Cleaning before the heating season: vents, returns and the smell of the first run

The first cold evening in Oakville or Mississauga is usually the evening the thermostat gets turned up for the first time since April. Whatever settled in the registers, on the return grilles and on the hot parts of the furnace over the summer gets pushed around the house or heated as soon as the system starts. A short round of cleaning in the days before, plus two safety checks that take a minute each, changes how that evening goes.

What to clean in the days before the heat goes on

In a house with forced-air heat, warm air leaves through the supply registers in each room and comes back to the furnace through the larger return grilles, usually lower on a wall or in a central hallway. CMHC's guide on heating ducts describes the return air trunk as the big rectangular duct along the basement ceiling that enters the bottom of the furnace, and notes that dust and debris collect over time, particularly in the return ducts. That is where to spend your effort.

1. 1Lift off the supply register covers and vacuum the openingFloor registers in kitchens and front halls catch crumbs, grit from boots and small toys. Pull the cover, vacuum as far down the opening as the hose reaches, and wash the cover in warm soapy water. Let it dry fully before it goes back in.
2. 2Vacuum the return grilles with the brush attachmentReturn grilles pull air in, so the dust sits on the room side of the slats. Brush them while they are still screwed on, then take them off and vacuum the first half-metre of the duct behind. Wipe the slats with a damp cloth last.
3. 3Move furniture and storage off the returnsA sofa pushed against a return grille, or a rug laid over a floor return, stops the furnace pulling back the air it is meant to reheat. Check every room, including the basement, where boxes tend to end up stacked in front of the return.
4. 4Clear the floor around the furnace and the water heaterSweep or vacuum around the base of the furnace, and move paint cans, cleaning products, cardboard and laundry away from it. Gasoline and solvents belong outside the house entirely; our post on what may be stored in the garage over winter covers that.
5. 5Look at the filter and write down its sizeSlide the filter out of its slot beside the furnace. The size is printed on the frame. If you can no longer see the filter material through the dust, replace it before the season starts, and buy the next one at the same time.
6. 6Electric baseboard heaters: breaker off, then vacuum the finsIn houses and condos heated by baseboards, dust on the element is what scorches. Turn the circuit off at the panel, let the heater cool, then vacuum the fins gently with a brush attachment so they do not bend.

Health Canada's indoor air advice adds two habits that matter more once the windows stay shut: vacuum carpets regularly, because they trap dust, dust mites and allergens, and clean hard floors with a damp mop, which removes particles a dry broom pushes back into the air. The difference between a routine clean and a deep one, and which of these jobs falls into each, is set out in our post on what a deep clean adds to a standard clean. After a renovation the dust is finer and the job is different again: see post-renovation dust and why normal cleaning misses it.

Which smell on the first run is dust, and which one means leave

A dry, dusty, slightly scorched smell the first time the heat runs comes from dust warming up on hot surfaces: inside the furnace, on a baseboard element, on a register cover. Cleaning what you can reach beforehand leaves less of it to burn. Two other situations look nothing like that and need a different response.

Carbon monoxide alarms: what changed on January 1, 2026

This is the first autumn start-up under Ontario's updated Fire Code rules on carbon monoxide alarms. According to the Government of Ontario, more than 65% of CO-related injuries and deaths in the province happen at home, and CO comes from fuels that do not burn completely in furnaces, fireplaces, water heaters, stoves and similar appliances.

In a house

A CO alarm is required if the house has a fuel-burning appliance, a fireplace or an attached garage, and since January 1, 2026 also if it is heated by air from a fuel-burning appliance outside the home. The alarms go next to each sleeping area, meaning the hallway outside the bedrooms, and since the same date on every storey, including a main floor or basement with no bedroom on it. A house that met the old rule with one alarm in the upstairs hall may now need one on each other level too.

In a condo or a rental

In an apartment or condo unit the rule also applies when the unit sits directly above, below or beside a building service room or a garage, or when it is heated by air from a furnace elsewhere in the building. Landlords install and maintain the alarms in rental units and test them every year, when a battery is replaced and at each change of tenancy. Tenants have to tell the landlord about a problem with an alarm, and removing its batteries is against the law.

• Press the test button on every CO alarm once a month, as Ontario's page recommends.
• Replace the batteries every year, and the whole alarm when the manufacturer's instructions say it has reached the end of its life.
• Learn which chirp means a low battery, which means end of life, and which is the emergency alarm. A combined smoke and CO unit sounds differently for each.
• When you buy one, look for the mark of a recognized testing agency such as CSA, ULC or ETL.

Are the heating ducts worth paying to have cleaned?

For cleaner air, it usually is not worth it. CMHC's guide on heating ducts was written because homeowners are regularly solicited for duct cleaning with promises of cleaner air, less mould and dust, better airflow and lower energy bills. It reports that research by CMHC and the US Environmental Protection Agency in the 1990s tested houses before and after cleaning, and it puts the conclusion plainly.

Duct cleaning will not usually change the quality of the air you breathe, nor will it significantly affect airflows or heating costs.

The same guide accepts that a thorough professional cleaning does clean the ducts, removing pet hair, paper clips and toys, and that you may want it for that reason alone. What it does not support is the promise of better air. The parts that you breathe past every day are the registers, the return grilles and the first stretch of duct behind them, and those are within reach of a household vacuum.

What a homeowner can clean, and what belongs to a heating technician

Everything in the cleaning list above is safe to do yourself or to have a cleaner do. The inside of the furnace, the burners, the venting and the chimney are a different job. Ontario's carbon monoxide page asks for fuel-burning appliances, chimneys and vents to be cleaned and inspected by professionals every year before cold weather sets in, and Enbridge Gas asks for gas appliances to be checked every year by a licensed professional. If that inspection has not been booked yet, mid-October is the time to do it.

One more check belongs outside. Ontario asks that the outside vents for the dryer, furnace, stove and fireplace stay clear of snow and debris. In autumn that means leaves and the lint around the dryer hood; in winter it means the drift that builds against the side of the house. While you are along that wall, check the draughts around the windows too: our guide to caulking and weatherstripping before the heat goes on explains how to find them.

If you would rather hand over the registers, baseboards and trim, the deep clean on our home cleaning page covers baseboards, edges and detailed trim work, with extra attention on vents, corners and the places where dust builds up. The filter, the furnace and the alarms stay with you and your heating contractor.