Furnace Blowing Cold Air? What to Check in Lower Mainland Homes

You turn the heat up on the first cold, wet October morning, the furnace kicks on, and the air coming out of the vents is cool. Not warm. Not even lukewarm.

Sometimes that's normal for a minute or two. Sometimes it's a thermostat setting or a filter you can sort out yourself in five minutes. And sometimes it's the furnace telling you something is wrong. Here's how Lower Mainland homeowners can tell which one it is, and when to stop poking around and call a licensed gas contractor.

First: give it a couple of minutes

Mid- and high-efficiency gas furnaces don't fire up instantly. FortisBC points out that these furnaces have an intentional delay built into ignition, so the sequence of fan, igniter and burner can take a moment. The blower may also push some cool air sitting in the ductwork before the warm air arrives.

If the air turns warm within a few minutes, you're fine. If it stays cold, or the furnace starts, stops and starts again without ever warming up, keep reading.

Five things to check yourself

None of these involve opening up the burner compartment or touching gas parts. If a step makes you uncomfortable, skip it.

1. The thermostat fan setting

It is one of the most common causes of cold air from the vents, and the easiest to fix. If the fan is set to ON instead of AUTO, the blower runs all the time, including between heating cycles when the burners are off. That means cool air blowing through the vents for long stretches. Switch it to AUTO and the fan should only run when the furnace is actually heating.

While you're there, make sure the thermostat is on HEAT, set a few degrees above room temperature, and has fresh batteries if it uses them.

2. The air filter

FortisBC lists a clogged or dirty filter as one cause of a furnace blowing cold air. A choked filter restricts airflow, the furnace can run hot, and a built-in safety limit may shut the burners off to protect the equipment. The fan keeps moving air, but there's no heat behind it.

FortisBC recommends checking your filter at least every three months and more often with pets or smokers in the house. Turn off power to the furnace before you pull the filter. If it's grey and matted, replace it. A lot of furnaces haven't had a filter changed since last winter, so October is a good time.

3. Power and the furnace switch

Your furnace has a service switch, usually on or near the unit and often looking like a regular light switch. It's easy to bump during storage clean-outs. Check that it's on and that the breaker for the furnace hasn't tripped. FortisBC also suggests making sure the furnace doors and access panels are fully closed. Many furnaces won't run properly with a panel out of place.

4. Vents and returns

Furniture, rugs and boxes over supply vents or return grilles cut airflow. Natural Resources Canada recommends keeping return-air grilles and warm-air vents clean and clear. If only one or two rooms feel cold while the rest of the house is warm, blocked vents or leaky ductwork is more likely than a furnace fault.

5. The gas supply

If your gas stove, fireplace or water heater isn't working either, the problem may be the gas supply rather than the furnace. Don't try to fix anything on the gas side yourself. If you've had recent FortisBC work on your street, our post on a furnace not working after FortisBC work covers what to expect.

When cold air means call a professional

FortisBC's own advice is blunt: a furnace blowing cold air isn't something most customers experience, and because it's abnormal you should get someone to look at it. If the checks above didn't fix it, these are the usual culprits a technician will look for:

• Overheating and a tripped limit switch. The furnace shuts the burners down to prevent a fire risk, often because of restricted airflow. If it keeps happening after a filter change, something else is wrong.

• Ignition or flame-sensing problems. The burners light briefly and shut off, or don't light at all, while the blower keeps running. Flame rectification is on Technical Safety BC's standard furnace service checklist for a reason.

• A blocked condensate drain on a high-efficiency furnace. These furnaces produce water that drains away through a small line. Natural Resources Canada notes a plugged drain can cause water damage, and on many furnaces it will also stop the furnace from heating.

• Venting or pressure switch issues. If the furnace can't confirm it's venting safely, it won't fire.

• Leaky ductwork. The furnace may be producing heat that never reaches the rooms.

If you want a sense of pricing before you call, we've broken down what furnace repairs cost in BC.

Stop right away if you notice any of these

Cold air is annoying. Some symptoms are dangerous. FortisBC and Technical Safety BC flag these warning signs with gas appliances:

• A rotten-egg smell (natural gas)

• A yellow or wavering flame instead of a steady blue one

• Soot, streaks or scorch marks around the furnace or vent

• Headaches, nausea, dizziness or flu-like symptoms in the house, which can point to carbon monoxide

If you smell gas, leave the house first and call from outside. FortisBC's 24-hour emergency line is 1-800-663-9911, or call 911. Our guide on what to do if you smell gas in your BC home walks through the steps. If a CO alarm goes off, FortisBC says to turn off appliances, get everyone out, including pets, and call 911 or their emergency line.

No CO alarm? FortisBC recommends installing them in bedrooms or outside each sleeping area on every level of the home.

Why it's worth getting the furnace serviced this month

Technical Safety BC recommends annual servicing of home heating appliances to help prevent fires and carbon monoxide poisoning. Its furnace checklist covers the parts behind most cold-air calls: safety limit controls, thermostat operation, blower speed, pressure switches, burners, flame rectification, venting and a carbon monoxide check.

Booking that in October, before the cold snaps, beats finding out on a frosty night in December that the furnace won't heat. Our post on how often to service a furnace or boiler in BC has more on timing.

Furnace still blowing cold? We can help

Ark Plumbing & Heating handles furnace service and repair across Surrey and the Lower Mainland, from diagnosing a no-heat call to annual tune-ups. Call us at 604-441-3411 or request a quote online, and we'll get back to you within 24 hours.