In a conventionally built house with average airtightness, natural air leakage through the building envelope provides some fraction of the fresh air that occupants need. This leakage is uncontrolled — it is driven by wind, stack effect, and mechanical exhaust — and it carries whatever quality of outdoor air is available at the point of infiltration. It also carries heat out of the building in winter at a rate determined by the outdoor temperature rather than by occupant need.
A passive house or similarly airtight building eliminates most of this leakage deliberately. The envelope is sealed to 0.6 ACH50 or less, which means natural air changes drop to negligible levels. Fresh air supply then depends entirely on a mechanical system — and the choice of that system determines both air quality and the energy penalty for delivering it.
How heat recovery works
A heat recovery ventilator (HRV) is fundamentally a counterflow heat exchanger with two balanced airstreams. One stream pulls stale air out of the house — typically from bathrooms, kitchen, and utility spaces — and exhausts it outdoors. The other stream pulls fresh outdoor air in and distributes it to living spaces and bedrooms. The two streams pass through the heat exchanger core in opposing directions without mixing.
As the warm exhaust air passes through the core, it transfers heat to the incoming cold fresh air. The exhaust air leaves the building at a lower temperature than the room temperature; the incoming fresh air enters the living space at a higher temperature than the outdoor ambient. The thermal efficiency of this exchange — called sensible heat recovery efficiency — typically ranges from 75% to 93% in residential units under standard test conditions.
No fuel is consumed in this process. The energy recovered is heat that was already in the building, captured at the point of exhaust and returned to the supply air stream. The electrical energy input to run the fans is the only operating cost, and modern units achieve this at 0.2 to 0.4 Watts per cubic foot per minute (W/CFM) or better.
HRV vs ERV: the moisture distinction
An energy recovery ventilator (ERV) adds moisture transfer to the heat exchange process. The core material in an ERV is hygroscopic — it absorbs moisture from the more humid airstream and releases it into the drier one. In summer, this means outdoor humidity is partially removed before entering the building. In winter, indoor humidity is retained rather than exhausted with the outgoing air.
The choice between HRV and ERV for Canadian climate zones is not universal. The prevailing guidance from organizations including PHIUS and Natural Resources Canada suggests HRVs for most Canadian climates, particularly in Climate Zones 6 and colder, for the following reasons:
- In a well-constructed passive house, internal moisture gains from occupants, cooking, and bathing are already managed by the ventilation rate. Retaining additional moisture in winter can push relative humidity above recommended levels, increasing the risk of condensation on cold surfaces and window frames.
- At very low outdoor temperatures, ERV cores can transfer moisture in directions that are counterproductive — a condition that requires careful commissioning to manage.
- HRV cores are generally easier to clean and maintain, particularly the polypropylene-core units common in the Canadian market.
ERVs are better suited to mixed-humidity climates or buildings with lower airtightness where moisture retention in winter is a comfort priority. In humid continental climates such as southern Ontario, ERV performance in summer can meaningfully reduce cooling loads by pre-treating incoming air.
Cold-weather defrost cycles
At outdoor temperatures below approximately -15°C to -20°C, the exhaust airstream can drop below the freezing point of water before it exits the HRV core. Moisture from the exhaust air condenses and then freezes on the core, gradually blocking airflow. All HRVs intended for Canadian cold-climate use must include a defrost mechanism.
Common approaches include:
- Recirculation defrost: The unit periodically bypasses the heat exchanger and circulates warm indoor air through the exhaust side to melt accumulated frost. During this cycle, fresh air supply is interrupted briefly — typically 5 to 10 minutes per cycle. The frequency depends on outdoor temperature and indoor humidity.
- Preheating coil: A small electric resistance coil pre-warms the incoming air before it enters the heat exchanger core, preventing the exhaust airstream from reaching freezing temperatures. This approach maintains continuous fresh air supply but consumes a small amount of electrical energy during operation.
- Earth tube or ground-coupled pre-conditioning: Outdoor air is drawn through a buried tube before entering the HRV. Ground temperatures at typical burial depths in Canada remain above 4°C year-round, providing pre-warming without active energy input. The capital cost of installation is higher, but operating costs are zero.
The Canadian Standards Association tests HRV performance at -25°C in its standard (CSA C444-19), which is the relevant reference for evaluating cold-climate suitability. Units carrying this certification have been verified to maintain performance or enter controlled defrost cycles at that temperature rather than blocking completely.
Sizing and airflow rates
ASHRAE 62.2-2022 and the National Building Code of Canada both provide ventilation rate calculations for residential buildings. The PHIUS 2021 standard uses 0.03 CFM/ft² of occupiable floor area plus 7.5 CFM per occupant as a baseline, with adjustments for actual occupancy and building airtightness.
In a typical 2,000 ft² Canadian passive house with four occupants, this produces a continuous ventilation rate in the range of 60 to 90 CFM. Most residential HRV units are sized from 50 to 200 CFM, so a mid-range unit in the 80–100 CFM class covers most single-family applications without oversizing.
Oversizing an HRV unit is a common error. A unit running at 30–40% of its rated capacity to meet actual flow requirements will operate at reduced heat recovery efficiency, higher unit-level energy consumption per CFM delivered, and increased noise. Selecting an appropriately sized unit is more important than purchasing a large unit and throttling it down.
Duct design and distribution
HRV duct systems differ from forced-air heating duct systems in important ways. HRV ducts carry relatively small volumes of air at low static pressure — typically 0.1 to 0.3 inches of water column. Short, straight duct runs with minimal fittings maintain this pressure budget. Long runs with multiple elbows can exceed the available static pressure of the fan and reduce delivered airflow below design levels.
Supply air from the HRV is typically distributed to bedrooms and living areas through short, insulated runs from a central distribution point. Exhaust is drawn from bathrooms and the kitchen through dedicated exhaust branches. In some designs, the HRV distribution system connects to the home's central air handler, which then distributes tempered fresh air through the existing duct network — a configuration called an integrated or air-handler-integrated HRV. This approach reduces dedicated duct runs but requires careful commissioning to ensure the HRV operates independently of the air handler fan.
Commissioning and maintenance
An HRV system that has not been commissioned — that is, one where airflow rates have not been measured and adjusted at each supply and exhaust branch — may deliver significantly different airflow to different rooms than the design intends. Commissioning is a brief process that uses a flow hood or rotating vane anemometer to verify delivered flow at each register and adjust dampers to match the design rates.
Maintenance is straightforward: filters require cleaning or replacement every 3 to 6 months depending on outdoor air quality, and the core should be inspected and cleaned annually. Polypropylene cores can be washed with warm water. Aluminium cores require care around cleaning solutions. Most manufacturers provide detailed maintenance instructions in the product documentation.