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Furnace turns on and off constantly (short-cycling)

Short-cycling is hard on the furnace and on your patience — but it's usually the furnace protecting itself from something simple. Most of the time the first thing to check is the filter, and the classic repair is a twenty-minute cleaning job.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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Pull the filter first. A clogged filter chokes airflow until the furnace overheats and its high-limit safety cuts the burners — then it cools, restarts, and does it all again. Hold the filter up to a light: if light won't pass through, a fresh one (under $25 CAD) is the first fix to try.

Ranked from real-world failure data. The order updates as you select what you’re noticing above.

Likely causes, in the order to check them

Showing the typical order
Beginner-friendly Doable, with care Safety-critical Call a pro

visibilityHow to check

  1. Slide the filter out of its slot (between the return duct and the furnace) and hold it up to a light. If light won't pass through, that's the prime suspect.
  2. Time one cycle. An overheating furnace typically runs five to ten minutes, cuts the burners while the blower keeps running, then relights a few minutes later — that pattern points to a limit trip.
  3. Check the vents around the house too: more than a few closed supply registers, or furniture over the returns, starves airflow just like a clogged filter.
buildflashlight schedule10 min

handymanHow to fix it

Fit a new filter of the same size with the airflow arrow toward the furnace, open closed registers, and let the furnace run a full cycle. If it still trips with a clean filter and open vents, book a technician — a limit that keeps tripping has a reason.

  1. Note the size printed on the old filter's frame (e.g. 16x25x1) and buy the same. Avoid jumping to a much denser filter than the furnace was designed for — that chokes airflow too.
  2. Slide the new filter in with the airflow arrow pointing toward the furnace, and reseat the panel door.
  3. Open any closed supply registers and clear the return grilles, then let the furnace run one full cycle: it should now run comfortably past the ten-minute mark.
schedule10 min savings$10–25
Furnace filter (1-inch or 4-inch media)Match the size printed on your old filter's frame (e.g. 16x25x1); avoid jumping to a much denser filter than the furnace was designed for

visibilityHow to check

  1. Turn the thermostat up so the furnace starts, then watch the burners through the small sight window on the front panel — no need to open anything.
  2. Healthy: burners light and stay lit until the thermostat is satisfied. Suspect: flames light, hold for a few seconds, then drop out — and the furnace retries this two or three times before locking out.
  3. That light-then-drop pattern is the flame sensor's signature. A furnace that runs long, steady cycles but still can't warm the house points back to airflow instead.
buildNo tools schedule10 min

handymanHow to fix it

report Power and gas off before you open the burner compartmentYou're working inches from the burners and the gas train. Kill power at the furnace switch and close the gas shutoff before removing the door, and don't force or bend anything — the sensor bracket and its ceramic insulator are brittle. If you smell gas at any point, stop, leave, and call your gas utility from outside. And if any of this is outside your comfort zone, a flame-sensor cleaning is one of the quickest, cheapest calls an HVAC tech makes.

With the power and gas off, remove the flame sensor (one screw, one wire), polish the rod gently with a fine abrasive pad (Scotch-Brite type), and reinstall it. A $0–$10 job that fixes one of the most common furnace complaints.

  1. Turn off the furnace at its power switch (the light-switch-style box on or beside the furnace) and close the gas shutoff valve — handle a quarter-turn, crosswise to the pipe.
  2. Remove the burner compartment door. The flame sensor is a thin metal rod on a small bracket at the opposite end of the burners from the igniter, with a single wire attached.
  3. Remove its mounting screw and slide the sensor out. Polish the rod gently with a fine abrasive pad (Scotch-Brite type) or 0000 steel wool until the metal is bright — not sandpaper or emery cloth, whose grit can pit the rod and foul the flame signal; wipe off the dust and avoid touching the rod with bare fingers afterward.
  4. Reinstall the sensor and door, reopen the gas valve, restore power, and run a full heat cycle — the flames should now light and hold steady.
schedule30 min savings$0–10
Flame sensor rodMost are near-universal, but match the rod length and mounting bend to the old one Fine abrasive pad (Scotch-Brite type)Fine polishing pad; avoid coarse sandpaper, which pits the rod
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: licensed professionals only
Maintenance that does not disturb gas piping, the gas valve, or combustion settings. Anything beyond the flame sensor is licensed gas work in every province. REVIEW: confirm this province's position on homeowner work inside the burner compartment.

visibilityHow to check

  1. Look at what surrounds the thermostat: a heat register below it, direct sun in the afternoon, an exterior door, a big window. Any of these skews what it "feels."
  2. On a smart or programmable thermostat, find the cycle-rate setting (often "cycles per hour," "swing," or "differential"). Three to six cycles per hour is typical for a furnace; more points here.
  3. Count your furnace's cycles for an hour on a cold day. Eight or more short runs with a clean filter points to the thermostat — its placement or its settings.
buildNo tools schedule10 min

handymanHow to fix it

Set the cycle rate (or swing/differential) to the furnace-appropriate value, shield the thermostat from the draft or sun where practical, and if the location itself is the issue, have it relocated — low-voltage work, but routing cable cleanly is fiddly.

