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Achieving net zero in a row home with one minisplit per floor?

user-242891 | Posted in Mechanicals on

I would like some opinions from all you smart people.

I have read many of the articles/posts on this site about a one head per floor minisplit approach for a net zero buildings. I do gut renovations of row homes in DC and have LEED Platinum certification on some, but have not achieved net zero. But I really want to!

Since these are existing ~100 year young brick rowhomes, mostly with attached frame porches, we cannot achieve a passive house tightness with our current construction techniques. We achieve in the neighborhood of 3.5 ACH50.

I have a project starting next week that is a classic example. It’s 20′ wide x 30′ deep with an 8′ porch addition that we will enclose. 2 floors (750 per floor) + basement.

I’ve attached two schematic designs: A) Exhaust only and B) ERV I realize there are many flavors of options in between and I’m open to all comments.

Specific questions:

1) Given our likely 3.5 ACH50 is getting heating and cooling along the entire length with one head feasible? and or reasonable?

My sense is that if it were <1 ACH50, a candle would heat the place…but I’ve got triple the infiltration…so I have concerns about comfort. In particular at the rear, where there are 3 exposed sides, as opposed to the main house where there are party walls. 2) There is a due west exposure on the rear (DINING AND MASTER BR). DC Summers are HOT and humid. However, we are a heating dominated climate, when you look at the numbers. 3) Given our likely 3ish ACH50, is exhaust only a reasonable approach? Would you do it in your own home? It seems to me that with that amount of leakage, which is only somewhat tight, my money would be better spent on an exhaust only system (forgoing the whole house ERV and associated ducting) and put that in higher efficiency mechanicals. I am working with an experienced installer/designer and the energy model and J are forthcoming which will help answer some of these questions. However, i’d appreciate folks chiming in from a real world installation perspective on actual comfort. Best regards, Tanya

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  1. GBA Editor
    Martin Holladay | | #1

    Whether or not your suggested strategy will work depends partly on your window specifications. If you are installing new double-glazed or triple-glazed low-e windows, with low-SHGC glass on the west elevation, you'll be in better shape than if you are using old, historic windows.

  2. user-242891 | | #2

    They will be new windows. Not spec'd yet, but minimum low-e low SHGC.

    I will be exploring triple-glazed, given the exposure. I will also be exploring exterior and interior shading as well. For the intents of this this discussion, you can assume a high performing western facade.

  3. Expert Member
    Dana Dorsett | | #3

    Hitting net-zero without at least SOME insulation upgrades to walls and roof (even foundation) seems implausible, and I'm not sure you'd be able to get enough without a serious build-out (or build-in) of the walls inside the brick facade, in which case good air tightness MIGHT be possible.

    I'll be curious to read what the energy use modeling & manual-J come in at.

  4. user-242891 | | #4

    I apologize, for brevity I omitted what we do for insulation...and the drawings are just an in progress layout. Like I mentioned, we gut renovate. Not to get too far into the weeds, but we achieve R20-30 on exterior walls, R60 in the attic with a combination of blow in an dense pack cellulose and rigid poly-iso. We do not insulate party walls nor basement slab. And we use other things to help with energy efficiency (drain waste recovery, i have used heat pump hot water heaters on projects...maybe this one). A weak point has been the 14-15 SEER air to air standard split system heat pumps. The ducting has been a constant installation quality issue and the equipment performance is quite low. Working in minisplits would address a performance weakness in the overall energy efficiency package...i just want to get a sense of what other peoples experience is with them. I want to see how few I can use. Also, these are not historic rowhomes. Just because they are old, does not mean they are historic. Thank God!

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