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Thread: Furnace has no fan relay, which makes it difficult to install HRV

  1. #1
    DIY Junior Member jessel's Avatar
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    Default Furnace has no fan relay, which makes it difficult to install HRV

    Since installing a used furnace, my Fantech HRV unit does not function properly. The HRV unit is now routed through the furnace ductwork, the furnace however does not have a fan relay, so it wasn't intended to be used with an air conditioner (or in this case an HRV unit).

    furnace model is: NUGG100EA02

    from the user manual it appear that the NUGG100EH01 model's wiring is quite similar except for a multi-speed blower and a fan relay.

    My question is whether or not it is acceptable practice to install the relay used in the NUGG100EH01.


    Thanks,

    Jesse

  2. #2
    DIYer, not in the trades LLigetfa's Avatar
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    I don't understand why the HRV would need to be interlocked with the furnace. Generally, a HRV runs independent of the furnace.

    I also don't understand what you mean about the HRV being routed through the furnace ductwork. I've seen where the HRV will dump into the furnace cold return duct and while the furnace fan can upset the balance of the HRV a little, it should not be enough to matter.

  3. #3
    DIY Junior Member jessel's Avatar
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    Here is what I am trying to do.

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    It seems like your're correct. I took another look at the furnace ductwork: the HRV fresh air to is connected to the return air line and the HRV exhaust air is connected to the hot air plenum. I think things will work without the furnace if I simply route the HRV exhaust line to what I had intended to be the bathroom exhaust...

    Thanks for the help. I'll let you know how it goes. I won't have time to do the work until this weekend.

  4. #4
    In the trades Dana's Avatar
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    An HRV should never be hooked up to the furnace output, but rather only the return. Even in the best of circumstances using the furnace ducts is not the ideal way to run an HRV, but it is often done that way for cost reasons. However, the extra power use of running the furnace's air handler to guarantee distribution can use more energy than the heat recovered in circumstances.

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    DIY Junior Member jessel's Avatar
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    I have connected the HRV per the attached schematic (so the fresh air is connected to the furnace return, and the exhaust line is connected to the appropriate bathroom register. This works to an extent, but not very well. When the bathroom door is closed there is basically no return line. It is necessary to either turn the furnace blower on, or connect the HRV exhaust line to the appropriate bathroom register (the bathroom has two, one for fresh air, one for exhaust).

    The advantage of having the furnace in the loop is that there will be a hot air register in the bathroom. I do not run the HRV continuously so I don't think that the energy difference will be significant. The only problem is that my furnace does not have a fan relay.

    Thanks

  6. #6
    DIYer, not in the trades LLigetfa's Avatar
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    HRVs are really meant to be a whole house ventilator, not just a bathroom fan. Tee it so that it has adequate flow with the door closed.

    There should be almost an inch of gap under the door so that even with the door closed, there should be 30+ sq in of capacity. Cut some off the bottom of the door if not.

    Furnace cold air returns are not zoned unless you have multiple furnaces. If there is a cold return in the bathroom, the air should flow. Find out why it doesn't.

    The HRV should connect to the furnace cold return far enough away from the furnace so as not to be seriously affected by the furnace blower. A HRV needs to be properly balanced, and the furnace blower will unbalance it if piped in too close to it.

  7. #7
    DIY Junior Member jessel's Avatar
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    Thanks for the help.

    The final installation is such that the fresh air line is connected to the return air line via a tee along the line that goes to a return register. This tee, I think, is enough that the pressure at the furnace blower is balanced. I don't have the means to test the pressure balance. I use the HRV daily, however I very seldom use the furnace (as I heat with wood). I certainly don't want to set up a system which is unsafe under some operating conditions, my thinking is that furnace blower imbalance would lead to strain on the blower, and would shorten the blower life, but wouldn't create a hazardous situation...

    The HRV does work well now.

    I still think that having the furnace blower kick on would be ideal. If I wanted to install air conditioning, would an HVAC company have to recommend replacement of the furnace? Or, would they simply install a relay into the existing furnace?

  8. #8
    DIYer, not in the trades LLigetfa's Avatar
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    It is the HRV that needs to be "balanced", meaning regulating the amount of air entering and leaving it. If not enough outside air is allowed to enter the home relative to how much is taken out, you can have a pressure deficit in the house that can induce flue reversal. If too much air enters, such as when the furnace blower boosts it, the HVR could ice up and/or you could feel cold drafts. If your home is drafty and leaks air out via stack effect, that too can affect the balance. A wood burning appliance without a dedicated outside air supply (OAK) can provide the equivalent to stack effect.

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