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ERV – combined one or two vs independent one or two?

Bruce_Davis | Posted in Green Building Techniques on

Building a reverse 1.5 with walkout basement. 2300 sq ft finished on main level and 2000 finished on lower level. Looking at two conventional HVAC systems. If we go with ERV intergrated with air handlers, presumably need two ERVs. If we go with an independent duct ERV system, do we need two ERVs duct systems or will one independent ERV duct system work for the whole house? Which system makes the most sense? Thanks.

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  1. Trevor_Lambert | | #1

    To get 0.3ACH, you'd need about 195cfm, assuming 9 foot ceilings. You should have no problem finding a single ERV capable of providing that plus a lot head room for boost. A lot of people will tell you that even that is more cfm than you need. Why are you looking at two hvac systems, one for each floor?

  2. Bruce_Davis | | #2

    Yes, one air handler unit for each floor. Main level ceilings are 10 ft with a vaulted ceiling in the great room. Lower level ceilings are 12 ft.

    Because it's just two of us, I was thinking one HVAC unit with 6 electronic damped zones. LL won't be used much. Recommendation is skip that complication and go with two units. LL unit could be more builder grade and basement will stay cool on its own in summer. In the winter, LL heat can stay on 60 degrees, 90% of the time.

    One more question, does the ERV eliminate the need for separate humidification and/or de-humidification units? Thanks.

  3. Trevor_Lambert | | #3

    So if you're conditioning the two floors independently, you'd presumably want to ventilate them that way as well, otherwise there may be balancing issues. I'm no expert in any of this. ERV does not humidify or de-humidify. It really depends on the specific scenario as to whether it would eliminate the need for either. It can actually make humidity more of an issue. Someone with a lot more knowledge will come along and give some more detail eventually.

  4. lance_p | | #4

    Bruce, what climate zone are you in?

    ASHRAE 2010 ventilation standards are based on # of bedrooms (potential occupants) and square footage:

    CFM = ((# of bedrooms + 1) x 7.5) + (sqft x 0.01)

    If this is a four bedroom house it would only require 37.5 + 43 = 80.5 CFM of continuous ventilation.

    During cold winter weather an HRV/ERV will dehumidify the house (ERV will dehumidify less), and during hot humid weather an HRV/ERV will humidify the house (the ERV will humidify less). During the shoulder seasons the effect is lessened as is the difference between an HRV and ERV. ERVs tend to require less intrusive and/or inefficient defrosting strategies during very cold weather.

  5. Reid Baldwin | | #5

    If you go with dedicated ventilation ductwork, one HRV/ERV is sufficient regardless of how you handle heating and cooling. Dedicated ventilation ductwork provides more effective ventilation than integrated, assuming that you are fairly logical about locating the stale air and fresh air inlets/outlets.

    If you decide to go integrated, putting your ERV on your upstairs systems might work. It depends on what you mean by the lower level not being used very much. Presumably, you wouldn't be paying to finish 2000 sf if you don't have plans to use it.

  6. Bruce_Davis | | #6

    I'm Zone 4.5 (right on the 4/5 border). Two bedrooms on main level, two on lower level. Only one bedroom permanently occupied on main level. The others are guest rooms or office space, etc.

    I'm confused by the humidity thing. I thought since this is supposedly helping the air quality, it would add humidity in the winter and reduce humidity in the summer.

    The lower level will contain a home theater and an art studio, both for occasional use. If we don't finish the basement, we only have the master bedroom and a flex bedroom/office on the main level. So two additional LL bedrooms/baths for resale. One of those bedrooms flexes as the art studio. Thanks.

  7. Expert Member
    Michael Maines | | #7

    Bruce, if you're a GBA Prime member, this is a good overview: https://www.greenbuildingadvisor.com/articles/dept/musings/misconceptions-about-hrvs-and-ervs.
    These are also good: https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system, https://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv.

    Briefly, neither HRVs or ERVs provide humidity. HRVs simply exhaust stale air and provide fresh, filtered air from outdoors, and heat is efficiently transferred in the process.

    ERVs do the same thing, except they also transfer some of the moisture from one air stream to the other. So if you're bringing in cold, dry outdoor air, the outgoing indoor air will humidify it to some degree. In the summer, outgoing cool, dry air (assuming air conditioning) tempers the incoming humid air, dehumidifying it a bit.

  8. user-2310254 | | #8

    Bruce,

    If you've previously lived in leaky, inefficient homes, you may have inaccurate notions about how your new, more efficient home is going to perform. If the home is tight, it needs to be ventilated. During the winter, introducing outdoor air may or may not reduce humidity to the point where you start to notice. I say may or may not because you will be cooking, showering, and breathing. All these activities generate moisture. If conditions become too dry, GBA's generally advises owners to temporarily reduce the ventilation rate. You shouldn't need a humidifier.

    During cooling season, ventilation will introduce air and humidity. Your HVAC should be able to keep this under control. If humidity levels increase too much, however, you could reduce the ventilation rate temporarily. If you live in a really humid region, as I do, you also could opt to install a dehumidifier and ventilator unit.

    I am not an expert. But I would encourage you to consult with one. It is important to pick the right HVAC and ventilation system for your climate, home, and personal preferences. See this article for additional information on this topic: https://www.greenbuildingadvisor.com/articles/dept/musings/who-can-perform-my-load-calculations

  9. Deleted | | #9

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