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HELP sizing a mini-split for my house!

NewOldHousePhilly | Posted in General Questions on

Hi all! This is my first question but I’ve been reading this site obsessively for the past few weeks, especially all of the info about mini-splits (BTUs, efficiency, cold weather performance, sizing, modulation, etc.). I’m purchasing two mini-splits for my new-to-me old house, and I’ve been struggling with sizing. It’s crunch time since I need to make a decision by tomorrow morning, so I was hoping for some advice. I’ll give as many relevant details as I can below, and am very grateful for any input!

– Rowhouse in Philadelphia. It shares north and south walls with neighboring houses. East and west walls face the outside (east to street, west to back patio).
– Nothing can be done to alter the east facade because it faces the street and house is historic.
– Has a footprint of about 27′ (E-W) x 15′ (N-S).
– House is essentially two rooms deep on each floor. In the east room, has large four-story brick masonry chimney that is currently wood-burning.
– Is 4 storys tall. First story is a self-contained 2-bed apartment of 400 ft2. Upper apartment (2 bed, 1 bath) is comprised of the upper 3 floors and is about 800 ft2: 2nd floor is 400 ft2 and is kitchen/living room; 3rd floor is about 250 ft2 for bedroom/bathroom, and I am hoping to add a 150 ft2 addition of another bedroom (bringing it to 400 ft2, same size as the previous floors). Attic bedroom is 150 ft2.
– Built in 1850, is plaster and lath, and I believe that any insulation is minimal. I am hoping eventually to insulate but am not at that stage yet.
– Currently heated by gas-powered steam boiler, which I am hoping to retire ASAP. Steam boiler has a crack in it, adding urgency. The house is also a duplex and the heat is not split, meaning that I pay for the heat, not the tenants. Part of the object here is to split the heating system so each tenant can pay for what he uses.
– Currently has no cooling except window units.

– Install 3 powerful and efficient direct-vent gas-powered fireplaces inserts into the masonry chimney in the east part of the house. First floor (separate one-bed apartment) will get a 6500 – 40,000 BTU insert in the living room that is about 85% efficient (it’s a used Mendota). Upper apartment will get a 17,000-33,000 BTU insert in the living room that is also about 85% efficient (Avalon insert); master bedroom of upper apartment will get some smaller insert or stove that produces something like 8000-20,000 BTU (Possibly a Regency 234 insert or a Jotul 100 Nordic stove). I am hoping that this will provide much of the heating, especially on the east side of the house. I will also be installing ceiling fans to help move heat.
– Install 2 Mitsubishi Hyperheat mini-splits (for now) on the west side of the house. One will go on the first floor, in the bedroom of the lower unit (semi-open to living room with gas insert in fireplace, when french doors are open – ultimately plan to demo most of the wall separating these two rooms). The other will go on the second floor, in the kitchen of the upper unit (semi-open to living room with gas insert in fireplace, when french doors are open). These mini-splits need to provide all of the cooling for the floors that they are on, and ideally could handle most/all of the heating, if required (but will have gas fireplaces to complement them). Hoping to use them most in shoulder seasons (spring/fall).
– I hope ultimately to put three more 6000 BTU mini-splits into the upper unit – one in each bedroom.

– I am not sure of the sizing. Basically, I am deciding between 9000 BTU vs 12,000 BTU units (Mitsubishi Hyperheat). I’m pretty sold on the idea of a 12,000 BTU mini-split in the upper-unit, since hopefully any extra heat will travel upstairs to the bedrooms (hoping to remove current stair and install open metal spiral staircase in 4′ x 6′ open well). I really don’t know about the sizing for the bottom unit.
– Efficiency: From what I’ve read on this site, mini-splits operate most effectively at low loads (rather than conventional boilers/furnaces, when oversizing is a detriment). Therefore maybe 12,000 is better? However I looked at the efficiency of the 12,000 vs 8000 BTU machines and it looks like the 12000 is more efficient (I was looking at SEER but I assume heating is similar)?
– Modulation: My big concern is that the heating or cooling load will go below the minimum modulating load and the machine will short-cycle. I know the machines can modulate very far down – but what if in Unit 1, the bedroom door is shut? The mini-split will be in the bedroom of that unit, which is only 10 x 10 (open to bathroom of 8 x 5 and open to living room by french doors).
– I had about 10 HVAC people in to give estimates. A few calculated the heating load. The most promising, an ex-engineer, gave me these numbers:

– First floor Apartment, including bedroom/kitchen:
Cooling: 8575
Heating: 17393
– First floor bedroom:
Cooling: 6100
Heating: 9470
– Upper apartment:
Heating: 38,402
Cooling 2nd floor: 9710
Cooling 3rd floor: 4372
Cooling 4th floor: 4918

Thanks for any advice you can provide!!

