A modern, airtight home cannot rely on leaks for fresh air, so it needs a mechanical ventilation system, and in most cold-climate homes that system is a heat recovery ventilator (HRV). An HRV only protects indoor air if it runs continuously, stays balanced and gets its filters, core and drain cleaned on schedule. For housing managers, the biggest gains usually come from a simple service routine and from showing every household how the unit works before they move in.
This guide pulls together what the National Building Code, CMHC, Natural Resources Canada (NRCan) and Indigenous Services Canada (ISC) say about ventilation, HRVs, moisture and mould, and turns it into steps a housing department can use. It is general information, not engineering advice: the manual for the specific unit in each home always comes first, and a qualified contractor should handle repairs and balancing.
Why does an airtight home need mechanical ventilation?
Older houses got their winter air through gaps in the walls, windows and doors. The National Building Code's own explanatory note says houses were long built without mechanical ventilation and "relied on natural air leakage through the building envelope for winter ventilation," but that houses "have become progressively more airtight" through tighter sheathing, better air and vapour barriers and tighter windows and doors (National Building Code of Canada 2020, NRC). The same note says this raised concern that natural air change "might be insufficient in some instances to provide adequate indoor air quality," along with condensation problems from higher humidity.
NRCan's HRV guide puts it plainly: new homes are built to be airtight, and "to complement this airtightness and ensure a healthy living environment, modern homes use a mechanical ventilation system" that replaces stale indoor air with fresh outdoor air. It adds that "in most homes, the mechanical ventilation system is an HRV" (Heat Recovery Ventilators, Natural Resources Canada). In winter, the HRV takes heat from the outgoing air and uses it to warm the incoming fresh air, which is why it suits cold climates.
In NBC 2020, Part 9 (housing and small buildings) requires every residential occupancy supplied with electrical power and a heating system to have heating-season ventilation, and a self-contained system serving a single dwelling unit must comply with Subsection 9.32.3. That subsection calls for a principal ventilation system, supplemental exhaust fans and protection against depressurization. The principal fan's normal exhaust capacity is set by bedroom count, from 16 to 24 L/s for a one-bedroom unit up to 30 to 45 L/s for five bedrooms, and the code allows that requirement to be met "by the exhaust side of a heat recovery ventilator." The NBC is a model code, so confirm with your lands office or ISC regional office which code and edition apply to your project.
What should a housing manager know about how an HRV works?
Three points from the NRCan guide explain most of the problems housing staff see in the field.
- It is meant to run all the time. NRCan says an HRV "should be operated on low speed continuously year-round, especially in airtight homes," and that if the HRV is turned off, the household should make other provisions for ventilation, such as opening windows. High speed is for cooking, bathing, cleaning products, stale or humid air, or when "there are many people in the house."
- It must be balanced. HRVs are designed so the same amount of air comes in as goes out. Too much exhaust creates negative pressure, which NRCan warns can pull carbon monoxide and other combustion by-products back into the house from a furnace, water heater or fireplace. Too much supply creates positive pressure, which can push moist air into walls and roofs, where it condenses and damages materials, often unnoticed until the damage is severe.
- It has to defrost in cold weather. Most HRVs have an automatic defrost mode that starts when incoming air is below -5°C, because frost can build up on the core and block airflow. As winter gets colder, the defrost cycle gets longer.
NRCan also cautions that an HRV is not an air cleaner and should not be relied on to remove combustion by-products or to supply combustion air for fuel-burning appliances (NRCan). Imbalance is usually caused by ductwork differences, but NRCan lists a clogged filter, a blocked hood or a faulty damper or fan as other causes, which is why routine cleaning and balancing go together.
How often should filters, cores and drains be serviced?
Federal sources give slightly different intervals, so treat these as a starting range and follow the manufacturer's schedule where it is stricter. CMHC's Indigenous housing maintenance page says to clean or replace filters every 2 months, noting that on an accessible unit this "takes only 5 minutes" (How to maintain a heat recovery ventilator, CMHC). NRCan's maintenance chart says every 1 to 3 months; CMHC's guide for property managers and its occupants' manual on mould both use 3 months (CMHC property manager guide).
A practical schedule built from the NRCan maintenance chart, unless the manufacturer says otherwise:
- Every 1 to 3 months: clean or replace air filters; clear outside intake and exhaust hoods and screens (more often in fall leaves and winter snow and frost).
- Every 3 to 6 months: service and clean the fans.
- Every 6 months: inspect and clean the heat-exchange core; inspect and clean the condensate drain and pans.
- Seasonally: adjust the dehumidistat.
- Annually: clean grilles and inspect ductwork; check the defrost system; general servicing and airflow balancing by a qualified contractor.
For the condensate drain, CMHC suggests slowly pouring about 2 litres of warm, clean water into each drain pan and clearing the drain if it backs up. NRCan adds that the drain needs a trap (an S or loop in the tubing) and recommends checking the core during very cold weather to confirm defrost is working. Always turn the HRV off and unplug it first.
NRCan recommends annual servicing by a contractor accredited by the Heating, Refrigeration and Air Conditioning Institute of Canada (HRAI), ideally on the same visit as the furnace, and says the contractor should measure airflows, record them on the unit's maintenance label and rebalance if needed. It also warns against skipping repairs to save money: in airtight homes, "poor indoor air quality, reduced comfort and moisture problems may result" if the system is not properly operated and maintained. Post the maintenance chart on the unit so the household and staff can log each visit.
How can staff check airflow without special tools?
