Heating, Ventilation and Indoor Air
The furnace works harder here than in most of Canada, and the air inside gets very dry. This page covers furnace filters and servicing, outside vents, the heat recovery ventilator (HRV), humidifiers, window condensation, the air conditioner, chimneys and fireplaces, and testing your home for radon.
On this page
Furnace filters
The filter is the cheapest, easiest part of looking after a forced-air furnace, and the one most often forgotten. It catches dust before it reaches the blower and the ducts.
- How often: Canada Mortgage and Housing Corporation’s (CMHC) home maintenance schedule says to check and clean or replace furnace filters monthly during the heating season. In Lethbridge, that’s a long stretch of the year.
- Write the date on the edge of each new filter, so you know how long it’s been in.
- Arrow the right way. Most disposable filters have an arrow on the frame showing which way the air should flow. Match the old filter, or check the furnace manual.
MERV basics
Filters are sold with an efficiency rating, often a MERV number printed on the frame or package. The right one depends on your furnace. Check the furnace manual, or ask the technician at your next service which size and type your system is designed to take, and stick with that.
What it means for you: buy a season’s worth of the right size at once, and keep them beside the furnace with a marker. If bending down to the furnace is hard, ask for the filter to be changed at the fall service, and have a family member take over the monthly checks.
Servicing the furnace
Two federal agencies give slightly different advice, and both are worth following:
- Inspection, every year. Health Canada says to have a professional inspect your furnace, chimney and vents at least once a year, or as the manufacturer recommends, to help prevent carbon monoxide problems.
- Full service, on schedule. Natural Resources Canada (NRCan) says natural gas and propane furnaces should be serviced at least every two years, and oil furnaces cleaned and tuned every year. CMHC’s schedule agrees, or says to follow the manufacturer.
Fall is the time for it. CMHC’s fall list also includes checking the furnace fan belt and blades, vacuuming electric baseboard heaters and the forced-air ducts you can reach, and running the furnace to test it before the first cold night.
Gas work is for licensed gasfitters. The City of Lethbridge says installing a new furnace, boiler or water heater, or changing gas piping, needs a gas permit, and only licensed gasfitters can get one. Homeowners can’t install or modify gas lines. Your part is the filter, keeping the area around the furnace clear, and booking the service.
ATCO’s safety guide adds a check you can do yourself: look behind the dryer and stove for a gas line left uncapped after an appliance was removed. If you find one, have a qualified gasfitter or heating contractor seal it.
Outside vents and snow
High-efficiency furnaces, and many water heaters, vent through plastic pipes in a side wall, often close to the ground. One pipe brings in combustion air and the other carries exhaust out. If snow or ice blocks either one, the furnace may shut down, and a blocked exhaust is a carbon monoxide risk.
- Health Canada says to inspect exhaust vents during and after snowstorms, to make sure they aren’t blocked by ice, snow or debris.
- CMHC says to keep snow clear of gas meters, appliance vents, exhaust vents and basement windows, and to watch them for ice building up.
- If the furnace stops after a storm, check the outside pipes before you call for service. Clear them by hand or with a broom, not a metal shovel. If a CO alarm is sounding or anyone feels unwell, don’t check anything: get everyone outside and call 911.
Our Winterizing your home page has the full winter routine for the furnace and its vents, and the carbon monoxide warning signs are on Power outages.
Looking after the HRV
A heat recovery ventilator (HRV) brings fresh air in and sends stale, moist air out, while recovering much of the heat. It’s usually a box near the furnace with ducts to the outside. CMHC’s maintenance guide for HRVs gives these steps:
- Filters every two months. CMHC says to clean or replace the HRV’s air filters every two months.
- Outside hoods. Clear the outside intake and exhaust hoods of leaves, nests and debris. In winter, clear away snow or frost blocking them.
- Condensate drain. If the unit has a drain, pour about 2 litres of warm water into each drain pan to make sure it flows freely.
- The core. Following the owner’s manual, take out the heat exchange core, vacuum it and wash it with soap and water.
- Grilles and ducts, once a year. Inspect them, and vacuum inside the duct openings you can reach.
- Fans. Gently brush the dust off the fan blades.
CMHC’s general schedule adds a fall check of the HRV’s ducts and seals. If you turned the HRV off for the summer, clean the filters and core and test the drain before you turn it back on.
What it means for you: unplug the HRV before you open it. If yours hasn’t been cleaned in years, or the manual is long gone, have it done at the same visit as the furnace service and ask the technician to show you how.
