ERV/HRV Balance Calculator
Supply/exhaust airflow balance for a heat/energy recovery ventilator.
Enter your values
Enter the measured supply airflow in CFM.
Enter the measured exhaust airflow in CFM.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Imbalance
- 10%
- Net pressure tendency
- Slightly negative
- Within 10% tolerance?
- Yes
- Average airflow
- 100 CFM
Quick answer
Balanced ventilation systems should have supply and exhaust CFM within about 10% of each other to avoid pressurizing the home.
Imbalance % = |supply - exhaust| / average x 100
At a glance
| What it does | Supply/exhaust airflow balance for a heat/energy recovery ventilator. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Measured supply airflow, Measured exhaust airflow |
| Main output | Imbalance |
| Formula | Imbalance % = |supply - exhaust| / average x 100 |
| Cost | Free — no sign-up, no download |
How to use the ERV/HRV Balance Calculator
- 1Measure supply and exhaust airflow with a balometer or flow hood at each register.
- 2Total each side and compute the percentage imbalance.
- 3Adjust dampers or fan speeds until within about 10%.
Inputs explained
- Measured supply airflow(CFM)
- Enter the measured supply airflow in CFM.
- Measured exhaust airflow(CFM)
- Enter the measured exhaust airflow in CFM.
Worked example
Using the values the calculator loads with:
Inputs
- Measured supply airflow95 CFM
- Measured exhaust airflow105 CFM
Results
- Imbalance10%
- Net pressure tendencySlightly negative
- Within 10% tolerance?Yes
- Average airflow100 CFM
Frequently asked questions
Is some imbalance ever intentional?
Yes — some codes call for a small negative bias to reduce moisture intrusion into wall cavities.
What causes drift over time?
Dirty filters or clogged cores slow one airstream more than the other, so recheck balance annually.
What do I need to enter into the ERV/HRV Balance Calculator?
Just 2 values: measured supply airflow, measured exhaust airflow. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the ERV/HRV Balance Calculator work out the answer?
Balanced ventilation systems should have supply and exhaust CFM within about 10% of each other to avoid pressurizing the home. It applies the formula Imbalance % = |supply - exhaust| / average x 100 and shows the working so you can check each step by hand.
Is the ERV/HRV Balance Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.