Yes. A clogged filter starves the furnace of air, so it overheats and a safety switch shuts the burners off. You may get cool air or no heat at all. Checking and changing the filter takes a few minutes and is one of the few furnace fixes that's safe to do yourself.
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 stop to check the filter.
How does a dirty filter stop my furnace from heating?
Your furnace pulls house air through the filter, warms it and pushes it back out. When the filter is packed with dust, not enough air moves past the hot parts inside. The furnace gets too hot, and a safety switch called the high limit shuts the burners off to protect it. The blower may keep running to cool things down, which is why you can feel cool air from the vents.
Signs your filter may be the problem
- The furnace starts, runs a short time, then the heat stops.
- Air from the vents feels weak or cool.
- Rooms are slow to warm up even though the furnace sounds like it's running.
- The filter is grey, matted or bowed in the middle.
If your furnace has a status light, a limit or airflow code points the same way. Our guide to furnace blinking light codes explains how to read it before you touch anything.
Where is my furnace filter?
Most filters sit where the return air enters the furnace. That's usually a slot on the side or bottom of the cabinet, or a separate filter box on the return duct beside it. Some homes have a filter behind a return grille in a wall or ceiling instead. Look for a thin removable cover, often marked with the filter size.
If you can't find it
- Check the owner's manual or the label on the furnace.
- Follow the large return duct to where it meets the furnace.
- If the only way in is to remove furnace panels and you're not comfortable doing that, stop and call.
How do I check and change the filter safely?
Turn the furnace off first, then slide the filter out and hold it up to a light. If you can't see light through it, or it's grey and matted, replace it with the same size. Furnace manuals warn against running the furnace without a filter, so have the new one ready before you start.
Step by step
- Set the thermostat to off, then turn off the furnace power switch near the unit.
- Open the filter cover and slide the old filter out, dirty side up, so dust doesn't spill.
- Read the size printed on the frame (for example, 16 x 25 x 1) and match it exactly.
- Find the airflow arrow on the new filter. Point it toward the furnace, in the direction the air travels.
- Slide it in fully, close the cover and make sure any furnace panels are back in place.
- Turn the power switch back on and set the thermostat to heat.
If you have a washable or electronic filter, follow its own manual instead of these steps.
How often should I change my furnace filter in Barrie?
Check it monthly during the heating season, and change it when it looks dirty. Natural Resources Canada suggests changing or cleaning filters about every three months, or sooner when they load up (government advice can change, so check the current page). Your owner's manual gives the interval for your model. Long Barrie winters mean long run times, so filters often fill faster from November to March.
Change it sooner if you have
- Pets that shed
- Renovation, drywall or new-build dust
- A wood stove or fireplace in use
- Central air conditioning, since the furnace blower also moves air in summer
A note on your calendar or phone each month makes it easy to remember.
Which filter should I buy?
Buy the size printed on your old filter or listed in your manual. A filter that's too small lets dust go around it, and one that's forced into place can bend and leak. Very dense filters can trap more dust, but some also restrict airflow on furnaces that weren't designed for them. If you're unsure, start with the type the manual lists.
Quick buying checklist
- Same length, width and thickness as the old one
- Fits the slot without bending
- Airflow arrow clearly marked
- A spare on hand for the next change
What if the heat still doesn't come back after a new filter?
Give it one normal start. If the furnace still won't heat, shuts off again, or shows the same fault light, leave it off and call. The cause may be something a filter can't fix, like the blower, the vents or an ignition part. Don't keep resetting it. Our furnace not working checklist covers the other safe checks.
Call right away if
- The house is getting cold and it's below freezing outside
- You notice a burning smell after the change
- The furnace keeps shutting off within minutes
- Water is pooling around the furnace
In a cold snap, overnight no heat can lead to frozen pipes. Our emergency furnace repair page explains what happens when you call. Furnace Repair Barrie answers 24/7 at (249) 209-2216.