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  1. NewOldHousePhilly | | #1

    Sorry, all, correction:
    The wall I am ultimately planning to demo is the kitchen/living room wall on the 2nd floor (Unit 2). The bedroom/living room wall in Unit 1 will stay put, which is part of the dilemma (is 12,000 BTU oversized in a bedroom where the doors may be closed, even if when the doors are open it needs to heat/cool the whole apartment)?

  2. Expert Member
    Dana Dorsett | | #2

    The first floor heating load of 17,393 BTU/hr @ +15F(Philly's 99% outside design temp) can be fully covered by a cold-climate 1.5 tonner, or even a 1.25 tonner if you can peel a bit off a bit.

    The minimum-modulated output of the Mitsubishi FH15 and FH18 are a bit high for the cooling load- you probably want to do something else. Even the 1 ton FH has too high a minimum modulation in cooling mode to be optimal, and doesn't fully cover the heating load.

    If it's going to be just one minisplit for the first floor, the Fujitsu 15RLS3H , which would be a better choice, since it can still deliver 18,000 BTU/hr in heating mode @ +17F which means it covers the 17K @ +15F, and it throttls down to 3100 BTU/hr in both heating & cooling modes.

    vs the FH15:

    Or, you could do it with a pair of Mitsubishi FH09s, which are good for 12,200 BTU/hr heating, 12,000 BTU/hr cooling, but throttle way down to under 2000 BTU/hr in either heating or cooling mode.

    The minimum cooling output the FH15 is 6450 BTU/hr, which is fully 75% of the cooling load, which means it it will almost always be cycling and rarely modulating during the cooling season, whereas the Fujitsu -15RLS3H has a minimum cooling modulation of a bit more than 1/3 peak, which means it will run much longer and more efficient cooling cycles.

    The upper apartment is 3 floors?

    A Mitsubishi FH09 per floor would be able to deliver 12,200 BTU/hr (x3= 36,600 BTU/hr), yet would still be able to throttle back to 1600 BTU/hr heating, 1700 BTU/hr cooling. The FH09 can also deliver 12,000 BTU/hr of cooling to a doored-off high gain room:

  3. NewOldHousePhilly | | #3

    Thanks so much for this - lots to look into and think about! Last threadsit, I promise, but I meant to say that I'm not entirely sure those load calculations are correct: the other estimates from other companies had different load calculations. The bottom unit is really only about 350 ft2 because there is an entryway to the house as well as a stairway down to the basement that are not included in the apartment so need to be subtracted from the 400 ft2 footprint. Do you really think I would need 15,000 BTU or 18,000 BTU?

  4. Expert Member
    Dana Dorsett | | #4

    The mini-split suggestions were based on the load calculations. If the calculated loads are garbage, so are the recommendations.

    Before you accept any proposals, hire an energy nerd like an engineer or RESNET rater to run a proper load calculation using a code-min 68F as an indoor heating design temp 80F as a cooling design temp, and the 1% and 99% outside design temps. Then YOU can decide what equipment choices work the best, and put them out to competitive bid. (I'm sick to death of letting HVAC "pros" come up with one truly insane ductless proposal after another and having to push back, HARD.)

    Loads are not a function of square feet of floor it's all about exterior surface area, air leakage, window types, etc.

    It's impossible to figure this out from mere verbiage, but a drawing with the room dimensions indicating which walls are exterior, and the previous estimated loads from various people would at least be a place where the discussion can start.

  5. NewOldHousePhilly | | #5

    Thank you! Yes, I think that's definitely a good recommendation. How would I go about finding someone to hire for that job? As in, is there a database of certified people, or is there a particular search term that I could search by, etc.? Thanks!

  6. Expert Member
    Dana Dorsett | | #6

    Or you can look up local P.E.s (certified Professional Engineers) and ask if they run Manual-J calculations as a service.

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