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Community and Q&A

Rig an HRV to act as a makeup air unit

Mark Helmrich | Posted in Mechanicals on

I’ve got some greentec HRV’s in my new house.

From what I understand, its possible to change fan speed on the intake/exhaust as to balance the system.

I’ve seen units like the Best by Broan MD6TU 6″ UNIVERSAL MAKE-UP AIR DAMPER what you can put a a sensor in your range hood duct or dryer duct, that if they are activated a flap opens to allow air to directly flow in thereby compensating for the air being removed.

It would be neat if it could be rigged into the HRV that if an appliance like a range hood or dryer is turned on it automatically increases just the intake air.

Does such a controller exist or is this not a good idea?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    In theory, it wouldn't be too hard to design an HRV with an intake fan with an adjustable speed, and a control that forced the unit into "unbalanced" mode.

    In practice, there are at least two problems.

    Whole-house ventilation requirements are generally in the 60 cfm to 120 cfm range. That's not much air. If someone has a big honking exhaust fan on their range hood that needs a makeup air system, the range hood fan is usually in the 400 cfm to 1,200 cfm range. The air flows are much higher than those of an HRV. That's problem number one.

    You could design an HRV that could ramp up to 1,000 cfm, I suppose. But what's the point? If the HRV is introducing 800 cfm of outdoor air to the house, and the 800 cfm is not balanced by exhaust air (because the exhaust air is going out the range hood), you don't end up with any heat recovery. So you might as well keep things simple and just install a makeup air fan. That way the design of the HRV can be optimized with its true purpose in mind: whole house ventilation with heat recovery.

  2. Hobbit _ | | #2

    The question about range-hood makeup air has come up a couple of times
    recently ... since it's sort of a special case in terms of volume and
    the pollutants in the exhaust stream, why hasn't anybody suggested
    simply having a makeup inlet *into* or very near the kitchen itself,
    and possibly relatively close to the range? Sort of like bringing
    combustion air from outside directly to where it gets used, without
    affecting the rest of the interior. Sure, it might make part of the
    kitchen a little cooler, but there's presumably already a lot of heat
    generation going on there so that would matter far less than, say,
    bringing makeup air into the HVAC system and having that cold outdoor
    air dump into other living spaces.

    Seems like a no-brainer. The point is to ventilate the stove area
    specifically; it should thus be handled as a special situation. Most
    residential kitchens have at least one exterior wall, and it wouldn't
    be hard to place both the range-hood outlet and its makeup inlet
    fairly near each other going through that wall and appropriately
    insulated/dampered/etc.

    _H*

  3. Mark Helmrich | | #3

    Good answers guys. Thanks :-)

  4. GBA Editor
    Martin Holladay | | #4

    Hobbit,
    Q. "Why hasn't anybody suggested simply having a makeup inlet *into* or very near the kitchen itself?"

    A. That suggestion has been made by Broan, a manufacturer of makeup air equipment, and I reported that suggestion in my 2010 article on the topic, "Makeup Air for Range Hoods." I wrote, "Broan sells motorized dampers for this purpose, and also provides detailed and useful installation instructions. Broan recommends installing an outdoor air duct connected to a grille mounted on the kitchen wall or connected directly to the return plenum of your furnace."

    Below is an image from the Broan installation instructions showing a wall-mounted grille to introduce makeup air.

    .

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