  1. In the thermostat's settings, set the cycle rate to the value recommended for a gas furnace (often 3 cycles per hour), or widen the swing/differential slightly — half a degree of swing buys a lot of calm.
  2. Redirect a register that blows on the thermostat (a $5 deflector works), and keep sun off it with a blind during the afternoon.
  3. If the thermostat sits somewhere hopeless — over a register, in a sunroom — relocation to an interior hallway wall is the durable fix. It's low-voltage wiring; comfortable DIYers can do it, and it's a quick visit for a pro.
schedule20 min savings$0–50

visibilityHow to check

  1. Find the two plastic pipes coming through the side wall of the house (usually white PVC, near the furnace's location). One breathes in, one breathes out.
  2. Look for drifted snow at or over the openings, a "frost beard" of ice around the exhaust, or icicles hanging into the pipe mouths. In freezing rain, a glaze over the intake screen is enough.
  3. While the furnace tries to run, listen at the exhaust: a healthy one puffs steam and warm air. Silence during a call for heat, plus a pressure-switch error code inside, points here.
buildflashlight schedule10 min

handymanHow to fix it

Clear snow to about a metre around both pipes, gently knock ice off the rims by hand, and keep the openings clear for the rest of the winter. Never jam tools or de-icers inside the pipes, and never modify the venting itself — that's licensed work.

  1. Shovel snow away to about a metre around both pipe openings, down below the pipe level.
  2. Break off frost and ice at the rims gently, by gloved hand — never with a tool inside the pipe, and never with salt or de-icer, which corrodes the vent.
  3. Cycle the furnace service switch off and on to clear the lockout, then watch one startup. After every big snowfall this winter, give the vents a ten-second glance.
  4. If ice keeps re-forming at the exhaust within days, have a technician check the vent's slope and termination — chronic icing has a fixable cause.
schedule15 min savingsusually free
warning The shutdown is protecting you — don't fight itWhen a vent is blocked, the furnace locks out to keep exhaust — which carries carbon monoxide — from backing up into the house. Clear the blockage outside; never bypass a lockout, never run the furnace with a blocked vent "just to get some heat," and never disassemble or shorten the vent pipes — vent work is licensed gas-fitter work in every province. If lockouts continue after the vents are clear, or you smell exhaust indoors, stop and use the escalation above.
gavel Check your province first AB: homeowner may do this · BC: homeowner may do this · ON: homeowner may do this · PE: homeowner may do this · QC: homeowner may do this
Clearing snow and ice from the exterior vent terminations only. Any disassembly, shortening, or re-routing of the venting is licensed gas-fitter work in every province.

visibilityHow to check

  1. Think back: has the furnace short-cycled since the day it was installed, in every season? A problem that was born with the furnace points to sizing, not a part.
  2. Time the cycles on a mild winter day with a clean filter: burner runs shorter than about five minutes that still satisfy the thermostat point to oversizing.
  3. Notice the comfort pattern: the room with the thermostat gets warm fast while far rooms stay cool, and the temperature see-saws. That's the oversized signature.
buildNo tools schedule10 min

verifiedWhy this one goes to a pro

Nothing here is dangerous to observe, but there's nothing to fix yourself either: correcting an oversized furnace means professional heat-loss calculations, and any derating or replacement is licensed gas work in every province. An HVAC pro can confirm the sizing with a heat-loss calculation, soften the symptoms with blower and cycle settings now, and right-size the furnace when it's due for replacement.

gavel Check your province first AB: licensed professionals only · BC: licensed professionals only · ON: licensed professionals only · PE: licensed professionals only · QC: licensed professionals only
Furnace installation and replacement is licensed gas-fitter work in every Canadian province.

Your notes travel with the request — what you saw, what you ruled out, when it started — so the pro arrives with a head start instead of a mystery.

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Why the list is in this order

We don’t sort by likelihood alone. We sort the way a good, honest tradesperson thinks: rule out the free, safe, common causes first. That’s why the cheap checks outrank the expensive repairs — and why anything dangerous gets a hard stop instead of a pep talk. The order exists to save you money, not to sell you anything.