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🍁 Conçu pour les foyers canadiens — rédigé selon les codes canadiens et les règles provinciales. N’est pas destiné à un usage hors du Canada.
ÉBAUCHE — cette page n’a pas encore été révisée par un professionnel licencié. Aperçu éditorial seulement.

Central AC won't shut off (runs even when the house is cool)

A system that keeps running is annoying and pushes up your bill, but nine times out of ten it's a thermostat setting — not a broken air conditioner.

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In Canada, check the thermostat fan setting first — it's free and the most common answer. If the fan is set to ON, the indoor blower runs all the time, even after the house is cool and the compressor has stopped. Switch it to AUTO and the fan will run only while the system is actively cooling. If air keeps blowing but the outdoor unit is quiet and off, this was it.

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visibilityHow to check

  1. At the thermostat, find the fan setting — it's usually a switch or menu item labelled ON and AUTO, separate from the Cool/Heat/Off mode.
  2. If it's set to ON, the indoor blower runs continuously by design. Note whether the air from the vents is actually cool, or just moving room-temperature air.
  3. Go outside (or listen) and check whether the outdoor unit is running. If the outdoor unit is off and quiet while air still blows indoors, the "won't shut off" is just the fan on ON — switch it to AUTO.
buildAucun outil schedule5 min

handymanHow to fix it

Switch the thermostat fan setting from ON to AUTO so the blower runs only while the system is actively cooling instead of all the time.

  1. On the thermostat, locate the fan control (a physical switch on older units, a menu setting on smart ones) — it reads ON / AUTO.
  2. Set it to AUTO. The blower will now start only when there's an active cooling call and stop shortly after the house reaches the setpoint.
  3. Wait for the next cooling cycle to finish and confirm the fan stops. If it does, the system was working correctly all along.
schedule2 min savingssouvent gratuit
gavel Vérifiez d’abord votre province AB: permis au propriétaire · BC: permis au propriétaire · ON: permis au propriétaire · PE: permis au propriétaire · QC: permis au propriétaire
Changing the thermostat fan setting, raising the setpoint, and swapping batteries or a compatible low-voltage thermostat are routine homeowner tasks across Canada. Anything past the thermostat — the contactor, relays, control board, or the refrigerant side — is licensed HVAC work.

handymanHow to fix it

Raise the setpoint above the current room temperature and confirm the thermostat isn't sitting somewhere that makes it misread the room (direct sun, above a vent, a warm corner).

  1. Raise the setpoint a few degrees above what a separate thermometer reads for the room, and wait: the system should cycle off once it thinks it's satisfied.
  2. Look at where the thermostat is mounted. Direct sunlight, a spot above a supply vent, near a lamp or appliance, or an outside-wall cold pocket all make it read the room wrong.
  3. If placement looks like the problem, a licensed tech can relocate the thermostat — but first confirm the simple setpoint change makes the unit cycle off normally.
schedule5 min savingssouvent gratuit
gavel Vérifiez d’abord votre province AB: permis au propriétaire · BC: permis au propriétaire · ON: permis au propriétaire · PE: permis au propriétaire · QC: permis au propriétaire
Changing the thermostat fan setting, raising the setpoint, and swapping batteries or a compatible low-voltage thermostat are routine homeowner tasks across Canada. Anything past the thermostat — the contactor, relays, control board, or the refrigerant side — is licensed HVAC work.

handymanHow to fix it

report Power off first — and know when to stopCut power at the breaker before you pull the thermostat off the wall. The low-voltage thermostat wiring is only 24 volts, but the air handler it connects to carries 120/240 volts, and opening it up is another job entirely. Swapping a thermostat is a reasonable DIY task; chasing a shorted wire inside the equipment is not. If a known-good thermostat doesn't fix it, that's your signal to hand it to a licensed HVAC technician or electrician.

Rule out dead batteries, then — if you're comfortable — replace the thermostat with a compatible model, matching the low-voltage wires terminal-for-terminal. If the trouble is a shorted wire behind the wall rather than the thermostat itself, stop and hand it to a pro.

  1. First, the free step: if the thermostat takes batteries, put fresh ones in — a low battery can behave erratically and hold a call for cooling.
  2. Turn the system off at the breaker before removing the thermostat. Photograph the existing wiring so every low-voltage wire goes back on the same lettered terminal.
  3. Fit a compatible thermostat, transferring the wires terminal-for-terminal. If the system still won't stop with a known-good thermostat, the fault is likely a shorted wire or the equipment control — stop here and call an HVAC technician.
schedule40 min savings$30–120
gavel Vérifiez d’abord votre province AB: permis au propriétaire · BC: permis au propriétaire · ON: permis au propriétaire · PE: permis au propriétaire · QC: permis au propriétaire
Changing the thermostat fan setting, raising the setpoint, and swapping batteries or a compatible low-voltage thermostat are routine homeowner tasks across Canada. Anything past the thermostat — the contactor, relays, control board, or the refrigerant side — is licensed HVAC work.

verifiedPourquoi confier ceci à un pro

The contactor switches line-voltage power to the compressor, and the control board sits in the same energized cabinet. Diagnosing a welded contactor or a control/sizing fault means working on live high-voltage components — and if the compressor has been running non-stop, it may be dangerously hot. In Canada this is licensed HVAC work. If the unit won't stop, cut power at the outdoor disconnect or breaker and let a technician take it from there.

gavel Vérifiez d’abord votre province AB: réservé aux licenciés · BC: réservé aux licenciés · ON: réservé aux licenciés · PE: réservé aux licenciés · QC: réservé aux licenciés
Replacing a contactor means working inside the live high-voltage section of the outdoor unit; in every Canadian province this is licensed HVAC (and, where applicable, electrical) work.

Vos notes accompagnent la demande — ce que vous avez vu, ce que vous avez éliminé, quand ça a commencé — pour que le pro arrive avec une longueur d’avance.

verifiedPourquoi confier ceci à un pro

The contactor switches line-voltage power to the compressor, and the control board sits in the same energized cabinet. Diagnosing a welded contactor or a control/sizing fault means working on live high-voltage components — and if the compressor has been running non-stop, it may be dangerously hot. In Canada this is licensed HVAC work. If the unit won't stop, cut power at the outdoor disconnect or breaker and let a technician take it from there.

Vos notes accompagnent la demande — ce que vous avez vu, ce que vous avez éliminé, quand ça a commencé — pour que le pro arrive avec une longueur d’avance.

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Pourquoi la liste est dans cet ordre

Nous ne classons pas seulement selon la probabilité. Nous classons comme le ferait un bon artisan honnête : éliminer d’abord les causes gratuites, sûres et courantes. C’est pourquoi les vérifications peu coûteuses passent avant les réparations chères — et pourquoi tout ce qui est dangereux reçoit un arrêt net plutôt qu’un encouragement. Cet ordre existe pour vous faire économiser, pas pour vous vendre quelque chose.