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Community and Q&A

Increasing ERV Air Flow

tim_st | Posted in Mechanicals on

I’m living in a newly built, fairly-tight home with a Broan ERV, and the unit seems to be slightly undersized; in particular we’re getting lower levels of fresh air than I’d like coming into the bedrooms at night (leading to CO2 buildup), even when it’s running at max.

I’m wondering if there’s anything cheap/easy/DIY I can do to help boost the airflow, short of just keeping the filters as clean as possible, which I’m already doing.

One specific question — the bedroom outlets for the ERV are just a bare 4″ duct coming up vertically from the floor into a 90-degree elbow which vents out into the closets. The end of the elbow is just the bare duct, no diffuser or anything.

I feel like this is probably HVAC 101, but wondering — if I add a diffuser, will it do anything to reduce the static pressure by improving the aerodynamics at that outlet duct, or not make much of a difference? Similarly, would removing the 90-degree elbow make enough of a difference to try?

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Replies

  1. Jon_R | | #1

    Also try closing non-bedroom supply vents at night. This will increase bedroom flow despite it also causing higher static pressure.

  2. exeric | | #2

    I found on my home that enlarging the filter from the smaller standard filter that came with mine helped out substantially. The smaller the filter the more restrictive to air flow that they are. The larger filters have more surface air area so they flow more air and get clogged up slower.

    To do this you would have to install an outboard filter box and plumb it into input of the ERV with a short length of duct. You can get the boxes for about $100 -$200. Even though the large filter work much better than the proprietary filter that come with ERV they are still much cheaper because they are standard MERV 13 furnace filters. Over a few years of replacement filters the savings will pay for the filter box. I was able to turn down my ERV motor 2 clicks after installation and it runs substantially quieter and more efficiently now for the same measured CO2 in the house.

    1. this_page_left_blank | | #3

      Did you do this for both air streams, or just incoming air?

      1. exeric | | #4

        I did it just for the fresh air coming from outside going to the input on the ERV. I suppose it might also help to have one on the supposedly dirty air coming back into the ERV from inside the house. However, I took my cues from the very thin sieve-like filter for that function that is already there. It seemed like there was very little restriction caused by it that I could improve on. In fact a regular big filter might have been a bigger air restriction that what exists.

    2. tim_st | | #5

      Oh nice! Unfortunately all that ductwork is behind drywall so not sure I want to cut into the ceiling to install a filter box (yet).

      Did you use an insulated one or not worry about it? And would a filter box with an inline fan be worth trying too, or do you think that's overkill

      1. exeric | | #6

        I used an uninsulated filter box. (I'm in California) You could situate the filter box anywhere in on that fresh air duct, even right at the cap inlet on the exterior. It wouldn't matter if you didn't insulate it then. Wherever it's easiest to get at and install.

      2. exeric | | #7

        Here's the insulated filter boxes. They're more than double the uninsulated average prices. They may be worth it depending on where you live. You'd also need a duct size adapter.

        https://www.hvacquick.com/products/residential/Air-Filters/Inline-Filter-Boxes/HVACQuick-IFVB-Series-V-Bank-Insulated-Inline-Filter-Boxes

          1. exeric | | #14

            I have the first one you mentioned, the one using 14" x 14" filters. It's plenty large enough for the low volume of air that ERVs/ HRVs have. I hope you're not in wildfire country because the MERV 13 isn't enough for smoke.

          2. exeric | | #15

            Here's a picture of it installed. I attached a wooden bracket across two studs then screwed the back flat plate of the box to the bracket. I then attached two picture hanging metal wires between the ceiling and the screws on the top of the filter box to support it on the other end. You can't see them in the photo but they are there and are required to hold the other side up. I like that box because I think it's more aerodynamically smooth to the air flow than the other one.

        1. tim_st | | #16

          Thx for the photos! We're nowhere near wildfire country but a bit of the smoke drifted over here last week even.

    3. tim_st | | #8

      Sorry to keep asking so many questions, but do you think an inline duct fan at the bedroom outlets would help too, or is that gonna cause too much of a pressure differential for the ERV?

  3. exeric | | #9

    Others here will know more than I but I would say no, don't do it. You will be taking the ERV out of its design specs and putting positive pressure in the house. I don't want to hog this space though as I'm a DIYer myself.

  4. Expert Member
    MALCOLM TAYLOR | | #10

    Tim,

    I wonder if the problem is exacerbated by the incoming air not being supplied directly to the bedrooms?

    1. tim_st | | #11

      You're right that it probably is; I had been concerned about noise in the bedrooms but the actual duct noise is so small that it would be fine to have them vent directly into the bedrooms. But even when we leave closet doors open there's still not enough airflow so I don't think extending the duct run to have it vent directly into the bedrooms would 100% solve the problem.

  5. jkumon | | #12

    Hard to gauge the problem without the model number of the broan unit and the size of the space it serves. That leads to the design amount and then the curve of the efficiency of the unit. Running ERVs on blast all the time can be very energy inefficient depending on the fan. The best practice is for the air to go directly to the bedrooms or as close as possible.

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