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Minisplit or air-source heat pump in a remodeled 1920 bungalow?

smashburn | Posted in Mechanicals on

Remodeling a 1920 Craftsman Bungalow. The home is in Zone 4A. The old Gas Furnace and AC System was working, however due to its age and a ducting system that is a disaster, it is being removed. The 2 second story bedrooms are being served by a non functioning Air Source Heat Pump located in the Attic with ducts run in the unconditioned space as well. The home is 3500 square feet with 2300 Sq Ft on the main level. The main level is open plan with a central hallway opening to the kitchen,living room, dining room. The master bedroom and bath are on the same level as well. The project will include new low-E window replacements, adding attic insulation, general air sealing. The walls and attics had blown in rock wool insulation previously installed it still is in in great shape.

As see it I have options:

Option A: Main Level served by a conventionally ducted Split System Air Source Heat Pump on the main level with a dual head Mini-Split serving the second floor guest bedrooms?

Option B: Main Level served by a conventionally ducted Split System Air Source Heat Pump on the main level with a zoned ducting system serving the second floor guest bedrooms?

Option C: Main Level served by a 4 Head Mini-Split System Air Source Heat Pump on the main level with a dual head Mini-Split serving the second floor guest bedrooms?

I have 3 quotes pending on these 3 options. I prefer the full mini split option as from my research its may be the most cost effective and compatible with my plan to install solar panels on garage after I complete the remodel.

Whats the best option in your opinion?

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  1. GBA Editor
    Martin Holladay | | #1

    You definitely don't want Option B, which would (apparently) use ductwork that is located in an unconditioned attic. Using ductwork located in an unconditioned attic is always a disaster.

    That leaves Option A or C.

    I find it hard to imagine, however, the you really need 4 ductless minisplit heads to serve your first floor. That sounds like overkill.

    Before you design a new heating system, you need to do a heating load calculation and a cooling load calculation. Once these calculations have been performed, you can begin to design your system.

  2. Dana1 | | #2

    Take the load calculation point seriously. Oversizing a ductless head by more than 25-50% results in a lot of cycling, low efficiency, and lower comfort. A multi-split compressor also has minimum load issues, which can make it cycle rather than modulate when only one head is calling for more refrigerant, making even worse. It's better to undersize it by 10% than oversize it by 50%, from both a comfort & efficiency point of view.

    So how big is your load likely to be?

    A reasonably tight 1.5-2 story craftsman bungalow with at least some storm windows over single pane wood sash will typically come in at about 15-18 BTU/hr-ft^2 @ 0F, or 10-13BTU/hr-ft^2 @ +20F. In zone 4 your 99% outside design temp will be somewhere between 15-25F, so for 3500' of conditioned space you're looking at a design heat load something like 35,000- 45,000BTH/hr, unless its' unusually tight and you have really great replacement windows, or it leaks a ton of air.

    The smallest 4-head systems are 3-tonners, which have a max output of ~40,000 BTU/hr @ +17F- which could easily be more than your whole house heat load. The first floor spaces typically have lower BTU/hr-ft^2 numbers, since they have conditioned space rather than a cold attic above them.For 2300' of space you're probably looking at about 20,000 BTU/hr, which is HALF the output of a typical 4 head multi-split compressor.

    Odds are pretty good that you can get by with 2 or at most 3 heads on the first floor- a 3/4-1-ton head for the big open connected spaces and a 1/2-3/4 ton head for the master bedroom. A half-ton head can deliver about 8000-8500 BTU/hr @ +17F, which is a very decent amount of heat. A typical 3/4 tonner can deliver on the order of 10-11KBTU/hr@+17F.

    You may be able to do the whole-shebang with a 3-head multisplit, using a 1 to 1.5 ton mini-duct cassette for the upstairs, if there is a reasonable place to put it (or if you can build an air-tight insulated space over it in the attic, making the duct runs as short as possible, with a 2.5-3 ton multi-split compressor. Since the upstairs rooms are "guest rooms" the modest efficiency hit of a mini-duct is of little consequence, since most of the time you will be leaving the cassette off, or at the lowest possible temperature setting.

    So do the math, a real room-by-room, zone by zone Manual-J (the gold standard) or an I=B=R type heat load calculation (easy, using spreadsheet tools), of the heat load at your 99% outside design temp, using 68F (or even 65F) as your interior design temp, even if you usually like to keep it at 72F. The is inherent bias in the calculations to the high side, so be aggressive rather than conservative on your assumptions, or you'll end up oversizing it.

    FWIW: I live in a not-superinsulated but reasonably tight 2400' atrs & crafts bungalow built in 1923 with clear glass storms over wood sash double hungs (about U0.5), with a lot of 2x6 raftered cathedral ceiling. My heat load @0F is about 35-36,000, (about 15BTU/hr-ft^2) as measured by both fuel use and an aggressive Manual-J. At +17F the heat load would be about 27K (about 11 BTU/hr-ft^2). With about 40K of heating system (limited by water temp & radiation) it cruises through negative single digits without losing ground.

    You'll probably have better insulation and definitely will have better windows than me, for a lower BTU/hr-ft^2 ratio. The insulated foundation (a retrofit) may be a slight performance advantage that you may not (yet) have, but you probably won't need 3-tons of compressor for the whole house (again, about the smallest 4-head multi-splits), so a 4-heat(3ton) + 2-head( 1.5-2 ton) solution would be about 2x oversized for the whole house, which would be a BAD idea.

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