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Is it better to combine an ERV with an air handler or ductless mini-splits?

User avatar
Eric Whetzel | Posted in Green Building Techniques on

We’re building a tight house (the Passive House metric of .6 ACH is the goal) with a significant amount of insulation (slab to attic R-values: 16-20-38-65) in the Chicago suburbs. Our house will be about 1,550 sq. ft. of finished space, single story, 2-BR, 2 bath, with a full basement.

I always assumed the HVAC system would be a basic combination of ERV and ductless mini-splits (for heat and AC), but our HVAC contractor is suggesting the following set-up:

Goodman air handler with 15K.W. Heat strips, for supplemental demand
Goodman 2 ½ ton Heat Pump 14.0 seer
Honeywell ERV 70 CFM Ventilator

He believes this set-up will avoid cold spots in winter, and be more comfortable on the hottest summer days. He’s considered an HVAC guru in the Chicagoland area (both commercial and residential), so I’m reluctant to disagree with him (What do I know?).

Building Science Corp. has an article on their website: Info-611: Balanced Ventilation Systems (HRVs and ERVs) http-//buildingscience.com/documents/information-sheets/info-611-balanced-ventilation-systems

The article details the most common HVAC set-ups for HRV/ERVs, and, based on the diagrams of various configurations, I can see why the use of an air handler could offer significant benefits in regards to overall comfort.

Any HVAC designers/installers see a problem with his set-up? Any serious energy penalty with his set-up — we’re trying to lower demand as much as possible in order to justify adding solar on the roof?

Any homeowners out there happy or miserable with a similar system, or with a basic ERV and min-split set-up?

What questions should I be asking?

I’ve read a few HVAC horror stories about tight houses recently, so I want to avoid a system that’s too big for actual demand, or — God forbid — mold problems because a system fails to properly handle moisture and humidity.

Any input or suggestions would be greatly appreciated. Thank you!

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Replies

  1. User avatar
    Dana Dorsett | | #1

    15kw (=51,000 BTU/hr) of heat strip backup is completely ridiculous overkill for a way-better-than-code 1550' PassiveHouse-tight decently insulated house. Your true whole house load @ 0F is likely to be under 15,000 BTU/hr (= 4.4 kw), and the 2 ton heat pump is probably going to cover the heating load completely all on its own at your local 99% outside design temp (which above -5F even in the coldest of the Chi-town 'burbs.)

    The 2.5 ton Goodman might be overkill too, but I'd need to see both the Manual-J and the extended temperature capacity tables to make that call. Some crap out on capacity at temps below 15F faster than some others. To be sure, 2.5 tons of cooling is a lot for most 1550' houses. Most would come in under 2 tons, many would come in under 1.5 tons. If you don't have a lot of west facing window area and high performance window it's probably 2x overkill for cooling, and may have been sized that big based on it's low temp heating capacity (TBD).

    Ideally the ERV and it's ducts would be independent of the heating ducts, but if that R38 is a whole-wall number (rather than center-cavity) the only way you'll ever get cold spots in the winter would be if you had a room with a substantially higher window/floor area ratio than the house average. Mixing/redistributing the house air by cycling air handler without the heat pump engaged adds a bit of wind-chill during those cycles, making it FEEL cooler everywhere, at least during those mixing cycles. During the cooling season that wind chill isn't a bad thing, but the other half of the year it is.

    The advantages of ERV vs. HRV are fairly small at 70cfm, but your sensible cooling loads are likely to be small enough that you'll need to run a room dehumidifier in the summer, even with an ERV dialed back to a lower ventilation rate.

    The basement will have a miniscule heat load, and very different gain/loss characteristics from the first floor. If fully conditioned it usually needs to be it's own zone.

  2. Steve Knapp CZ 3A Georgia | | #2

    Perhaps Dana will weigh in, but I have a suspicion that the contractor is specifying a vastly oversized system. My house is 1.1 ACH/50, and I use a 2 ton air-source heat pump (17 SEER) to condition 3,200 square feet over three zones. It is quite comfortable.

  3. User avatar
    Eric Whetzel | | #3

    Steve and Dana --- Thanks so much for the quick responses and all the info.

    I was afraid the current set-up would be overkill, so your replies aren't that surprising.

    Should we forget about the air handler altogether? Instead, just go with an ERV and separate heat pump set-up? And then concentrate on figuring out actual demand (1.5 or 2 ton heat pump)?

    Our house will be a rectangle, and the long, east-west axis separates the BR's and baths from the "public" spaces (kitchen and family room) that face south. Can we get by with just one distribution box in the kitchen/family room area for the whole house?

    Thanks again for the help!

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Eric,
    If your "HVAC guru" is suggesting wildly oversized equipment, that calls into question the guru's expertise.

    Step one is to get an accurate room-by-room heating load and cooling load calculation, using Manual J aggressively (with no fudge factors). Don't depend on an HVAC contractor to perform this calculation; find a trustworthy mechanical engineer, energy consultant, or certified Passive House consultant to help you.

    Like Dana, I strongly suggest that your ventilation ductwork be kept separate from your heating and cooling ductwork. For more information on ventilation systems, see these articles:

    Designing a Good Ventilation System

    Ducting HRVs and ERVs

    For more information on heating and cooling with ductless minisplits, see this article:

    Rules of Thumb for Ductless Minisplits

  5. Steve Knapp CZ 3A Georgia | | #5

    Eric. Consider contacting Optimal Building Solutions or HomeEnergyPartners for your manual J. It's likely the savings on a properly sized system will more than pay for this service. Plus, you won't end up with an HVAC system that makes you miserable.

  6. User avatar
    Eric Whetzel | | #6

    Martin --- Agreed. We may have to move on to someone with more direct experience with this kind of build. And thank you for the article suggestions --- they always help a lot.

    Steve --- Thank you for the company suggestions. I'll definitely be checking those out. Also, is your system ductless? If ductless, when you say "three zones" does that mean there is a separate distribution box in each zone? In the articles Martin suggested, along with pieces I've read elsewhere, a common theme is the need to keep doors open to help the distribution box spread the heat/cooling beyond the room in which it sits. Is this a strategy you employ in your own home? If so, does it work effectively?

    Thanks, once again, for everyone's input. Much appreciated!

  7. Steve Knapp CZ 3A Georgia | | #7

    Eric. I considered a split mini, but the HVAC designer convinced me that it would not perform well with my floor plan. I ended up with a conventional air-source heat pump. A good designer can look at your floor plan and determine the best strategy for your structure and budget.

  8. User avatar
    Eric Whetzel | | #8

    Steve --- Thanks for replying. I've already contacted Home Energy Partners, so hopefully they can help me figure out a system that will work, then I can try to find an HVAC installer in my area who feels comfortable putting it in. Thanks for the recommendation!

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