Number and positioning of indoor units in mini-split heat/cool system
We are planning to replace our existing gas/steam heat system with a heating/cooling mini split system, and are now in the process of getting site visits and bids. We’re motivated to do it by 1. we have solar panels, 2. availability of big rebates, 3. our steam pipes have a complicated leaking problem that would require major repairs. We have already gotten somewhat conflicting advice about the number of units we need and placement.
The house is a very old semi-attached 3-story in Brooklyn, NY (zone 4a), brick on the ground floor, frame walls with brick fill on the 2nd and third floors, windows only on east and west sides, all insulated, and insulated attic crawl space. We are taking some steps to improve weather sealing and insulation, but there’s only so much that’s practical. Floor plans and dimensions are attached.
All agree that the single 500 sq ft ground-floor room will have one ceiling unit near the front of the house. 8′ ceiling, exposed joists, 3 brick walls, front wall is sheetrock with lots of insulation. All contractors propose 18k air handler (excessive?).
2nd floor has one 400 sq ft room, where bedroom area is separated from living area by a large bookcase – air can flow across the 2′ gap below the ceiling and through the 4/ wide doorway. 12 ft. ceiling. 2 contractors advise a single 18k wall-mounted air handler in the large font section. The third suggests 2 9k wall-mounted air handlers, one in front the other in back. The front and back corners of the east wall are recesses and would aesthetically be ideal locations. If we install only one unit, would there be enough airflow to the bedroom area? Would a ceiling fan there help? And are those sizes overkill?
3rd floor would have a wall-mounted unit in each bedroom. All 3 contractors advise 9k for each. Would 6k be enough?
Finally, also trying to decide whether the small 130 sq ft office at back of the ground floor needs its own unit since the door connecting it to the larger room is often closed. If so all advise a 6k unit.
These would all connect to a single condenser in the alley on the east wall.
Any advice mush appreciated, especially about size and whether 1 or 2 units on the second floor.
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Replies
Heating and cooling load isn't determined by square footage. It's determined by how well insulated the building is, the weather outside, and any other sources of heat. Square footage isn't useful for rules of thumb or even for sanity checks.
Anyone designing a system should do something called a Manual J, which will model heating and cooling needs on a room-by-room basis. Once you have that you can start looking at equpiment.
Mini-split systems are often terribly over-sized, because the heads are meant to serve a single open space. The smallest mini-split head I'm aware of is 6K, which is quite a bit more than many rooms require. An oversized system will be uncomfortable and inefficient.
There are ducted mini-split heads available, they allow you to use one head for several rooms. Of course you have to run ducts, which takes away some of the appeal of mini-splits.