📊 Worst-Case Imbalance
🌡 Pressure Tendency
Where the Imbalance Comes From
Full Calculation Table
| Source | Side | Rate | Airflow |
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How to Use the Balanced Ventilation Calculator
Choose HRV, ERV, or a simple balanced fan pair. This only changes the recommendation text since all three are balanced by design.
Enter the continuous airflow your ventilation unit moves on both the supply and exhaust side. This is the rate from the unit's rating or commissioning report.
Add bathroom fans, the range hood, and the clothes dryer. These almost never have a matched supply, so they are the usual source of imbalance.
Click Check Balance to see the worst-case imbalance, whether the house tends to pressurize or depressurize, and whether dedicated makeup air is worth adding.
Why a Balanced System Can Still Run Unbalanced
An HRV or ERV is balanced by design. The core moves the same airflow in and out, so on its own it does not change the building's pressure relative to outside. The trouble is that almost no Canadian home runs the HRV alone. Bathroom fans, the kitchen range hood, and the dryer all exhaust air to the outside without any matched supply, and none of them are usually tied into the balanced core.
Worst-Case Stacking
This calculator looks at the worst case where the core is running at its continuous rate and the bathroom fans, range hood, and dryer are all running at the same time. That is not the average condition, but it is the condition that matters for combustion safety. Natural Resources Canada's guidance on combustion spillage warns against running several powerful exhaust devices at once, because a furnace or water heater can backdraft during a short, high-demand moment just as easily as during a sustained one. A high-capacity range hood, especially anything above roughly 190 L/s (400 CFM), is the most common single cause of a serious imbalance in a modern Canadian home.
The arithmetic is simple because the core cancels out. With the Load Example inputs, the core moves 45 L/s each way, the two bathroom fans add 50 L/s, the hood 140 L/s, and the dryer 47 L/s. Total exhaust is 282 L/s against 45 L/s of supply, a 237 L/s imbalance, or 84% of exhaust. A 140 L/s makeup air damper interlocked with the hood cuts that to 97 L/s (34.4%), since the fans and dryer still exhaust without a matched supply.
Pressurizing vs. Depressurizing
When total exhaust exceeds total supply, the house depressurizes. In a Canadian winter, that pulls cold air in through every crack and gap, raises heating costs, and can pull combustion gases back down a chimney or flue instead of letting them vent. The infiltration load calculator shows what that extra leakage air costs in heating load. When supply exceeds exhaust, the house pressurizes, which pushes warm, moist indoor air into wall and ceiling assemblies, where it can condense against cold sheathing. Neither direction is free of risk, which is why balanced systems target close to zero imbalance.
When Makeup Air Helps
If this calculator shows a meaningful imbalance, a dedicated makeup air device, often a motorized damper that opens when the range hood or a high-capacity exhaust fan runs, can bring the worst case back toward neutral. Use the makeup air calculator to size that device, or the transfer air calculator if the better fix is moving air between rooms instead of bringing in more outside air.
Canadian Code Context
The National Building Code and CSA F326, the standard most provinces reference for residential mechanical ventilation, deal with depressurization from large exhaust devices. CSA F326 has used a limit of 5 Pa with the ventilation system at full capacity and a clothes dryer plus any exhaust device of 75 L/s or more running, and the code's makeup air provision targets dwellings with fuel-fired appliances that are not direct-vented or mechanically vented, or with soil gas concerns. Codes are published by the National Research Council but only become law once a province or territory adopts them, so check your local authority having jurisdiction and the current edition if this calculator flags a significant imbalance. For the underlying continuous ventilation rate itself, see the whole-house ventilation calculator.
HRV vs. ERV for This Calculation
The balance math is identical for an HRV and an ERV since both move matched supply and exhaust air through the core. The difference is moisture: an ERV transfers some humidity between the airstreams, which matters for comfort and condensation risk but does not change the pressure balance calculated here. Check the resulting indoor humidity against the humidity comfort calculator, and size either core with the HRV calculator or ERV calculator.
Frequently Asked Questions
A balanced ventilation system supplies and exhausts air at matched rates, usually through an HRV or ERV core, so it does not pressurize or depressurize the building on its own. This is different from exhaust-only systems, which remove air without a matched supply, and supply-only systems, which add air without a matched exhaust.
Yes. Bathroom fans, range hoods, and dryer exhaust are almost always separate from the HRV or ERV core, and they only exhaust air without a matched supply. When several of them run at once, the house can depressurize even though the main ventilation system is balanced by design. This calculator adds those intermittent loads to find the worst-case imbalance. Try the bathroom fan calculator if you still need to size those fans.
Canadian codes limit pressure, not a percentage. CSA F326 has used a depressurization limit of 5 Pa with the ventilation system at full capacity and a clothes dryer plus any other exhaust device of 75 L/s or more running, so check the edition your project follows. This calculator cannot measure pressure, so it screens with percentage bands instead: under 10% of total exhaust reads as well balanced, 10 to 20% is worth addressing, and above 20% is a significant imbalance. The bigger concern is depressurization strong enough to backdraft a furnace, water heater, or fireplace, which is a combustion safety issue, not just a comfort one.
It depends on the exhaust devices and on what burns fuel in the home. The National Building Code's makeup air provision for large exhaust devices is aimed at dwellings with fuel-fired appliances that are not direct-vented or mechanically vented, or with soil gas concerns, and a high-capacity range hood is the usual trigger. Provinces may add their own rules, so confirm with your authority having jurisdiction. Use the makeup air calculator to size a dedicated supply if this calculator shows a significant imbalance.
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