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When installing an HRV for indoor air quality, is an independent duct system better than a “simple” installation?

Patrick345 | Posted in General Questions on

I’m installing an HRV in my new house to improve air quality and mitigate outgassing of pressed wood cabinets, carpet, etc. (Climate zone 5a, so mixed humid conditions; hot summers, chilly winters but not too cold)

Initially, I was going to install a simple exhaust-only system with Panasonic bath fans, but then I came across a study that suggested that exhaust-only systems aren’t as helpful for indoor air quality as an ERV (or an HRV):

Ventilation System Effectiveness and Tested Indoor Air Quality Impacts
Building America Report – 1309, March 2013
Armin Rudd and Daniel Bergey
http://www.buildingscience.com/documents/bareports/ba-1309-ventilation-system-effectiveness-and-indoor-air-quality-impacts

I’ve attached a plot from that study that shows formaldehyde concentration vs. ventilation type. The ERV gives the best results for formaldehyde reduction, but the air changes per hour for each ventilation type were different (see second attached plot). So it’s not exactly a direct comparison even though I think they tried to make the ventilation rates equal in terms of CFM. I was very surprised at the results, but maybe I’m misinterpreting them?

Anyway, in that study, the ERV was ducted independently of the heating and cooling system. I’m wondering if a “simple” installation in which the HRV/ERV is tied into the HVAC return would be significantly different (better or worse) for indoor air quality than an independently ducted HRV? I’m considering both options and only want to pay for independent ducting if it would be better for air quality.

Follow-up question: If I did the “simple” HRV installation I would try to retrofit a Gentech Evergreen IM motor in the air handler. It claims 95 W power usage when run continuously in the fan-on mode. Combined with a good HRV, energy consumption wouldn’t be ideal but should be good enough for continuous HRV operation. Does anyone have opinions or experience on doing things this way?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Patrick,
    Q. "When installing an HRV for indoor air quality, is an independent duct system better than a 'simple' installation?"

    A. Yes, for a variety of reasons. If you use your furnace fan or air handler fan to distribute ventilation air, your operating costs will be higher than if you use the fan that comes with the ERV. A dedicated duct system will ensure that you deliver the fresh air exactly where you want to deliver it, and pull exhaust air from the rooms that you want to pull it from.

    A few comments: it sounds like you are worried about formaldehyde emissions. If that's your worry, the best approach for a new house is to choose cabinets and other building materials that are formaldehyde-free.

    I advise you not to obsess about this issue. As long as your ventilation system is well designed and well commissioned, the approach you suggest will work, and your indoor air quality is likely to be much better than that in the average new home. If you can't afford dedicated ventilation ductwork, it's not the end of the world.

  2. Patrick345 | | #2

    Yes, the house some formaldehyde emission issues. Standard builder's cabinets, engineered wood building materials, etc. When I have the opportunity to build from scratch, I'll select formaldehyde-free options.

    I'll get prices for simple installation and dedicated ductwork and go with whichever one makes the most sense for the house. Thanks for the reply.

  3. Kevin_in_Denver | | #3

    The most efficient HRVs use regenerative heat exchangers, which work well with no ducts at all.
    http://www.greenbuildingindenver.blogspot.com/2014/06/the-future-of-residential-heat-recovery.html

  4. Patrick345 | | #4

    Kevin, thanks for the link. I have been considering that type of HRV (both the Lunos e2 and the Vents model that you discuss). They look like a great option from an energy perspective, but I am concerned that they might not work as well as a standard HRV for improving indoor air quality.

    Mostly I'm concerned about the lack of synchronization of the Vents TwinFresh model. I agree with you that this won't affect efficiency very much in most situations, but from an air quality perspective I wonder what would happen if the units were anti-synchronized? (i.e. all units pushing or pulling air at the same time, creating negative or positive pressure in the house). In the negative pressure/exhaust-only situation, at least some of the "new" air coming into the house would be "filtered" through nooks and crannies, flowing across building materials that are outgassing and picking up pollutants along the way. Not being able to control the source of the incoming air seems like a big disadvantage for iaq.

    I've also looked at the Lunos e2, which are synchronized, but they are more expensive and only 2 pairs can be hooked up to a controller for synchronization. I'm not sure that the 40 CFM total from 2 pairs would be enough for my house

    But then again…. I have no real-world experience with any of this so maybe my indoor air quality concerns about the point-source HRVs aren't valid?

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