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How best to balance an HRV's intake and Exhaust?

I got my ecoEnergy Retrofit final energy evaluation back, it's a Canadian retrofit program. The house scored an 84 out of a 100. The evaluation report doesn't actualy provide an meaningful information, just the score which is used to calculate your rebate. Most of the marks were for the geoexchange system I put in!

I called up the evaluator and was able to get some useful information from him. The ACH50 was 1.85, the ACHnatural was 0.080 and the equivalent leakage was 72 in^2. It's not a passive house, two additions and a retrofit of the existing house.

And finally he told me that the HRV needs to be set at 82.8 cfm.

Unfortunately I wasn't home when the HVAC guys came to balance the HRV, my wife was! And her description of it makes it sound as though the balancing did not involve any measurement of air flow, that said I'm not sure she was down in the mini-basement to see what they did.

What is the proper way to commission an HRV to ensure that intake and exhaust are balanced and that the flow is set to my houses requirements?

There are five fresh air vents in the living areas and bedrooms, and two exhaust vents for the bathrooms. Also the mini-basement which is completely separate from the house has at least a fresh air vent and possibly an exhaust, might this unbalance the basement air pressure?

Asked by Anonymous
Posted Nov 20, 2009 4:44 PM ET


7 Answers

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I'll leave it to others to explain the procedure, but I hope the HVAC guys got that 82.8 cfm, since surely 83 would not do ;-)

You're right, though, to be concerned about pressure imbalances in a very cold climate since any positive pressure would tend to drive air into the thermal envelope, in spite of its tightness, and create the potential for condensation.

What seems to be a closely-held secret is that a perfectly balanced system, in terms of CFM, creates an imbalanced condition when the cold outside air warms and expands, thus increasing its volume and causing an interior positive pressure.

For instance, 0° air warmed to 68° will expand by 13% and, in a 2,000 SF home will create 182 pounds of bouyancy on the ceiling and 9 pascals of positive pressure.

Which makes me wonder whether HRV commissioners compensate for this by increasing the exhaust flow relative to intake. I suspect not.

Answered by Robert Riversong
Posted Nov 20, 2009 8:21 PM ET


Basically, you adjust the airflows with dampers, and verify a balanced condition with a magnehelic pressure gauge.
Here is an example of an HRV manufacturer's balancing instructions (these are Venmar instructions from http://www.venmar.ca/AxisDocument.aspx?id=494&langue=en&download=true ):

• A magnehelic gauge capable of measuring 0 to 0.5 inch of water (0 to 125 Pa) and 2 plastic tubes.
• The balancing chart located on the unit door.

• Seal all the unit ductwork with tape.Close all windows and doors.
• Turn off all exhaust devices such as range hood, dryer and bathroom fans.
• Make sure the integrated balancing dampers are fully open.Turn the thumb screw (A) clockwise
to manually open the dampers.Both are located on the Exhaust air to outside port and on Fresh air to building port.
• Make sure all filters are clean (if it is not the first time you balance the unit).
1. Set the unit to high speed.
Make sure that the furnace/air handler blower is ON if the installation is in any way connected to the ductwork of the cold air return. If not, leave furnace/air handler blower OFF. If the outside temperature is below 0°C / 32°F, make sure the unit is not running in defrost while balancing.
(By waiting 10 minutes after plugging the unit in, you are assured that the unit is not in a defrost cycle.)
2. Place the magnehelic gauge on a level surface and adjust it to zero.
3. Connect tubing from gauge to EXHAUST air flow pressure taps (see diagram beside).
Be sure to connect the tubes to their appropriate high/low fittings.If the gauge drops below zero, reverse the tubing connections.
NOTE: It is suggested to start with the exhaust air flow reading because the exhaust has typically more restriction than the fresh air, especially in cases of fully ducted installations or source point ventilation. Place the magnehelic gauge upright and level.Record equivalent AIR FLOW of the reading according to the balancing chart.
4. Move tubing to FRESH air flow pressure taps (see diagram). Adjust the fresh air balancing damper until the fresh air flow is approximately the same as the EXHAUST air flow.If fresh air flow is less than exhaust air flow, then go back and adjust the exhaust balancing damper to equal the fresh air flow.
5. Secure both dampers thumb screw in place with tape.
6. Write the required air flow information on a label and stick it near the unit for future reference (date, maximum speed air flows, your name, phone number and business address).
NOTE:The unit is considered balanced even if there is a difference of ±10 cfm (or ±5 l/s or 17 m3/h) between the two air flows.

Exhaust air flow
To avoid balancing, the difference between stale air ducts total length and fresh air ducts total length must not exceed 50 ft. However, even if the stale air ducts and fresh air ducts lengths are almost equal, your local building codes may require balancing the unit.
If the unit does not need to be balanced, shut all the pressure taps (located on the unit door) with the small plastic plugs included in the hardware kit.

Answered by Martin Holladay
Posted Nov 21, 2009 7:03 AM ET


Hi Martin and Robert,

Thanks for the info. I 'm surprised at the note...

"NOTE:The unit is considered balanced even if there is a difference of ±10 cfm "

Does it matter if the house is slightly pressurized or slightly de-pressurized? I guess it doesn't matter and following from Robert's posting the stack effect will add some pressure to the house anyway.

I notice that my dryer vent at the moment leaks air, the flap is blown open a good quarter inch. That's not an efficient air leak!! Would a house with a properly balanced HRV have this kind of leakage?

Thanks again,


Answered by Andrew Henry
Posted Nov 22, 2009 9:54 PM ET


Did you sort this out? I purchased a Magnehelic gauge to balance my system as I purchased it wholesale and installed myself. If you are in my area I would gladly lend it too you.

Answered by chris
Posted Sep 14, 2010 8:30 PM ET


Hi Chris,

Not yet. Too many other things on my plate. I live north of Ottawa, Canada. How close are you! : )

I would love to borrow your gauge but I won't be able to do the balancing just yet as I have a bunch of other projects to get done first.

Thanks for offering. Very considerate of you.



Answered by Andrew Henry
Posted Sep 14, 2010 10:10 PM ET


Layman question: is the goal of balancing an HRV to make sure the CFMs in both directions are the same or that the supply CFM is more to make sure there's a slight positive pressure? TIA.

Answered by Venkat Y
Posted Jan 23, 2014 7:36 AM ET


Q. "Is the goal of balancing an HRV to make sure the CFMs in both directions are the same or that the supply CFM is more to make sure there's a slight positive pressure?"

A. When an HRV system is balanced, the exhaust airflow in cfm equals the supply airflow in cfm. The goal is to make sure that the ventilation system neither pressurizes nor depressurizes the house.

Answered by Martin Holladay
Posted Jan 23, 2014 8:14 AM ET

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