When a furnace overheats, a safety switch called the high-limit switch shuts the burners off so the unit doesn't get hotter. A common cause is restricted airflow: a dirty filter, closed or blocked vents, or furniture covering a return grille. Check the filter and open the vents first. If it still shuts off, stop resetting it and call.
Safety first: If a CO alarm goes off, leave the home now and call 911 from outside. If you smell gas, leave and, from outside, call 911 and Enbridge Gas's emergency line at 1-866-763-5427. On propane, call 911 and your propane supplier. Don't stay inside to reset the furnace.
What does it mean when my furnace overheats and shuts off?
It means a high-limit safety switch has opened. Natural Resources Canada warns that restricting the air going back to the furnace can cause it to shut down in the heating season by triggering the high heat limit switch. The burners stop. Depending on the furnace, it may try again once it cools, then overheat and shut off again. That pattern leaves rooms cold on a Barrie winter night even though the furnace "tries."
Can a dirty filter cause my furnace to overheat?
Yes. A clogged filter starves the furnace of air, so heat builds up inside and the high-limit switch trips. Natural Resources Canada says to keep return-air grilles and warm-air vents clear, and to change or clean filters every three months or when they look loaded. Enbridge Gas also advises cleaning or changing the furnace filter regularly. For how to find and change the filter safely, use our dirty furnace filter guide instead of opening furnace panels.
Can closed vents make my furnace shut off?
Yes. Closing supply vents in unused rooms, blocking returns with furniture, rugs or drapes, or covering floor registers can cut the airflow the furnace needs. Natural Resources Canada says to keep return-air grilles and warm-air vents clear of obstructions. Enbridge Gas says to keep air vents unobstructed by carpets or furniture. Opening closed vents and clearing returns is safe. Don't seal off large parts of the house; that can cause another high-limit trip.
Quick airflow checks
- Open supply vents in the rooms you use.
- Make sure return grilles aren't blocked by furniture, boxes or curtains.
- Lift rugs off floor registers.
- Check the filter and replace it if it's dirty, following the filter guide above.
How is overheating different from short cycling?
Overheating is one reason a furnace can short cycle, but short cycling has other causes too, such as flame sensing, thermostat placement or an oversized furnace. Our short cycling guide covers that wider pattern. This page is about the overheating path: restricted airflow that trips the high-limit switch. If the furnace lights and runs for a while, then the heat stops, check airflow before anything else.
What should I check safely before I call?
Stick to the outside of the furnace. Confirm heat is set above room temperature, open the vents, clear the returns and change a dirty filter. Note any status-light pattern before you reset. Don't open panels, bypass switches, touch wiring or jump the limit. For a furnace that never lights, see our guide for a furnace that won't ignite, which also covers the single reset to try.
Stop and call if
- The furnace shuts off again after you've opened the vents and changed the filter
- You smell burning or hot wiring
- The status light keeps flashing after one reset
- Rooms are getting colder and the furnace won't finish a cycle
- Anyone at home is very young, older or unwell
Is an overheating furnace an emergency?
It can be. A furnace that keeps shutting off on a cold Barrie night leaves you with little or no heat, and repeated overheating isn't something to ignore. Call sooner if the house is dropping in temperature or you smell burning. If a CO alarm sounds, leave and call 911 from outside first. Furnace Repair Barrie answers 24/7 at (249) 209-2216. Our 24/7 emergency furnace repair page lists what helps on the call.
Tell us
- Whether you changed the filter and opened the vents
- How long the furnace runs before it shuts off
- Whether the fan keeps running after the heat stops
- The status light pattern, if any
- Which town you're in, and your fuel (natural gas, propane or oil)