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Community and Q&A

Backdraft damper

ervrelay | Posted in Building Code Questions on

I am working on a retrofit installation of an ERV. It is going to be connected to the return side of the furnace for both the return air and the fresh air supply. The exhaust is on the same side of the house as the exhaust of the furnace, so I have concerns that a backdraft may cause the CO to be pulled into the house when the ERV unit isn’t running and the furnace blower unit is running. The exhaust vent has a backdraft damper built in, but it is not very airtight. The vent is Broan Nutone 641 ( The furnace exhaust is still more than 1 meter from the erv exhaust, so perhaps it is good as it is. Is there a need for an extra backdraft damper?

As a background, I was thinking I could add a motorized damper just in case, but I am running into difficulties with creating the relay circuit for both the damper and the furnace blower. There is a timer control with furnace interlock (Soler&Palau SFM timer control), but it will only allow me to turn on the furnace fan, no separate relay switch to open the damper. I had difficulty with finding a relay with two separate switches that would be activated by the timer control (Soler&Palau SPTL timer control).

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ervrelay.

    Hopefully another GBA member will be able to help you with the motorized backdraft damper install, if you decide to go that route. I wonder if you can monitor CO levels in an effective way to determine if this is actually something you need to be concerned about?

  2. Expert Member
    BILL WICHERS | | #2

    If you are OK modifying your furnace slightly, add a relay so that it's coil is activated by the power supply to the inducer blower (the one that blows combustion air). The inducer blower will come on (prepurge) before the unit begins combustion and turn off (post purge) after combustion since the unit is trying to make sure there are no residual fumes in the combustion chamber. This would also mean your ERV is always running before, and for a period of time after, there is combustion going on in your furnace. I think that's what you're trying to achieve here.

    You can get any required relay from Grainger. I like to use the octal base relays with separate sockets since it makes replacement very easy in case of a relay failure.


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