Is it normal for your furnace to smell the first time you turn it on?

It happens every fall. The first chilly evening comes, someone turns the heat on, and a few minutes later the whole house smells faintly like something is burning. It is a reasonable thing to worry about, and most of the time it is nothing.

Here is how to tell the normal smell apart from the ones that are not.

Why does my furnace smell the first time I turn it on?

Because it has been sitting all summer. While the furnace is off, household dust settles on the parts inside it, including the heat exchanger and the burners. The first time the furnace heats up in the fall, that dust burns off, and the smell comes out of your vents.

It usually smells dry and dusty, a bit like a hot iron or a space heater that has not been used in a while. Carrier describes this burning dust smell as temporary, typically lasting from about 30 minutes to a few hours on the first run of the season.

If that is what you are smelling, let the furnace run. The smell should get weaker as it goes and be gone the same day.

How long is too long?

A normal dust smell does three things: it fades while the furnace runs, it clears within a few hours, and it does not come back on the next run.

If it is still there after a few hours, gets stronger instead of weaker, or returns every time the furnace kicks on, something else is going on. A dirty filter is the first thing to check, since a clogged filter holds onto the smell and makes the furnace work harder. If a fresh filter does not fix it, have the furnace looked at.

Which smells mean something is wrong?

Burning plastic, hot wiring, or a gunpowder smell. This is different from dust. It is sharp and chemical, and it points to a part inside the furnace overheating or an electrical problem. Turn the furnace off at the thermostat and call a technician. If the smell is coming from one vent rather than the furnace itself, something may have fallen into the duct.

Rotten eggs or sulphur. Natural gas has no smell on its own. The gas company adds that rotten egg smell on purpose so that you notice a leak. Treat it as an emergency, not a furnace problem to troubleshoot. FortisBC's instructions are:

• Stop what you are doing.
• Do not use light switches, phones, or anything else that could make a spark while you are inside.
• Go outside, leaving the door open behind you.
• From outside, call the FortisBC Emergency Line at 1-800-663-9911 (24 hours) or 911.

FortisBC says it would rather you call even if you are not sure. Once they have made the home safe, that is the time to have the furnace checked.

A musty or damp smell. This tends to come from moisture in the ductwork or a dirty filter rather than the furnace itself. Start with the filter. If it keeps up, the ducts may be worth a look, and we wrote up how to tell whether a house actually needs a duct cleaning.

The one you cannot smell

Carbon monoxide is the reason a furnace deserves respect. Health Canada is clear that it has no colour, smell or taste, and the only way to detect it is a carbon monoxide alarm. A furnace can be producing it without any smell at all.

Fraser Health recommends a certified carbon monoxide alarm on every floor of your home and at least one near the bedrooms, tested monthly. If an alarm goes off, Health Canada's instructions are simple: leave right away, do not try to find the source, and call 911 once you are outside. Go back in only after a professional has fixed the problem.

The start of heating season is a good time to press the test button on every alarm in the house.

What about a brand new furnace?

A new furnace can smell for a different reason. Trane notes that on its first runs, a new furnace may give off a chemical smell from the manufacturing process and the protective coatings on its parts. Like dust, it fades as it burns off. If it does not fade, call whoever installed it.

How to keep the first run quiet next year

Two things cut down that first run smell. Change the filter before you turn the heat on for the season, so the dust has somewhere to go. And have the furnace serviced before the weather turns, so anything that is off shows up before you are relying on the heat. Health Canada also recommends having fuel-burning appliances inspected by a professional at least once a year, which is about safety as much as smell. And if you are still deciding when to turn the heat on at all, we answered when to turn on your heat in Vancouver.

Where we land on it

A dusty smell on the first cold evening of the year is almost always just that, and the best thing to do is let it run and open a window if it bothers you. The smells worth acting on are the ones that are sharp and chemical, the ones that do not go away, and the rotten egg one, which always means get out first.

Air Comfort Systems services, repairs and replaces residential furnaces across Metro Vancouver. If your furnace is giving off a smell that does not fit the normal pattern, give us a call during business hours and describe what you are noticing. For a gas smell or a carbon monoxide alarm, call FortisBC or 911 first.

Sources: Carrier, Burning Smell in House? Causes and Solutions for Furnace Odors; Trane, Why Your Heater Smells Like It's Burning; FortisBC, Gas leaks and odours; Health Canada, Carbon monoxide: Preventing exposure; Fraser Health, Carbon monoxide poisoning: Know the signs, prevent the risk.