Humidity, humidifiers and window condensation
Winter air in Lethbridge is dry, and it’s tempting to add moisture. But the inside of a window on a −25°C night is very cold, and humid air condenses on it, runs down onto the sill and can lead to mould.
How humid is right?
- Health Canada suggests keeping indoor humidity at around 30% in winter and around 50% in summer, to prevent mould growth.
- CMHC says humidity should not go above 45% in the heating season, and that in very cold weather, 30% or lower may be needed to prevent condensation on windows.
An inexpensive hygrometer (an indoor humidity gauge) takes the guessing out of it. Put it in a living area, away from the kitchen and bathroom.
Window condensation
A little condensation at the bottom of the glass on the coldest nights is normal. A lot, every day, means there’s too much moisture in the air. CMHC suggests:
- Wipe it up when it forms, so water doesn’t soak into the sill.
- Open drapes and blinds so warm room air can reach the glass.
- Run the bathroom fan during showers and baths, and for 15 minutes or longer afterwards. Make sure it vents outside.
- Use the kitchen fan when cooking, especially on a gas stove.
- Don’t humidify unless you really need to, and then only for short periods.
CMHC’s winter checklist includes watching for excess moisture and condensation on windows. If mould does appear, CMHC says small areas (fewer than three patches, each under 1 square metre) can be cleaned with unscented detergent and water. It doesn’t recommend bleach. Larger areas, or mould that keeps coming back, need a professional.
Caring for humidifiers
Dirty humidifiers can put bacteria and mould into the air you breathe. Health Canada’s advice for cool-mist humidifiers:
- Empty the water reservoir whenever you turn the humidifier off, and refill it only when you’re about to use it again.
- Clean and disinfect the reservoir regularly, following the manufacturer’s instructions.
- Consider distilled water. Health Canada says it releases far fewer particles into the air than tap water. The fine white dust that settles near a humidifier is usually minerals from tap water.
CMHC suggests cleaning humidifiers two or three times over the winter. If your furnace has a built-in humidifier, CMHC’s spring list says to shut it down, drain and clean it, and close its damper if you have central air.
Air always holds some invisible water, called water vapour. Warm air can hold more than cold air. When warm, damp air touches something cold, like a window in winter, the vapour turns back into tiny drops of water. That’s condensation.
Wonder: why do windows fog up more in the kitchen and bathroom than in the bedroom?
Try this: take a glass of ice water out of the fridge and set it on the counter. Watch the outside of the glass for a few minutes. Where did those drops come from? (Hint: not from inside the glass.) Then, on a cold morning, check which window in your home has the most condensation and tell a grown-up.
The air conditioner
Central air gets a rest for most of the year here, so it needs a little attention at each end of the summer. CMHC’s schedule gives this routine:
| When | What to do |
|---|---|
| Spring start-up | Switch the power to the outdoor unit back on. Clean or replace the air-conditioning filter. Have the system serviced every two or three years. |
| Summer | Clean or replace the air-conditioning and ventilation filters. |
| Fall shut-down | Check that the drain pan under the indoor coil drains properly and is clean. Shut off the power to the outdoor unit and cover it. |
Any work inside the unit, or on the refrigerant, is a job for a qualified technician.
Spring frosts can carry on into late May in Lethbridge. Watch the forecast before you uncover the outdoor unit, and leave the switch off until the hot weather is actually coming.
Chimneys and fireplaces
- Before the first fire: CMHC says to check the chimney for obstructions, such as birds’ nests, before you turn on the furnace or light the fireplace in the fall.
- Wood-burning fireplaces and stoves: Health Canada says to have the chimney cleaned every year. CMHC’s spring list includes having the fireplace, wood stove and chimney cleaned and serviced as needed.
- Gas fireplaces: include them in the annual professional inspection Health Canada recommends for all fuel-burning appliances. CMHC suggests turning off gas fireplace pilot lights over the summer where possible.
- Chimney cap: CMHC’s summer list includes checking the chimney cap and its caulking, from the ground.
Leave the chimney to a pro. Sweeping a chimney means working on the roof, and inspecting one properly needs training and equipment. Hire a qualified chimney professional. Installing a gas fireplace needs a gas permit from the City of Lethbridge, which only licensed gasfitters can get. Every home with a fireplace or any fuel-burning appliance needs working carbon monoxide alarms (see alarms).
