HVAC guide
Why Is My AC Running But Not Cooling?
Four causes explain almost all of them. Two you can check yourself in ten minutes.
An air conditioner that runs continuously without cooling is one of the most common calls we take between June and August. It nearly always comes down to one of four things, and two of them you can rule out before anyone charges you for a visit.
1. The filter is choking the system
Start here, always. A filter clogged enough to restrict airflow drops the evaporator coil temperature below freezing. Ice forms, airflow drops further, and the system spirals — running constantly while delivering almost nothing at the vents. Pull the filter. If light doesn’t pass through it, that’s your first suspect.
If the coil or the copper line is already iced, switch the system to FAN ONLY and leave it for several hours to thaw before running it again. Running a frozen system risks liquid refrigerant reaching the compressor, which turns a cheap problem into an expensive one.
2. Airflow is blocked somewhere else
Closed supply registers, furniture over a return, a collapsed flex duct in the crawl space, or a failing blower motor all produce the same symptom. In older Chicago bungalows and Berwyn two-flats, the return side is usually the weak point — many were built with a single central return that was never adequate once the attic got finished.
3. There’s a refrigerant leak
Refrigerant isn’t consumed. A system low on charge has a leak, full stop. Symptoms include a system that cools a little but never reaches setpoint, ice on the suction line at the outdoor unit, and a hissing sound near the coil. This one needs gauges — and it needs the leak found, not just a top-up. Recharging without repair means paying for refrigerant again next season.
4. The compressor or capacitor has failed
If the outdoor fan spins but the compressor doesn’t engage, or the unit hums and trips the breaker, the run capacitor is the usual culprit — a genuinely inexpensive part. If the compressor itself has failed, you’re into a repair-or-replace conversation, especially on any system still using R-22.
Check the filter, confirm the thermostat is set to COOL with the fan on AUTO, and check that the breaker and the outdoor disconnect are both on. If all three are fine, you have genuinely useful information for whoever comes out.
What a proper diagnosis looks like
Measuring the temperature split across the evaporator coil, checking refrigerant charge against outdoor ambient, reading static pressure to confirm airflow, and testing the capacitor under load. That sequence separates a $200 fix from a $6,000 one, and it takes about half an hour.