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Need help puzzling through the details of outfitting my house with a mini-split system

iLikeDirt | Posted in Mechanicals on

Right now my house has a 19 year old 125,000 BTU 80% efficiency gas furnace with ducts in the unconditioned attic, partially buried under insulation. I’d like some help determining if mini-split heat pumps will work as replacements, and in what configuration.

The house is a 1972 1,300 sf single story slab-on-grade ranch almost literally right on the border between zones 4B and 5B (high desert New Mexico) with an open kitchen/living/dining space and 3 bedrooms off of a central hall. My winter design temperature is 18 degrees and summer design temperature is 98 degrees. In practice, it almost never gets below 15 degrees in the winter; only for a few hours a few days a year. Daytime winter temperatures are often in the high 50s or even 60s, and nighttime summer temperatures often fall to the 60s. No polar vortices here, and global warming is probably going to make the place hotter over time.

After envelope improvements, air sealing, new windows and doors, etc, I’ve calculated from my own Manual J-ish spreadsheet that if I abandon the existing ductwork (all in the attic), seal up the registers and returns, and use only ductless mini-splits or build new ducts in a dropped ceiling down the main hallway within the conditioned space, then my heating and cooling loads will be about 15,500 BTUs/14,000 BTUs (respectively) at the design temperatures. Room-by-room load calcs show the main living area at about 8,500/7,000, and each of the three bedrooms at roughly 2,000/1,500. One bathroom with an exterior wall is about 1,000/500 and the other bathroom and the main hallway have negligible loads.

My envelope isn’t that good, but it’s the best I could do. R-50 cellulose and fiberglass attic insulation, R-16 (calculated whole wall) mineral wool, fiberboard, & EPS foam insulation, U-0.26 double pane vinyl windows and u-0.19 triple pane fiberglass sliding glass door with a 13% total glazing ratio. Slab on grade with R-8 EPS foam insulation on the top 6″ of the sidewalls and no insulation below grade or under it (the house is ringed with concrete that would have to be cut and removed to trench around and insulate the footings; too much $$$). The house is oriented north-south with only two small south windows, and a large 32 square-foot west-facing window (0.2 SHGC). There is an unheated but insulated attached garage with a heat pump water heater in it. We blew 7.8 ACH50 (1350 CFM50) on the last blower door test.

Subjectively, I know my envelope isn’t A+ by the standards of a thoughtfully-built new construction PGH, especially the mostly uninsulated slab and the only-okay walls. So I have a few questions:

1. even though the numbers suggest that mini-splits will work, is my envelope good enough that doing so won’t cause comfort problems, thermal stratification, distribution issues, etc.?

2. If 1 or more mini-split will work, what size should I get? I keep hearing conflicting information about oversizing the units, modulation, turn-down ratios, etc.

3. Which of the following air distribution options would make the most sense?

1. Single mini-duct cassette (probably a Fujitsu 18RLFCD) with new ducts in a dropped ceiling going down the main hallway; registers above the doorways and a single return in the dropped ceiling (no space for per-room returns)
– Worry: with my non-high-performance envelope, will the air being blown from the center of the house and over the doorways be sufficient? Will the air get pulled right back through the doorways without adequately mixing in the rooms? Will the puny air handler of this thing have enough “throw” to really push the air far into the rooms?

2. Single outdoor unit driving two units: a mini-duct cassette as above to feed the bedrooms and bathrooms, and a ductless unit for the main living area (probably a Fujitsu 12RLFCD + 9RLS3)
– Worry: as above, plus… will two indoor units on the same compressor be more expensive or less efficient to run than the single ducted unit?

3. Single outdoor unit driving two ductless units (probably 2x Fujitsu 9RLS3 )on either side of the hall doorway header, one blowing air into the living area, and the other blowing air down the hall toward the bedrooms and bathrooms;
– Worry: even with open doors, will the bedrooms and bathroom get enough of the conditioned air? Will two indoor units on the same compressor be more expensive or less efficient to run than the single ducted unit, even though both individual units would be higher efficiency than a single ducted unit (2x SEER 33 vs 1x SEER 19.7, for example)?

Which one seems like the best option? Or will mini-splits not be a good fit for my house? Or is there something I haven’t considered that might be better?

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  1. iLikeDirt | | #1

    My picture didn't show up. I'm attaching it here.

  2. Dana1 | | #2

    The biggest comfort issue during the heating season relative to where the heat is being put into the rooms is the windows. But if you crummiest windows are U0.26 that won't be much of an issue.

    If using the mini-duct cassette implement the returns solely as door cuts directly below a supply register over the door will result in some short-circuiting. Making the return a jump-duct inside a partition wall stud-bay further along the wall on a different wall, would produce a more satisfactory result.

    Using a 9RLS in the hallway won't really cut it with design loads on the bedrooms as high as 2000 BTU/hr. Even at 1500 BTU/hr the bedrooms would run significantly cooler than the hall on design day, but at 1000 and less it becomes a non-issue, since a single sleeping human would be supplying ~25% or more of the design heat requirements (unless you already subtracted that out in your Manual-J.) If taking that approach, use a wall thermostat placed in one of the bedrooms, not the hall. The hall will be extra-warm when it's cold out if you do that, but you won't end up with a bedroom that's too cold.

    A 2-head multi-split solution is probably more expensive than a mini-ducted solution, but it's fair to have it quoted both ways.

  3. iLikeDirt | | #3

    Thanks Dana. Jump ducts are a good idea. I can easily do that for the bedrooms and bathrooms, but not for the main living area without building out bulkheads or something to conceal ducts.

    I'm sort of leaning towards using a ductless unit for the main living area and a ducted unit for the rest of the house but I'm not sure how this would affect the operating costs. And should they be on a single compressor? Or separate ones?

  4. Dana1 | | #4

    Looking at the floor plan more carefully it's clear that you don't have sufficient partition wall area for heat transfer between the hall and bedrooms with the doors close. A mini-duct cassette will be far superior approach for that zone with our layout.

    The HSPF efficiency of the 18RLFCD is still in the 11s, and would likely be at least as efficient as a dual head multi-split.

  5. Dana1 | | #5

    The 8,500 BTU/hr @ +18F heating load of the big open area is barely half the output capacity of the 9RLS3 at that temperature. The minimum modulation output is about 3,000 BTU/hr, which isn't much less than your average wintertime load for that zone, which means it'll do a lot of on/off cycling during the shoulder seasons rather than modulating.

    You would have the same issue with a 9RLFCD on the remaining 6500 BTU/hr of load for the bedroom zone.

    The 18RLFCD has the same 3K minimum, but that's at least well below the average winter load for the whole house, so it would be modulating most of the time. There will be some duct design issues and some flow tweaking to get it to balance reasonably with the bedrooms, but it seems like a more appropriate choice overall from a capacity & comfort point of view.

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