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Furnace runs but the house won't warm up

A furnace that runs without warming the house is usually starved for airflow, not broken — the top causes are free or under $25 to rule out.

tuneWhat are you noticing?
Select anything that matches and the checklist below reorders to fit. Not sure? Skip it.
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In Canadian homes, a clogged furnace filter is the most common cause to rule out when the furnace runs but the house stays cold. Pull the filter and hold it up to a light: if you can't see light through it, that's the prime suspect — a $10–$25 replacement, no tools needed.

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. Find the filter slot — usually a thin metal door where the big return duct meets the furnace, sometimes behind a return grille on a wall or ceiling.
  2. Slide the filter out and hold it up to a light. If you can't see light through it, it's spent.
  3. Note the size printed on the frame (e.g. 16x25x1) and the airflow arrow direction before you toss it.
buildNo tools schedule5 min

handymanHow to fix it

Slide out the old filter, buy the same size, and slide the new one in with the airflow arrow pointing toward the furnace. Repeat every 1–3 months during heating season.

  1. Match the size printed on the old filter's frame — hardware stores and Canadian Tire carry the common sizes for $10–$25.
  2. Slide the new filter in with the arrow on its frame pointing toward the furnace (the direction the air flows).
  3. Close the door, run a heat cycle, and feel the vents — stronger, warmer airflow within a few minutes points to the filter as the cause.
schedule5 min savings$10–25

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 the thermostat's fan setting. ON means the blower never rests; AUTO means it only runs while the furnace is actually heating.
  2. Feel the air at a vent between heating cycles: room-temperature air blowing steadily, with warm bursts now and then, points here.
buildNo tools schedule2 min

handymanHow to fix it

Set FAN to AUTO and the mode to HEAT with the setpoint a couple of degrees above the current room temperature. The blower will now run only when the air is actually warm.

  1. Switch FAN from ON to AUTO.
  2. Set the mode to HEAT and raise the setpoint above the room temperature so the furnace gets a real call for heat.
  3. Wait one full cycle: the vents should now blow warm whenever the blower runs.
schedule2 min savingsusually free

visibilityHow to check

  1. Walk every room and check each supply vent: louvres open, nothing on top — no rugs, beds, or couches parked over them.
  2. Find the return grille (the big one, often in a hallway) and make sure furniture or curtains aren't blocking it.
  3. With the furnace running, hold a hand over the farthest vents. Weak flow at several closed-off rooms points here.
buildNo tools schedule10 min

handymanHow to fix it

Open every supply louvre and clear the return grille. Keep at least 80% of vents open even in unused rooms — closing too many pressurizes the ducts and pushes your heat out through the leaks.

  1. Open the louvres on every supply vent — the little wheel or lever on the grille.
  2. Move anything sitting on or in front of vents and the return grille; leave a hand's width of clearance.
  3. Give the house an hour at a steady setpoint, then re-check the cold rooms.
schedule10 min savingsusually free

verifiedWhy this one goes to a pro

The heat exchanger is the wall between the flames' exhaust and the air you breathe. A crack in it can put carbon monoxide into your ducts — which is why inspecting it, and any blower or combustion work behind it, belongs to a licensed technician with the instruments to test for it. If your CO alarm ever sounds or you smell exhaust while the furnace runs, leave the house and call 911 or your gas utility. For everything short of that, this is a normal service call — book it, don't open it.

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
Work on the gas train, combustion chamber, heat exchanger, or venting of a gas furnace is licensed gas work in every province. REVIEW: confirm each province's exact licensing wording (e.g. TSSA in ON, RBQ in QC) before publish.

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.

visibilityHow to check

  1. Turn the thermostat up so the blower is running, then walk the accessible duct runs in the basement, crawl space, or attic with a flashlight.
  2. Look for joints that have pulled apart, flex duct that's crushed or come off its collar, and listen for air rushing where it shouldn't.
  3. Hold a hand near each joint — escaping warm air you can feel is exactly the heat your cold rooms are missing. An unusually warm basement or attic is a clue on its own.
buildflashlight schedule20 min

handymanHow to fix it

warning Watch your footing up thereIn an attic, step only on the joists or laid boards — the drywall between them won't hold you. Wear a mask around loose insulation, keep a light in one hand at most, and if the run you need is buried, cramped, or high, hand this one to an HVAC company. It's a routine visit for them.

Reseat any joint that has pulled apart and seal accessible seams with foil HVAC tape (not fabric duct tape). Big rips, crushed runs, or anything you can't reach comfortably is a job for an HVAC company.

  1. Push separated joints back together fully — sheet-metal sections seat about 3–5 cm into each other; flex duct clamps back onto its collar.
  2. Wipe the metal clean, then wrap each seam with foil HVAC tape, pressing it down smooth. Fabric "duct tape" dries out and falls off — don't use it.
  3. Run the furnace and re-check each taped joint for escaping air, then give the cold rooms a day to judge the difference.
schedule45 min savings$10–30
sort

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.