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HRV/Air exchanger connections with forced air duct system and interlock

user-5566269 | Posted in GBA Pro Help on

Hello, I recently bought a townhouse (2 stories + basement). There is a furnace and a heat pump (HVAC system) and there are ducts everywhere except the basement.

I want to improve ventilation using an HRV. I had several ventilation specialists come in and they all seem to think that putting in separate ductwork is not a consideration in my house. They all seem to suggest attaching the HRV unit to the furnace. I am ok with this idea except I am getting conflicting information that I was hoping you could help me with:
1 – Some say that they will exhaust stale air from the RETURN of the forced air duct system and supply the fresh air through the SUPPLY of the forced air duct system. is this ok? If not, why is it bad to supply fresh air to the supply of the forced air duct system (as opposed to supplying the fresh air to the return of the forced air duct system, as suggested by some other ventilation specialists).
2 – My furnace has an ECM motor… but I still hear the fan well enough through my 2 return ducts on floors 1 and 2. I dont want the furnace fan to work 24h a day so I can hear it constantly!! My main concern is basement ventilation in the first place… so I was wondering… Is it acceptable to have the HRV work say 20 min per hour (with the furnace fan) but then completely turn OFF and I can have quiet in the house for the next 40 minutes? Or does the HRV need to absolutely work 24/7 constantly?
3 – One contractor suggested that an interlock is NOT absolutely necessary between HRV fans and furnace fan. He says if he supplies to supply of furnace and exhausts from return of furnace then when the fan of the furnace is OFF, the fresh air will passively go through the ducts anyways… not as efficiently but that it will still get to desired locations. He says the new Fantech models are powerful enough with 175cfm or so, for this kind of design. And this way I can have a my furnace go on and off periodically as it is working now, without any HRV in the house – which is how i prefer it. Does this approach make sense?
4 – Between Lifebreath, Fantech or Minotair, is there one that you would suggest more for Montreal, QUEBEC weather?

Sorry in advance if this question has already been asked, but I couldnt find the answer to this question anywhere on the internet (especially the first question!). I really appreciate your input on this! Thank you in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to an article that discusses different ways to duct an HRV that is connected to a forced-air system: Ducting HRVs and ERVs.

    The article also discusses fan energy use, and the advantages and disadvantages of using a furnace fan or an air handler fan to distribute ventilation air.

    For more information on the Minotair, see Another North American Magic Box.

  2. user-5566269 | | #2

    Thank you for the interesting post on Minotair, it was very informative.

    I already read the article that discusses different ways to duct an HRV that is connected to a forced-air system. But it doesnt explain if there is a possibility to duct the supply from the HRV to the supply of the furnace and its advantages vs disadvantages (vs attaching the supply of the HRV to the return of furnace). Certain websites I found mention that this is possible and this way it will not require an interlock with the furnace fan. This way the air will dissipate through the house more passively through the supply grilles. A motorized damper will be installed on the supply HRV duct to prevent mixing of fresh and stale air that you mention in your post.

    Here is a link to what I was considering, without interlock:
    (part which is entitled: Multi-Point HRV or ERV with Full Connection to Central Air Handler)

    And if a Minotair would be installed, the risk of condensation in the supply duct would be greatly reduced as it recovers the heat much better than a classic HRV.

    Thank you in advance for your opinion.

  3. GBA Editor
    Martin Holladay | | #3

    I can't improve on the Building Science Corporation recommendations; these recommendations are based on experience and are reputable.

    The ducting system you describe -- the one the sends fresh air from the HRV to the supply duct of the forced air system, with a motorized damper installed between the HRV and the forced-air supply duct -- has disadvantages, as BSC notes: "Differing air flows through the HRV/ERV can occur due to pressures created by the central air handler in the trunks. The dueling pressures can significantly reduce ventilation effectiveness."

    Moreover, this type of installation requires an experienced ventilation contractor who knows how to set up the controls on the motorized damper.

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