Radon: test in the fall or winter
Health Canada describes radon as a radioactive gas that comes from uranium in the ground and can get into your home undetected. Health Canada says radon exposure is the number one cause of lung cancer in non-smokers, and that people who smoke and are exposed to radon have an even higher risk.
Is it a problem here?
Yes, it can be. In Health Canada’s Cross-Canada Survey of Radon Concentrations in Homes (final report, 2012), 9.0% of the 111 homes tested in the former Chinook Regional Health Authority, the southern Alberta health region that served Lethbridge at the time, were above the Canadian guideline, and 0.9% were above 600 Bq/m³. For Alberta as a whole, the figure was 6.6%. The survey reports by health region, not by town, and it tested only a sample of homes. The only way to know your own home’s level is to test it.
How to test
- Use a long-term test. Health Canada says to do a long-term test for at least 3 months (it doesn’t recommend tests shorter than 91 days).
- Test in the heating season. Health Canada says to test ideally during the fall or winter, with at least 91 days of the test in the heating season, because radon levels are usually higher in the colder months.
- Put it on the lowest lived-in level of your home, such as a finished basement where someone spends time.
- Do it yourself or hire it out. You can buy a do-it-yourself test kit, or hire a measurement professional certified under the Canadian National Radon Proficiency Program (C-NRPP).
What the result means
The Canadian guideline for radon is 200 becquerels per cubic metre (200 Bq/m³). Health Canada says to fix it within a year if your home is above 200 Bq/m³, and sooner if the level is higher. It recommends hiring a C-NRPP-certified radon mitigation professional, and says radon levels can be reduced by more than 80%, for about the same cost as other common home repairs. Health Canada says the most common method is called active soil depressurization.
What it means for you: start a long-term test kit in October or November and leave it in place for at least 3 months (91 days), as Health Canada recommends. A kit started in late November should stay out until at least the end of February. Health Canada says to place it on the lowest lived-in level, at least 30 cm above the floor and at least 1 m from vents, windows and doors. It’s one of the most worthwhile things you can do for your family’s health, and it’s easy.
The year at a glance
| Season | Heating and air jobs |
|---|---|
| Fall | Furnace serviced (gas every 2 years at least, oil yearly) and inspected; chimney checked for obstructions; HRV cleaned and restarted; portable humidifiers cleaned; air conditioner power off and covered; radon test started. |
| Winter | Furnace filter monthly; HRV filters every two months; outside vents cleared after every snowfall; humidity watched (about 30%); humidifiers cleaned two or three times. |
| Spring | Furnace humidifier drained and cleaned; wood-burning chimney cleaned; air conditioner power on and filter cleaned; gas fireplace pilot off where possible. |
| Summer | Air-conditioning and ventilation filters cleaned; basement humidity below 60%; chimney cap checked from the ground. |
Back to the printable year-round checklist.
Sources
- Canada Mortgage and Housing Corporation, About Your House: Home Maintenance Schedule (filters, furnace, HRV, humidifiers, air conditioner, chimney): assets.cmhc-schl.gc.ca
- Canada Mortgage and Housing Corporation, How to maintain your heat recovery ventilator (2018): cmhc-schl.gc.ca
- Canada Mortgage and Housing Corporation, Moisture and Air: A Guide for Understanding and Fixing Interior Moisture Problems in Housing (45% maximum; 30% in very cold weather; fans; mould cleanup), copy hosted by the Intact Centre on Climate Adaptation: intactcentreclimateadaptation.ca
- Health Canada, Cool-mist humidifiers (30% in winter; empty and clean; distilled water): canada.ca
- Health Canada, Carbon monoxide: preventing exposure (annual inspection; vents after snowstorms; wood-burning chimneys cleaned yearly): canada.ca
- Natural Resources Canada, ENERGY STAR new homes, Maintenance (furnace servicing intervals): natural-resources.canada.ca
- City of Lethbridge, Gas permits (furnaces, fireplaces; licensed gasfitters only): lethbridge.ca
- ATCO, Natural Gas 101 (uncapped gas lines behind appliances): gas.atco.com
- Health Canada, Radon: what you need to know (fact sheet, 2025): canada.ca
- Health Canada, Guide for Radon Measurements in Residential Dwellings (Homes) (91 days; heating season; lowest lived-in level; mitigate within a year; modified 2026-05-22): canada.ca
- Health Canada, Cross-Canada Survey of Radon Concentrations in Homes, Final Report (2012) (Chinook Regional Health Authority: 9.0% above 200 Bq/m³; 0.9% above 600): canada.ca