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Furnace Keeps Shutting Off in Cold Weather in Calgary

A furnace that keeps shutting off may be tripping one of its own safety controls. Check the safe things below, and don't keep pressing reset.

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What to do right now

  1. CO alarm or gas smell? Leave the checks for later

    Get out first. From outside, call 911, and ATCO at 1-800-511-3447 for a gas smell. Don't restart the furnace.

  2. Replace a clogged filter

    Carrier says a dirty filter can make a furnace overheat and shut itself down. Switch it off, then change the filter.

  3. Open every register

    Supply vents open, return grilles uncovered. Carrier lists blocked registers among the causes of short cycling.

  4. Check the thermostat

    Put in fresh batteries. A thermostat in direct sun or near a heat source can stop the furnace early. Moving it is a job for a pro.

  5. Clear the pipes outside

    From outside, look at the intake and exhaust pipes. Brush off snow and frost, and don't chip at ice on the pipe.

    Calgary Fire traced one 2024 carbon monoxide build-up to a vent that was blocked with leaves and debris and then iced over. If your CO alarm went off, stay outside and call 911. Have a professional check the furnace even if it restarts.

  6. High-efficiency furnace in a cold snap? Think condensate

    Trane's guide says that if condensate freezes in the lines, a drain switch shuts the furnace down, and only a furnace technician can reset that switch. Carrier adds that a clogged condensate drain or a frozen drain line can make a furnace leak water in winter.

    Water near outlets, cords, the electrical panel, the furnace or the water heater? Stay out of the water. Don't touch anything electrical.

  7. Stop pressing reset

    Don't keep pressing the reset button. If the furnace has shut off more than once, leave it off until a professional checks it.

    For a furnace that keeps shutting down before it reaches the set temperature, Carrier's advice is to turn the system off and contact a qualified HVAC professional.

Digital wall thermostat displaying 22.5 degrees Celsius.
A wall thermostat displays the room temperature.

Furnace keeps turning on and off repeatedly: what is happening

Short runs that start and stop are called short cycling. Carrier lists a dirty filter, blocked registers and a thermostat near a heat source among the causes a homeowner can check.

In deep cold, look outside as well. Snow or ice over the intake or exhaust pipe is on ATCO's no-heat checklist. On high-efficiency furnaces, frozen condensate is another cause Trane names. Environment Canada's 1981 to 2010 normals give Calgary about 22 nights a year below -20 C, and about 4 below -30 C.

Furnace not working in extreme cold: when it's an emergency

  • A CO alarm, a gas smell, or headaches and dizziness at home: get out, then call 911 from outside.
  • Someone vulnerable getting cold: go somewhere warm now. Call 911 if anyone is in danger.
  • The furnace has quit more than once: leave it off for a professional.

Calgary Fire says its crews see more carbon monoxide calls as the weather turns colder and furnaces work harder. It asks residents to keep the vents for the furnace, dryer, stove and fireplace clear of snow and debris.

Plugging in a space heater? Wall outlet only, no extension cord. Keep it well clear of anything that could catch fire.

High efficiency furnace keeps shutting off: what a professional does

Resetting a condensate drain switch is technician work, by Trane's own rule. Flame sensors, controls and questions about the furnace's size sit on the same side of the line. A professional diagnoses those with the furnace open.

Questions

Why does my furnace keep shutting off after a few minutes?

A clogged filter is one cause Carrier names: the furnace overheats and a safety switch shuts it down. Blocked registers and a thermostat sitting near heat are others you can check. If it keeps happening after that, turn the furnace off and call a professional.

Why does my furnace keep shutting off in extreme cold?

Look outside first. ATCO's no-heat checklist includes clearing snow and ice from the intake and exhaust pipes, and Trane says frozen condensate can shut a high-efficiency furnace down. Clear loose snow from the pipes yourself. A frozen condensate line is for a furnace technician.

My furnace keeps shutting off and needs to be reset. Why?

A furnace that only runs after a reset is shutting itself off for a reason, and the reset button doesn't fix that reason. Press it no more. Leave the furnace off after the second shutdown and have a professional look.

Why does my furnace keep shutting off before reaching temperature?

Carrier's advice for this exact symptom: turn the system off and contact a qualified HVAC professional. Before that, check the filter, the registers and the thermostat batteries.

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Call 587-880-3103

Calls are answered by an automated assistant and transcribed.