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Need a stealthy ERV installation

Ryan Griffin | Posted in Mechanicals on

Hi, I own a 1450 SQFT home that has been spray foamed throughout (walls, attic, etc.) and has decent triple pane windows, but minimal to no passive solar due to orientation and surroundings. Although the home is 75 years old, a blower-door test revealed its tighter than most new builds, so I need a ERV as soon as possible. I chose an ERV over an HRV due to the humid summers and long dry winters here in MN. The home has a 96% efficiency gas furnace and central AC. The furnace has a multispeed fan.

Here are the constraints: my wife is very sensitive to drafts and is not willing to accept any cool outside air coming through vents or our heating system in the winter (think negative 30 deg F here in Jan), and I am very sensitive to noise (the constant quiet hum of our radon mitigation system annoys me). We are not really willing to do much in terms of running new ducting or disturbing existing drywall. I’m looking for a lower cost solution here. An HVAC guy quoted us on a Venmar AVS C12 ERV, but it would be more of a standard installation, and I have no idea if that is an efficient ERV.

I have a few unconventional ideas on how we might still get the fresh air we need, but I’ll let you shoot holes in them or make other suggestions:

1) Set up (a larger CFM) ERV to run only when the furnace or AC is operating. I know this isn’t ideal for those times of year when we aren’t heating or cooling, but I figure we’ll be more likely to open windows at those times anyway.

2) If the above solution is no good, look instead to plumb the ERV to feed the air RETURNS instead of supplies, and only when the furnace fan is not running. The thinking here is that the returns are in locations that are much less noticeable if there were some air coming out. Not sure exactly how this could be done though.

3) Ventilate only to a non-living space like the basement (I don’t really see the point of this though)

3) Worst case: run the bath fans on a set frequency to ensure we’re getting some air flow. I don’t want to resort to this, but it’s better than bad air.

Thoughts?

Thanks,
Ryan
Minneapolis, MN

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ryan,
    This article may answer some of your questions: Ducting HRVs and ERVs.

    The article explains (a) conventional advice for those who want to connect an ERV to the ducts of a forced-air heating system, and (b) the advantages and disadvantages of this approach.

  2. Ryan Griffin | | #2

    Thanks Martin, but I'm looking for something different than these 3 common scenarios. I want to use existing ducts, but don't want to use the furnace fan to distribute the air. But maybe I'm too far 'outside of the box' with this idea.

  3. GBA Editor
    Martin Holladay | | #3

    Ryan,
    The system that comes closest to meeting your needs is the one described in my article as an "exhaust-ducted system." This system requires the installation of at least one exhaust-air duct (usually from a bathroom to the ERV), but it has a distinct advantage over most other systems that try to use forced-air ductwork for ventilation: namely, it does not require the furnace fan to operate unless the furnace is actually running.

    All systems that attempt to use forced-air heating ductwork for ventilation are somewhat jury-rigged, and all have disadvantages. The best approach is dedicated ventilation ductwork. If you don't want to install dedicated ventilation ductwork, compromises are inevitable.

  4. Charlie Sullivan | | #4

    Ryan, It's certainly possible to set up a different ERV system. As you suggest, you could set it up to use the forced air system vents in reverse, and interlock it to run only when the forced air system is off. You'd then need good backdraft dampers for both systems; Tamarack Technologies offers one that they claim is superior to the alternatives. The disadvantage of that setup would be that on cold days when the furnace is running most of the time, you won't get much ventilation. But there's some logic there--you probably get some air leakage somewhere in the envelope as a result of your forced air system running and you certain get some air leakage driving by stack effect. So your air change rate due to inadvertent leakage will be greater on the days when your ERV runs the least.

    By the time you get that system engineered and implemented, with the unconventional ducting and controls, you might spend as much time and/or money as you'd spend running dedicated ducts to a few "safe" places for your ERV, which could be right along side the furnace return locations that you think are OK.

    If you want to get into complex and expensive options, you can also install a low-power heater, electric or hydronic, the supply duct going from the ERV to the house so it delivers at at or above room temperature, or install a heat-pump-based heat recovery system:
    https://www.greenbuildingadvisor.com/articles/dept/musings/balanced-ventilation-system-built-heat-pump

    I believe your option 1) is in the scope of the article Martin pointed you to. Your option 3) might be worth considering, but your air intake, through random cracks, etc. will be at a much lower temperature than the air that the ERV would supply, so it would be worse from the perspective of cold drafts.

    If you run the ERV at very low speed with the air distributed over many vents, you should have no problem with cold drafts. The airflow rate is vastly different from a furnace.

  5. Ryan Griffin | | #5

    Thanks for the info here. This helps a lot. One last question (for now!): what would you guys think about exhausting basement air instead of bathroom air. The (finished/heated/AC'd) basement is always a bit musty, so I was thinking there could be some benefit to this. The ceiling is only partially finished as well, and that's where the HRV would mount, so it would be one less duct to run. Any thoughts on this? Pros/Cons vs. bathroom? Kitchen is out of the question due to no space.

    Thanks,
    Ryan

  6. Charlie Sullivan | | #6

    If you aggressively air condition in the summer and keep the humidity low in the house, running that air through the basement on its way out of the ERV could help. If you don't, the air in the house going into the basement could add humidity, because one that warm somewhat humid air gets cooled in the basement, its relative humidity goes up.

  7. GBA Editor
    Martin Holladay | | #7

    Ryan,
    Most ventilation systems exhaust air from bathrooms for the following reasons: (a) building codes often require bathroom exhaust; (b) bathrooms can be smelly; and (c) bathrooms can have too much moisture.

    If you don't want to exhaust air from your bathrooms with your whole-house ventilation system, you need to plan some other way to exhaust air from your bathrooms.

    It's possible to pull air from a basement if that's what you want to do. However, make sure that (a) you've done a good job of air sealing leaks at the rim joist area, and (b) your basement walls are well insulated. Otherwise you may end up pulling outdoor air into your basement, or increasing your energy bills by pulling conditioned air into a space that (in many homes) wouldn't necessarily need conditioning.

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