Why does my HRV seem to bring in cold air during Toronto winters?

If your HRV supply grilles are blowing noticeably cold air during Toronto winters, there is likely a specific, fixable cause — a properly functioning HRV should deliver incoming air that feels cool but not cold, even when outdoor temperatures drop to -15 or -20 degrees Celsius. This is one of the most common complaints from GTA homeowners, and the most frequent reaction — turning off the HRV entirely — is exactly the wrong solution because it eliminates your home's ventilation and leads to humidity buildup, window condensation, and poor indoor air quality.

The most common cause is dirty filters restricting airflow through the unit. When filters are clogged with dust, the HRV cannot move enough air through the heat exchanger core for efficient heat transfer. The incoming cold air passes through too slowly and does not pick up adequate heat from the outgoing warm air. Clean or replace your filters — if they have not been serviced in 6 months or more, this is very likely your problem. Filters should be cleaned every 3 months in a Toronto home, and more frequently if you have pets or live near a busy road.

A dirty heat exchanger core significantly reduces heat recovery. Dust, grease, and debris accumulate on the core plates over time, insulating them and blocking the heat transfer that is the entire purpose of the unit. Remove the core and wash it in warm soapy water, rinse thoroughly, and dry completely before reinstalling. If you have never cleaned the core in several years of operation, you may be shocked at how much buildup has accumulated and how much warmer the supply air becomes after cleaning.

Frosting and excessive defrost cycling cause cold air complaints during the coldest Toronto weather. When outdoor temperatures drop below -15 to -20 degrees, moisture in the warm exhaust air freezes on the cold incoming side of the core. The HRV's defrost cycle responds by temporarily shutting off the supply fan while keeping the exhaust fan running — warm exhaust air melts the frost. During this defrost period (typically 5-10 minutes), no fresh air enters the home, and when the supply fan restarts, the initial burst of air is very cold until the core warms back up. In extreme cold snaps, this cycle can repeat frequently, making it seem like the HRV is constantly blowing cold air.

Some HRV models have a recirculation defrost mode that is less noticeable — instead of shutting off the supply fan, they recirculate indoor air through the supply side while the exhaust thaws the core. If your unit uses the supply-off method and you find the cold blasts objectionable, upgrading to a unit with recirculation defrost may be worthwhile.

Blocked or partially blocked exterior hoods create airflow imbalances that reduce heat recovery. After Toronto ice storms or heavy snowfalls, check both the intake and exhaust hoods on your exterior wall. Ice buildup, snow, or debris blocking either hood disrupts the balanced airflow the HRV needs for efficient heat transfer. The intake hood is especially vulnerable to ice formation because cold outdoor air meets any residual moisture around the opening.

An aging or damaged heat exchanger core may simply no longer recover heat efficiently. If your HRV is 12-15 years old and you have cleaned the filters and core thoroughly but supply air is still uncomfortably cold, the core may need replacement ($200-$800 for the part) or the entire unit may be approaching end of life. A qualified HVAC technician can measure the actual heat recovery efficiency by comparing supply and exhaust air temperatures on both sides of the core. If efficiency has dropped below 60%, core or unit replacement is justified. Find local HVAC professionals through the Toronto Construction Network at torontoconstructionnetwork.com/directory?trade=hvac.