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!

DETAILS:
– 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.

MY PLAN:
– 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.

CONCERNS:
– 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:

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HEATING/COOLING LOAD:
– 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!!