Quick answer
Yes. Turn the AC off at the thermostat if it has run for 15 minutes or more without cooling the house. An air conditioner that keeps running on a frozen coil or a low refrigerant charge can send liquid refrigerant back to the compressor or overheat it, and the compressor is the most expensive part in the system. Run the fan alone only to thaw ice.
Yes, you should turn off your AC if it is not cooling, and sooner is better. A system that is blowing room-temperature air is not resting: the compressor outside is usually still running, drawing full power and working against whatever went wrong. Shutting it off costs you nothing, since it was not cooling anyway, and it protects the one part that decides whether this is a repair or a replacement.
Why running an AC that is not cooling damages the compressor
The compressor is built to pump refrigerant vapour, not liquid. When the indoor coil is frozen, air cannot give its heat to the refrigerant, so the refrigerant comes back down the suction line still partly liquid. Liquid does not compress, and when it reaches the compressor it washes oil off the bearings and can break valves or scrolls. Technicians call this liquid slugging.
A low charge does damage the other way. The compressor motor is cooled by the refrigerant vapour returning to it, and with too little refrigerant in the loop that vapour comes back thin and hot. The windings overheat, the internal overload trips, and every trip shortens the motor's life. Both routes end at a compressor replacement, which on an older unit usually means a new outdoor unit.
Five checks to make while the AC is off
- Thermostat mode: confirm it reads Cool, the setpoint is below room temperature and the batteries are fresh. A thermostat bumped to Heat or to fan-only is the cheapest fault there is.
- Filter: slide it out and hold it up to a light. If no light comes through, replace it. A plugged filter is the most common reason a coil freezes.
- Breakers: the outdoor unit has its own double-pole breaker. If it has tripped, the indoor fan keeps blowing and nothing is cooling. Reset it once and no more.
- Outdoor coil: look at the fins on the condenser. Cottonwood fluff, grass clippings and dryer lint mat the outside of the coil and stop it shedding heat. Keep about half a metre clear on every side.
- The large copper line: find the thicker, insulated pipe where it enters the furnace coil or the outdoor unit. Frost or ice on it means the indoor coil is frozen.
When running the fan only is safe, and when it is not
Fan-only is safe, and useful, when the problem is ice. Set the thermostat to Off and the fan to On, and the blower pushes house air across the frozen coil and thaws it far faster than waiting. Put a towel at the base of the furnace, because a thick block of ice makes more meltwater than the drain pan normally handles. Timing is covered in how long a frozen AC takes to thaw.
Fan-only is not safe if you smell burning, if the blower itself is grinding or humming, or if water is already on the floor around the furnace. In those cases turn everything off at the breaker. Fan-only also will not cool the house: it moves air, and on a humid day it puts moisture from the wet coil back into the rooms.
Signs that need a same-day call
- A burning or hot-electrical smell from the vents or the outdoor unit.
- A breaker that trips again after one reset.
- Ice that returns after a full thaw and a new filter, which points to low refrigerant or a failing blower rather than airflow you can fix.
- The outdoor unit humming or buzzing while its fan blade sits still.
- Anyone in the house who is elderly, an infant or medically vulnerable during a heat warning. See what to do if the AC fails during a heat wave.
What the repair usually costs once a technician has looked
Most no-cooling faults caught early are small. The diagnostic visit is $90–$150, credited to the repair if you proceed, and the common fixes sit in these bands:
- Failed capacitor or contactor: $150–$350.
- Blocked condensate drain that tripped the safety switch: $150–$350.
- Refrigerant leak repair and recharge: $300–$800+, depending on where the leak is.
- Condenser fan motor or blower motor: $300–$700.
The same faults left running for a week in July are how compressors fail. That is the whole case for the off switch: off is free, and on can turn a small repair into a new outdoor unit.
If the five checks do not bring the cooling back, leave the system off and call ZK Mechanical at (647) 801-1252. We test the charge, the capacitor and the airflow, show you the readings, and quote the fix in writing before starting. The AC repair page explains the visit, homeowners near the shop can start with AC repair in Burlington, and anyone can book online.