common reasons an AC can’t keep up
- A dirty air filter. Less air across the indoor coil means less heat picked up, and eventually a frozen coil.
- A dirty outdoor coil. Pollen, leaves, lint and dust on the fins hold heat in the system. Capacity drops most on hot afternoons, exactly when you need it.
- Low refrigerant. The charge is not used up, so low means leaking. See refrigerant leaks.
- A frozen evaporator coil. Ice blocks the air, and the system runs without end. See frozen AC coils.
- A weak compressor or capacitor. The unit runs but moves less heat than it was built to.
- Thermostat trouble. A thermostat in afternoon sun reads high and never lets the system rest; one that reads low shuts it off early.
- Leaky or disconnected ducts. Cold air pours into the attic or crawlspace instead of the rooms.
- Closed vents and blocked returns. A sofa against the return or a row of shut registers cuts total airflow.
- A system too small for the load. An addition, a converted garage or a wall of west-facing glass can add more heat than the original unit was sized for.
Numbers one and eight are yours to fix. The rest need measurements.
What a technician checks
We start with airflow (filter, blower, return and the temperature drop across the indoor coil) because airflow problems imitate a low charge on the gauges. Next the outdoor coil and fan, then the electrical parts under load: capacitor, contactor and motor amps. Only then do the gauges go on, to read pressures, superheat and subcooling, which show whether the charge is right and the compressor is doing its work.
If the equipment tests fine, the problem is in the house: duct leaks, disconnected runs, undersized returns or a heat load the system cannot meet. In homes with old ductwork in the crawlspace or attic, duct repair is often what finally lets the AC catch up. You get an itemized price after the diagnosis, and we recommend only the repairs that address what we found.
Turn it off and call when
- The AC breaker trips again after you have reset it once.
- Ice is forming on the copper lines at the outdoor unit.
- The outdoor unit buzzes loudly, screeches or hums without its fan turning.
- You smell hot plastic or burning insulation.
Running a system in any of these conditions puts the compressor at risk, and it is the costliest part to replace.
Keeping it from coming back
Change the filter every one to three months during the cooling season, keep two feet of clear space around the outdoor unit, and have the system checked each spring. HVAC maintenance includes the coil cleaning and electrical tests that catch a weak capacitor before the first hot week. Our blog has a longer AC troubleshooting walkthrough as well.
Why it shows up on certain afternoons
Inland valleys run warmer than the shoreline. Novato and the north side of San Rafael can bake on a summer afternoon while Sausalito sits under fog. A system that copes on a mild day can fall behind on the handful of hot ones, and those are the days a dirty coil or a slightly low charge finally shows itself.