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What size HRV do I need? (correction)

Jack Nimble | Posted in General Questions on

Hi everyone, I am looking to put an HRV into my house in the Toronto, Canada area.

New (5 yrs) house with pretty tight construction. 3200 sq ft on main and upper levels, 1400 sq ft basement, average ceiling height 9 ft.

My calculation (from the internet) says I need 217 CFM. Can someone clarify/ validate?


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  1. Stephen Sheehy | | #1

    Jack-My HRV supplier (Zehnder) did the design for us and specified 72 cfm for a house about half your size. You might solicit a design from your HRV vendor and see what they come up with. Even if you plan to install it yourself, getting the vendor to design it makes it more likely you'll get all the necessary parts.

    I was surprised at how little air gets exchanged, but my architect said we were fine. As I recall, about .25 air changes per hour is the usual standard, but that may vary according to factors specific to the home and occupants.

  2. Reid Baldwin | | #2

    You may want to check out the article at

    There are different views on the correct ventilation rate. Among those views, the one that sets the rate highest is the ASHRAE formula 0.03*area+7.5*(bedrooms+1). If your home has four bedrooms, that yields 176 cfm, counting the basement in the area. Other methods would come up with lower numbers.

  3. GBA Editor
    Martin Holladay | | #3

    Here's a link to another article you might want to read: Designing a Good Ventilation System.

    The ASHRAE formula works OK for houses in the typical range (2 or 3 bedroom houses measuring 1,600 to 2,400 square feet). Even then, many critics think that the ASHRAE ventilation rates are somewhat high.

    Once you get to a house as big as yours (4,600 square feet), you have to put on your thinking cap and experiment a little. If you have a 5-bedroom house and a family of 12, then you might need to ventilate at a high rate. If you have only 3 or 4 people in your family, then you probably don't need much ventilation.

    My own advice is to ventilate as little as possible. What you don't want is condensation on your window glass, or annoying odors. If you have licked those problems, your ventilation rate is probably good.

    Remember, ventilating at a high rate always incurs an energy penalty.

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