What Airflow Balance Means
An HRV is designed to move equal amounts of air in two directions simultaneously: fresh air from outside into the living space, and stale air from the living space to outside. When these two flows are equal, the unit is said to be balanced. The house maintains neutral pressure — it is neither positively pressurized (where air is being pushed out through gaps in the building envelope) nor negatively pressurized (where air is being drawn in).
In practice, exact balance is difficult to achieve and maintain over time. Filters load unevenly, dampers shift, and ducts develop partial obstructions. Most manufacturers specify an acceptable range: typically the exhaust flow should be within 5–10% of the supply flow. Outside that range, the imbalance creates measurable problems.
Consequences of an Imbalanced HRV in Canadian Conditions
Pressure imbalances have different effects depending on which side is dominant:
Excess Exhaust (Negative House Pressure)
When the HRV exhausts more than it supplies, the house develops negative pressure. Air is drawn in through any available gap: around window frames, through electrical outlets, through the gap at the sill plate. In Canadian winter conditions, this infiltrating air is extremely cold and can carry moisture with it. That moisture condenses inside wall cavities, leading to insulation degradation and mold over time.
Negative pressure also increases backdraft risk for combustion appliances. Gas furnaces, water heaters, and fireplaces all require draft to safely vent combustion gases. In a significantly depressurized house, that draft can reverse, drawing flue gases back into the living space.
Excess Supply (Positive House Pressure)
When the HRV supplies more air than it exhausts, the house becomes positively pressurized. Conditioned indoor air is pushed out through gaps in the envelope. In winter, this means warm, humid air exfiltrating through wall and ceiling assemblies, where it can condense inside the insulation layer and accumulate over months. This is a primary driver of interstitial condensation damage in Canadian homes.
Tools Needed
A basic flow measurement requires one of the following:
- Flow hood (capture hood): The professional standard. Placed over a grille to read volumetric flow directly. Accurate but expensive to purchase; renting from an HVAC equipment supplier is an option for one-time use.
- Anemometer with a shroud or bag: A less expensive approach. An anemometer measures air velocity; with a known cross-sectional area (from the grille size), velocity can be converted to volumetric flow. Accuracy is lower than a flow hood but sufficient for a balance check.
- Incense stick or tissue paper (qualitative): Not useful for measuring flow, but useful for confirming direction (supply vs. exhaust) at a grille and for identifying unexpected infiltration locations around the building envelope.
How to Measure HRV Airflow
The target is to measure flow at every HRV supply grille and every HRV exhaust grille in the house. The sum of supply flows and the sum of exhaust flows should be roughly equal.
- Run the HRV at its normal operating speed for at least 10 minutes before measuring to allow airflow to stabilize.
- Measure each supply grille individually. Record the reading. Note the grille location.
- Repeat for each exhaust grille.
- Total the supply side and the exhaust side separately.
- Calculate the imbalance: ((supply − exhaust) ÷ ((supply + exhaust) ÷ 2)) × 100%. A result within ±5% is generally considered acceptable.
| Imbalance Level | Practical Effect | Recommended Action |
|---|---|---|
| 0–5% difference | Negligible pressure differential | No action required |
| 5–10% difference | Minor pressure effect; may show in blower door test | Adjust dampers or check for partial blockage |
| 10–20% difference | Noticeable pressure; infiltration or exfiltration at envelope gaps | Damper adjustment and filter inspection; retest |
| >20% difference | Significant pressure imbalance; moisture and combustion risk | Professional inspection recommended |
How to Adjust Airflow Balance
Most residential HRV units have adjustable dampers or flow restrictors at the branch points inside the unit or at the grilles. The method varies by model:
At the Unit
Inside the HRV unit, fresh air and exhaust ports often have sliding or rotating dampers. Reducing the opening on the higher-flow side brings the two sides closer to balance. Make small adjustments (10–15% change in opening) and re-measure before making further changes. Damper positions can interact with each other in ways that make large single adjustments counterproductive.
At the Grilles
Some supply and exhaust grilles have built-in adjustable louvers or screw-adjusted baffles behind the face plate. Balancing at the grille level distributes flow between rooms within the same side (supply or exhaust), but does not change the total flow on either side of the unit.
Fan Speed Adjustment
Some HRV units have independent motor controls for the supply and exhaust fans. If the unit supports this, a small speed adjustment on the dominant side is a direct way to correct imbalance. Consult the unit's installation manual before adjusting motor settings, as the permissible range varies by model.
Seasonal Rebalancing
Filter replacement, duct cleaning, and changes in household occupancy or layout can all shift the balance point. A reasonable schedule is to check airflow balance once per year, ideally at the start of heating season, after filters have been cleaned or replaced. This ensures the system enters the high-demand winter months at its calibrated performance level.
In homes where a blower door test has been done recently (as part of an energy audit or new home certification), the test results can indicate whether the building envelope shows signs of pressure-driven air movement, which is indirect evidence of a chronically imbalanced HRV.