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ERV ducting

Kay_Esper | Posted in General Questions on

Good day,
I just finished installing my ERV ductwork it’s for a Daikin VAM 250 FC 90 CFM at low speed and 150 CFM at high speed, my house is a 1500 sqf with 9f ceiling system is 6inch × 9 inches rectangular rigid trunks for supply and exhaust and 5 inches branches out of them to 6 supply points and 6 exhaust points with dampers on each point close to main trunk now my question is I have an issue with the ducts going out of the house can I use flex insulated round ducts stretched to the max for 20 feet of exhaust and 18 feet for supply? And second is my duct sizing Okay?

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  1. Expert Member
    Akos | | #1

    Nothing wrong with flex duct for the outdoor supply and return provided you don't do any sharp bends with it.

    For more complicated runs I like to use semi rigid aluminum pipe (think dryer went but longer length) with an insulation sock pulled over it. Semi rigid is about 15% more restrictive than metal ducts but you can do tight bends with it unlike flex.

    Make sure you seal the stale air out well, leaks there could add a lot of cold air to the house in the winter.

    On your house side the ducts (trunk and branches) are oversized, which is never a bad thing. One thing to watch for is on a 5" branch at bedroom flow rates the dampers will be very sensitive to adjustment.

    1. Kay_Esper | | #2

      Thanks a lot for the precise info can you elaborate more on downside of oversized trunk and branches, and the bedroom dampers being sensitive in what way? Sorry for the basic questions but i basically did the homework myself so far

      1. Expert Member
        Akos | | #3

        The larger trunk just takes up more space, there is nothing wrong with it.

        The issue with the larger branches / low flow is the damper will only start effecting the flow right before it is closed. This makes it a bit hard to adjust as small changes in the angle of the damper make a big difference in flow. The leakage area of some standard round dampers is also pretty high. With 5" and 10CFM, the damper even fully closed might not do anything.

        If your branches are pretty balanced, then this might not be a big issue. If you really need to throttle back one of the branches, downsizing the takeoff there would help. Generally bedrooms need only 3" branches (4" if complex ducting). Good thing about the 3" is that easily fits into a 2x4 wall.

        1. Kay_Esper | | #4

          Thank you so much very helpful indeed now 1 last thing does connecting 2 solid ducts 6x9 via 2 feet of 6 inch flex duct at the far end with only 2 supply outlets going out of the last piece affect the system? And I'm trying to space at least 6 feet between outside supply and exhaust ports

          1. Expert Member
            Akos | | #6

            Not sure I understand the question correctly.

            If you are looking to connect two sections of the supply trunk with a 6" flex, that should be fine no matter how many supply feeds are on it. 6" flex pulled straight can flow a lot of air.

        2. Kay_Esper | | #5

          And I managed to replace 5 dampers with 3 inches branches to the bedrooms and toilet close to the beginning of the trunk

          1. Kay_Esper | | #7

            Yes correct, you've been very helpful thank you for your time and knowledge. I will get back to you in case I need anything else

  2. Kay_Esper | | #8

    Getting back to this question, update on some required info, so 5 inch fully closed damper can still leak enough air to mess with my balancing? And if one damper is open slightly more than the others closer to the main branch can it leak all the 150 cfm?

    1. Expert Member
      Akos | | #9

      The slightly closed damper will never leak the full 150cfm, but I wouldn't be surprised a box store damper leaking 10CFM fully closed. The problem is that if you try to set the flow for a bedroom say at 10CFM on a trunk that is near the ERV, with a larger damper even the smallest adjustments can change the flow by a factor of 2. This is more of an issue if your branches are unbalanced (not the same pressure drop at the design flow). If you have a hard time balancing the flow, add in a reducer with a smaller damper.

      1. Kay_Esper | | #11

        Thanks Again Akos, I don't have a problem with the pressure drop since my main trunk for fresh air is well balanced it contains only 1 90degrees turn while for the exhaust has 1 90 degree into straight then split left and right then another 90 degree but that's for the exhaust ports only. I guess I won't find out until I install the ERV next week

  3. user-5946022 | | #10

    Don't most ERV's have internal dampers?

    Seems from the description that Kay has a ducted distributed ERV. Are additional dampers needed at each supply and exhaust point?

    What about a spot ERV like the Panasonic Whisper Comfort? Those just duct to the exterior for both supply and return. Do the ends of those ducts need dampers?

    1. Kay_Esper | | #12

      I have built a fully dedicated ducting system for the ERV unit

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