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Dampers in ERV

Kay_Esper | Posted in General Questions on

Good day,
I’m set to install an erv and finished talking with the contractor but I had one question in mind, need your advice about Dampers, system is 8 inches main trunk galvanized steel with 6 inches flexible ducts branches 6 for fresh air and 6 for exhaust, should I install damper on each branch including exhaust to be able to adjust each port for the correct CFM and velocity?

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  1. Expert Member
    Dana Dorsett | | #1

    Only large rooms would need a 6" branch duct. The appropriate thing to do would be to size the ducts correctly/appropriately (typically 4" for a bedroom or small living room) by design rather than oversizing the ducts and dampering down. It could take forever to tweak with dampers, which could also skew the overall supply/exhaust balance. Hard piped or semi-rigid plastic ducts are more appropriate ventilation systems than the typical 6" flex used for heating/cooling ducts.

    1. Kay_Esper | | #2

      So a 125 cfm ERV with an 8 inch trunk should have 4 inches of supply ports? How would I calibrate air volume doesn't the closest branch to the start of the trunk get more cfm than the last branch off it? And what about the exhaust should the ports be 4 or 6 inches? Sorry for the newbie style questions but where I live ERVs are very rare to install at houses, basically doing my own homework

  2. Expert Member
    Akos | | #3

    125 CFM is very low flow rate for an 8" trunk, you will have very little pressure drop between the first takeoff and the last. Provided your branches are similar, the flow to each would be about the same.

    No matter what, you will still need some way of balancing the flow. This can be at the registers in the rooms (they sometimes get louder if you need to choke one a lot), or better have a damper installed in each takeoff by the trunk. You should only have to ever adjust this during commissioning, you do want to make sure that each area is getting the right amount of ventilation.

    1. Kay_Esper | | #4

      So basically an 8 inch trunk should be made smaller? And dampers at each take off branch? Including exhaust? I'm doing it once and would want to do it the right way can you advice of a main trunk and branches?

      1. Trevor_Lambert | | #5

        8" is bigger than what you need for 125cfm, but the advice he's giving you is based on the 8" trunk. It may change with only a 6" trunk, or it may not. 125cfm is still not a lot for 6", but if you'd already planned for 8" and you have the space for it, it's not going to hurt.

        Dampers at each branch is the best way to do balancing. You can do it at the registers, but that makes it too easy for someone to undo.

  3. Expert Member
    Akos | | #6

    The trunk really depends on the boost flow of your erv. With a typicall unit, that is probably around 200cfm, for that you can go down to 7" if needed. Generally there is nothing wrong with upsizing especially if you are running flex ducting.

    Balancing is mostly important on the supply side, you want to make sure you get the correct airflow to each bedroom. The returns, if you size them for the flow rate, you should get to close enough that I wouldn't bother adding in the balancing dampers.

    Assuming rigid ducting, you probably need 3" supply to each bed, 5" to the living/dining.

    On the returns, it doesn't hurt to put a larger return register with a filter. This will keep your ducts cleaner over time.

    The ERV should be installed near an exterior wall the the ducting that goes to the outside needs to be insulated and sealed. The rest of the ducting, if run inside conditioned space, doesn't need any insulation.

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