NRCan describes a rough field check using a large plastic trash bag taped to a bent coat hanger. Held over a supply register, the time the bag takes to inflate gives a rough airflow: about 13 seconds for 5 L/s, 8 seconds for 10 L/s, 5 seconds for 15 L/s and 3 seconds for 25 L/s. For a rough balance check, time how long a leaf-bag-size bag takes to fill at the outside exhaust hood, then how long it takes to empty at the intake hood. On a well-balanced system the two times "should be equal, to within a matter of seconds" (NRCan).
This is not a substitute for a contractor's measured balance, but it lets a housing inspector flag a unit that is clearly off. NRCan's troubleshooting table also helps sort calls: little or no airflow usually points to blocked hoods, dirty filters, a closed damper or a frozen core, while cold drafts can mean a blocked exhaust stream or a frozen core.
How do crowding and daily life affect moisture?
CMHC's guide for First Nations housing providers says indoor relative humidity should be between 30% and 50%. When it is below -10°C outside, indoor humidity should be about 30%, and "may have to be as low as 25% in extremely cold regions" to prevent condensation on windows (Mould in Housing: A Resource Guide for First Nations Housing Providers, CMHC). The guide recommends measuring relative humidity in every home inspected, using a hygrometer.
The same guide lists common causes of high indoor humidity:
- many people living in one home
- firewood stored indoors
- hanging clothes to dry indoors
- cooking
- lack of kitchen range hood and bathroom fans
- a large number of plants, water leaks and a damp foundation
Crowding matters here. CMHC's mould page notes that, according to the 2021 Census, people in First Nations communities are four times more likely than the non-Indigenous population to live in crowded housing (Mould in housing, CMHC). More people means more moisture from breathing, cooking, bathing and laundry. NRCan reminds homeowners that "bedrooms require more fresh air when occupied by more than one person." In practice, a home with more occupants than it was designed for may need its HRV run at a higher setting, more frequent filter changes and a contractor check of whether its minimum continuous rate is adequate. NRCan's troubleshooting guide lists that last point as a possible cause of poor air quality or excess moisture throughout the house.
What should occupants be shown at move-in?
An HRV that the household does not understand often gets switched off, especially if it seems noisy or drafty. CMHC's guide for property managers says tenants "might not understand how it works," that the unit "is intended to operate continuously," and that "shutting it off for a long time can lead to a buildup of pollutants and humidity" (CMHC). CMHC's housing providers' guide says to have the contractor or another knowledgeable person explain new heating or ventilating equipment, and to leave printed maintenance instructions for unfamiliar equipment, "such as heat recovery ventilators," with the occupants (CMHC).
A short move-in walkthrough can cover:
- Where the HRV and its controls are, and that the main control should stay on low continuous operation.
- When to use high speed or a bathroom booster: showers, cooking, cleaning and gatherings.
- Where the outside hoods are and why they must stay clear of snow, garbage and the barbecue. NRCan advises keeping contaminants away from the fresh-air intake.
- How to pull and wash the filters, and who to call for the core, drain and annual service.
- Everyday moisture habits from CMHC's occupants' manual: use bathroom fans and range hoods, store firewood outside, and keep the bathroom fan running for 30 minutes after a shower (Mould in Housing: An Information Kit for First Nations Communities, CMHC).
The CMHC guide also notes that a humidifier can cause excess moisture and should not be used where there are signs of dampness. Its advice for dry homes is to seal air leaks and "make sure you use your ventilation system for good indoor air quality."
When does a ventilation problem become a mould problem?
ISC lists condensation from "excessive humidity, lack of ventilation, or low temperature" as a common source of indoor moisture, along with steam from baths, showers and cooking (Mould and your health in First Nations communities, ISC). ISC says occupants can clean areas smaller than 1 square metre with unscented dish detergent and protective gear, after finding and fixing the moisture source. They should contact their housing manager or local Environmental Health Officer when there is a patch over 1 square metre, more than three smaller patches, mould that keeps coming back, or a mould problem they cannot solve.
For housing staff, CMHC's guide is clear that the moisture problem must be fixed before cleanup, because otherwise "the mould problems will likely come back." Among its general solutions is "supplying bathroom fans, an exterior vented kitchen range hood or a heat recovery ventilator to help control indoor humidity levels." When inspecting an HRV, the guide says to check hoods, filters, core, drain pans, controls and the last service date against the manufacturer's recommendations (CMHC). It also suggests checking that a bathroom fan pulls hard enough to hold a sheet of tissue paper to the grille. For repair funding, see Herauf's funding guide for First Nations and confirm details with CMHC or ISC.
Where does a builder fit in?
Ventilation performance starts with design and installation, then depends on operation and maintenance. The NBC requires ventilation equipment to be "accessible for inspection, maintenance, repair and cleaning," so ask any builder where the HRV will sit, how easy the filters and core will be to reach, and what manuals and maintenance labels come with each unit.
Herauf Modular builds homes and community buildings as finished sections indoors in its Saskatchewan and BC plants, then delivers and sets them on site. Every unit meets or exceeds CSA A277 and the National Building Code for climate zones 6 through 8. Most builds are manufactured and delivered in about 16 weeks, with foundations, pads and services prepared on site at the same time. For background on factory-built housing for First Nations, see modular Indigenous housing. To talk through a project, contact Herauf; Herauf replies to enquiries within one business day.
This guide is general information, not legal or financial advice. Program rules change: confirm the details with the program, your Indigenous Services Canada regional office or CMHC before relying on them. Herauf does not administer any program or apply on a community's behalf.