When an air conditioner runs but the house stays warm, the cause is nearly always one of five things: a filter packed solid, a tripped breaker or float switch, a condenser coil buried in cottonwood and grass, a failed run capacitor, or refrigerant that has leaked out. The first three you can check in ten minutes. The last two need a licensed technician, and most of those repairs are $150–$450 in a single visit.
The five checks that solve most of these calls
- Thermostat — set to Cool, setpoint at least three degrees below the room, fan on Auto rather than On. A fan left on On blows unconditioned air between compressor cycles and feels exactly like an AC that has stopped cooling.
- Filter — pull it out and hold it to a light. A blocked filter starves the coil, drops capacity, and eventually ices the whole thing over.
- Breaker and disconnect — a central system has two supplies: the furnace circuit at the panel and a disconnect box on the wall beside the outdoor unit. Check both.
- Outdoor coil — walk out and look at the fins. Cottonwood seed in June, lawn clippings, and lint from a dryer vent pointed at the unit will mat the coil so completely that the system cannot reject heat.
- Condensate — if the drain pan has filled and tripped its float switch, the system shuts down as designed. Water under the furnace or a pan with standing water in it is the tell.
Two more that take a minute each: make sure nothing has been stacked against the condenser, and that it has at least half a metre of clear space on every side with nothing overhanging it closely. And check the supply registers upstairs are open — a room that was closed off over winter often stays closed off.
Measure the temperature split before you call
This is the single most useful number a homeowner can hand a technician. With the system running for at least 15 minutes, hold a thermometer in the return-air grille, then in a supply register close to the furnace, and take the difference.
- A split of roughly 8 to 11 °C means the system is moving heat properly and the problem is elsewhere — sizing, ductwork, or heat coming into the house faster than the unit can remove it
- A split under about 6 °C usually points to low refrigerant, a failing compressor, or a coil that has iced
- A split well over 12 °C usually means airflow is restricted — filter, blower speed, or a return that is too small
- No difference at all with the outdoor fan spinning suggests the compressor is not running, which is often a capacitor
- No difference and a silent outdoor unit points at power, the contactor, or the low-voltage circuit from the thermostat
What the outdoor unit tells you
Stand next to the condenser while the system is calling for cooling. A healthy unit has a fan pulling air up through the top and a compressor humming steadily underneath it, and the larger insulated copper line running back into the house should be cold and sweating.
- Fan turning, no compressor sound — likely the compressor side of the run capacitor, or the compressor itself
- A hum with no fan movement — the fan motor or its half of the capacitor; do not spin the blade by hand with the power on
- Rapid clicking at the contactor — pitted contacts or a control problem, a $150–$350 part in most cases
- The unit blows warm air out the top but the copper line is warm too — the system is short of refrigerant
- Ice on the copper lines where they leave the house — stop and go to the next section
One safety note that matters: never touch the capacitor. It holds a charge after the power is off and it is not the kind of thing to learn on. The disconnect box is the boundary between what a homeowner should do and what needs a licensed technician.
Ice on the coil or the lines: what to do in the next hour
Free quotes, written pricing.
Switch the cooling off at the thermostat and set the fan to On. That runs room air across the coil and melts the ice without the compressor adding more. Give it two to four hours, put an old towel under the furnace, and do not chip at the ice or point a heat gun at it.
Then change the filter and open every supply register in the house, because restricted airflow is the most common cause. If it ices again after a full thaw with a clean filter, the system is short of refrigerant and the leak needs finding — the whole sequence is in why an AC freezes up.
Faults that need a licensed technician, and what they cost
- Run capacitor: $150–$350, the most common AC repair we do and usually a same-visit fix
- Contactor: $150–$350, the relay that switches the condenser on
- Condenser or blower fan motor: $300–$700, and which end of that you land on depends on the motor
- Refrigerant leak repair and recharge: $300–$800+, because finding and sealing the leak is the expensive part
- Control board: $400–$800 on newer units with electronic controls
- Condensate drain cleaning or trap replacement: $150–$350; a new condensate pump installed is $250–$450
Expect a diagnostic visit of $90–$150 on top, credited toward the repair if you go ahead. Refrigerant is never consumed by a healthy system, so anyone offering to simply top you up without looking for the leak is selling you the same repair again next summer — the ranges are set out in what AC repair costs in the GTA.
When to stop troubleshooting and call
Call rather than keep testing if you smell burning or hot insulation, if the outdoor unit is tripping the breaker repeatedly, if you hear grinding or screeching from either unit, or if water is running somewhere it should not be. A breaker that trips more than once is reporting a fault, and resetting it a third time is how a fan motor becomes a control board as well.
How to stop it happening again
Most of the calls in this article are preventable in an hour each spring: wash the condenser coil from the inside out with a garden hose, clear anything growing within half a metre of it, put a fresh filter in, pour water down the condensate line to confirm it drains, and run the system for a full cycle in May rather than discovering the fault in July.
If the house cools but never feels dry, that is a different problem with a different fix — start with why a house stays humid with the AC on, and read our spring HVAC preparation guide before the first hot week.
ZK Mechanical diagnoses and repairs air conditioning across the GTA and Hamilton with TSSA-licensed technicians and written pricing before the work starts. Tell us the split you measured and the flash code if there is one, and book a visit — that detail often decides which part travels on the truck.
Frequently asked questions
Why is my AC running but not blowing cold air?
The three that account for most of these calls are a blocked filter, a matted outdoor coil, and low refrigerant from a leak. Measure the temperature difference between a return grille and a supply register: a healthy split is roughly 8 to 11 °C. Under 6 °C points to refrigerant or the compressor; well over 12 °C points to restricted airflow. That one number tells a technician where to start.
Should I add refrigerant to my air conditioner?
No, and not just because handling refrigerant requires an ODP card in Ontario. A sealed system does not use refrigerant up — if it is low, it leaked, and adding more without finding the leak means paying for the same gas again next season. A proper repair locates the leak, repairs it, pulls a vacuum on the system and charges it by weight to the manufacturer's specification.
How long should it take my AC to cool the house down?
On a hot GTA afternoon, expect a correctly sized system to pull the house down by about half a degree an hour once the building has soaked up heat all day. Recovering from a deep setback can take most of an evening, which is why a schedule that starts cooling before the house gets hot works better than a large drop at 6 p.m. If it never reaches setpoint at all, the sizing or the charge is the issue.
Why is only my upstairs hot?
Warm air rises, second-floor duct runs are the longest in the house, and the return path from upstairs is often a single grille at the top of the stairs. Start by opening every upstairs supply, closing the basement supplies part way, and running the fan on Auto. If that is not enough, the fix is balancing the trunk dampers or adding return capacity, not buying a bigger air conditioner.
Is it worth washing the outdoor unit myself?
Yes, and it is the highest-value maintenance task a homeowner can do on a central system. Cut the power at the disconnect, clear the debris out of the base, then rinse the coil from the inside outward with a garden hose so the dirt exits the way it came in. Use no pressure washer and no chemical cleaner — bent fins and stripped coatings cost more than the cleaning saves.