Furnace Not Working? 8 Checks to Make Before You Call for Repair

The first properly cold morning in London is when furnace problems show up. The furnace sat unused all summer, and the first time it has to work hard, it doesn't come on. The good news is that a lot of "broken" furnaces turn out to be something you can sort out yourself in a few minutes. Work through this list in order before you pick up the phone.

1. Check the thermostat

Make sure it's set to Heat (not Cool or Off) and set a few degrees above the current room temperature. If it has a screen that's blank or faint, replace the batteries. Programmable thermostats sometimes reset to a schedule after a power flicker, so check that too.

2. Look for a tripped breaker or switch

Your furnace has a breaker in the electrical panel, and most also have a separate on/off switch near the furnace that looks like an ordinary light switch. Both are easy to bump by accident, especially during fall cleaning in the basement.

3. Make sure the front panel is closed properly

Modern furnaces have a safety switch behind the lower door. If that panel isn't seated properly after you changed the filter, the furnace won't start. Take it off and put it back on firmly.

4. Change the filter

A clogged filter is the most common cause of a furnace that runs for a few minutes, shuts off, and repeats. The furnace overheats and a safety limit switch turns it off. Filters should be checked every one to three months during the heating season. Our Tips & Guides page has a short video on changing a furnace filter.

5. Check the vent pipes outside

High-efficiency furnaces vent through two white plastic pipes in the side of the house rather than up a chimney. After a snowfall or a windy night, snow, ice or leaves can block them, and the furnace shuts itself off to stay safe. Clear the snow around the pipe ends and keep the area clear all winter.

6. Check the gas valve and water heater

Make sure the handle on the gas line going into the furnace is in line with the pipe (open). If your gas water heater is also not working, the problem may be the gas supply to the house rather than the furnace.

7. Check the condensate drain

High-efficiency furnaces produce water that drains through a small tube to a floor drain or pump. If that line is clogged or frozen, or the condensate pump has failed, many furnaces will shut down. Look for water pooling around the base.

8. Read the status light

Most furnaces have a small window on the lower panel with a blinking light. The pattern of blinks is an error code, and the chart explaining it is usually printed on the inside of the door. Note the code before you call. It can save time on the service visit.

When to stop and call

• If you smell gas (rotten eggs or sulphur), don't touch any switches. Leave the house and call Enbridge Gas on 1-866-763-5427 or 911 from outside.
• If you hear banging, screeching or grinding, or see scorch marks or soot.
• If your carbon monoxide alarm goes off. Get everyone outside first.
• If none of the checks above solve it.

Never take apart the burner area or the gas valve yourself. If you've gone through the list and the house is still cold, book a furnace service visit and tell us which checks you've already done.