Why an air conditioner runs but does not cool
A system that is running and not cooling is a different problem from one that will not start, and it usually has a cheaper cause than people expect. Four of the six common reasons cost nothing to rule out, and two of them are things you should stop doing immediately.
Before you call, four minutes of checks
- Set the thermostat to COOL and the fan to AUTO, not ON. A fan left on ON pushes warm air through the house between cooling cycles and feels exactly like a dead compressor.
- Look at the filter. A filter packed solid starves the coil, freezes it, and stops the system cooling while it carries on running.
- Look at the indoor coil or the refrigerant line for ice. If either is iced, switch the system to FAN only and let it thaw. Running it iced does damage.
- Look at the outdoor unit in daylight. If the fins are packed with dust, grass or cottonwood, the system cannot dump heat and will not cool no matter what else is right.
- If the breaker has tripped, reset it once. If it trips a second time, leave it off and call. Repeated resets on a shorted compressor can weld the contactor closed and are a genuine fire risk.
Most of the time it is airflow, not refrigerant
The instinct is to assume the system is low on refrigerant. Sometimes it is. More often the refrigerant is fine and the system cannot move enough air across the coil to do anything with it.
A air conditioner cools by moving warm indoor air across a cold coil and dumping that heat outside. Choke either side of that exchange and the symptom is identical: the system runs continuously and the house does not come down.
On the indoor side the usual causes are a blocked filter, a closed or crushed return, or duct that has come apart in an attic. On the outdoor side it is a dirty condenser coil or a condenser with no room to breathe.
Airflow problems are also the ones most likely to be free to fix, which is why they are worth ruling out before anybody is dispatched.
Why a frozen coil means the system stops cooling
Restricted airflow drops the coil temperature below freezing. Condensation on it turns to ice, the ice blocks what airflow was left, and the process accelerates until the coil is a solid block. At that point the system is running, the fan is blowing, and nothing cold is reaching the house.
Low refrigerant produces the same icing for a different reason, which is why a frozen coil on its own does not tell you which problem you have. What it does tell you is to shut the cooling off and run the fan until it clears, because running a compressor against a frozen coil pulls liquid refrigerant back to it and that is how a cheap repair becomes an expensive one.
Give it a few hours. If it ices again within a day of thawing, the cause is still there.
Sometimes nothing is broken and the system is simply outmatched
On the hottest afternoons of the year a correctly working system can fail to hold setpoint, particularly in a house with west-facing glass, an under-insulated attic, or duct running through an attic that is far hotter than the air inside it.
The tell is that the house tracks the outside temperature with a lag instead of holding a number: it keeps up at eight in the morning and loses ground by four in the afternoon, then catches up again overnight. A failing system usually does not cool well at any hour.
A condenser sitting in full afternoon sun against a masonry wall runs at higher head pressure and higher current draw than the same unit in shade. It does not cause the failure, but it shortens the life of capacitors and fan motors and it explains why two identical houses on a street lose equipment years apart.
This matters for what you ask for. If the answer is capacity or duct rather than a fault, adding refrigerant will not fix it and you should not be paying for refrigerant.
When it is refrigerant, and what to ask
Refrigerant is not consumed. A system that is low has a leak, and topping it up without finding the leak buys a season at most.
Ask what the measured pressures and the superheat or subcooling were, ask where the leak was found, and ask what the repair to the leak costs separately from the recharge. A technician who tops up without locating anything is selling you the same repair again next year.
Ask which refrigerant the system uses before approving anything. R-22 has not been produced or imported in the United States since 2020 and only reclaimed stock remains, which makes a large recharge expensive on equipment that is by definition old enough to be near the end of its life. On an R-22 system, price a replacement alongside the repair rather than after it.
When it can wait, and when it genuinely cannot
Heat is not an inconvenience for everyone. Infants, adults over sixty-five, people on certain medications, and anyone with a heart or respiratory condition lose the ability to regulate temperature well before a healthy adult does, and indoor heat illness develops faster than people expect in a house with no cooling.
If anyone in that group is in the house and the indoor temperature is climbing rather than holding, treat it as urgent, and in the meantime move them to the coolest room, keep fluids going, and close blinds on the sun side.
It can wait if the house is holding a liveable temperature, nobody is at particular risk, and it is late in the evening. You will get a better rate in the morning and every company in town is backed up on a heat-wave afternoon regardless.
Call rather than wait if the breaker trips repeatedly, if you smell burning, or if the outdoor unit is buzzing without the fan turning. That last one is a motor or capacitor that has failed with the compressor still trying to start, and leaving it is how a repair becomes a replacement.
Common questions
Can a dirty air filter really stop an air conditioner cooling?
Yes, and it is one of the most common causes. A blocked filter starves the coil of airflow, the coil ices over, and the iced coil blocks the little airflow that was left. The system runs the whole time and the house never comes down.
Why is my AC running but not cooling the house?
In rough order of likelihood: the fan is set to ON rather than AUTO, the filter is blocked, the indoor coil is frozen, the outdoor coil is dirty, a capacitor has failed so the compressor is not actually running, or the system is low on refrigerant because of a leak.
How long should it take to cool the house down?
A working system typically pulls an indoor temperature down by roughly one degree every ten to twenty minutes, and that slows considerably on a very hot afternoon. If nothing has moved in two hours with the system running continuously, something is wrong.
Should I just add refrigerant myself?
No. Refrigerant is not consumed, so a system that is low has a leak that will not be fixed by adding more. Overcharging also damages the compressor, and handling most refrigerants requires certification.
The outdoor unit is humming but the fan is not turning. What is that?
Usually a failed run capacitor or a seized fan motor. Shut the system off at the thermostat and at the breaker. Leaving a compressor trying to start against a stalled fan is how a repairable fault turns into a replacement.
If it is happening now
Each city page below carries one number, answered day or night, that reaches a licensed AC technician on call in that area.
Sources
- United States Environmental Protection Agency, phase-out of R-22. Production and import of R-22 ended in the United States in 2020
- United States Department of Energy, central air conditioning maintenance. Airflow, filter and coil effects on cooling capacity
- Centers for Disease Control and Prevention, heat and health. Groups at elevated risk during indoor heat exposure