HVAC for 2nd story
I’m seeking advice on heating/cooling the 2nd story of a 1950s Cape Cod that I am finishing. The main story is heated/cooled via forced air from a basement furnace, but there is no great place to run ducts, and I am advised (perhaps wrongly?) that zoning would be difficult. So I have been thinking that a mini-split may be my best option, that is, until I read a conversation about installation costs. Also some crude calculations indicate that even a 9000 BTU/hr mini-split is oversized.
I’ve attached the basic floor plan showing about where a mini-split head would go. If I were to use a mini-split I further wonder if a ceiling mount unit (or something else that could be more centrally-located) would be necessary for adequate distribution, or perhaps even a ducted unit.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
What are your 1% and 99% outside design temperatures? What sorts of load numbers were you coming up with?
The Mitsubishi half-tonner, the FH06NA is good for 8700 BTU/hr @ +5F, and has a decent low-end modulation @ +47F:
But with low ceilings a high-wall cassette is going to be a head-banger and take up some of the precious floor area.
The 4- way ceiling mount cassettes are physically pretty large, and would almost certainly need custom framing to install (assuming it would fit at all and still have enough room for adequate insulation above the cassette.)
A floor mounted 3/4 tonner set into a new cubby cut into one of the kneewalls so that the front face of the cassette is flush with the plane of the kneewall might be the better approach eg:
The floor mounted units take the air in near floor level and supply it out the top. If it's blowing at a sloped ceiling the mixing is quite good, winter or summer. High wall coils take the air in from the top and blow it out the front, which is usually OK in the cooling season, but not as efficient or comfortable during the heating season.
Dana, for some more context, note that I implemented your insulation suggestions in https://www.greenbuildingadvisor.com/question/weighing-insulation-options-for-attic-roof-and-gable-ends. In short, all exterior surfaces are flash & batted with Bonfiglioli strips. Actually I have a drawing for this--see attached. There is plenty of room above the flat part of the ceiling for mechanicals.
My design temperatures are 16.3 and 93.2 deg F. I based the crude heating load calc (i.e. conduction loss only...and emissive assuming that is rolled up in window U-factor) on the observation that over winter the 2nd story did not drop below 60 deg F due to heat rising from the first story. So I figured that the heat gain from downstairs is equal to the heat loss at 60 degree inside temp and 20 degree (conservative guess) outside temp. Subtracting that from the loss at 70 deg inside, 16.3 outside I get < 300 W. That does not include air exchange but it's theoretically pretty tight. So it would appear a space heater would easily meet my heating needs.
For cooling I used conduction and assumed sunlight is (continuously) normal to my 0.22 SHGC western windows as a simple limiting case (again, ignoring air exchange). But I just now realize my error of using outdoor air temp instead of surface temp. At the moment the western roof is 50 deg hotter than ambient and the western siding is 15 deg hotter than ambient. So adding those to the summer design temp I get ~5400 BTU/hr for cooling (yes, intentionally using different units here). So now a 9000 BTU/hr mini-split is not looking quite as egregiously oversized as I first thought.
A floor mount in a knee wall sounds pretty good. Are the different mounting types all roughly the same price?
Run a Loadcalc.net guesstimator on that space to come up with cooling loads. If you assume tight construction it'll at least be in the ball-park.
The exterior surface temps can be hotter than ambient, but not all exterior surfaces hit their peaks at the same time.
But a 3/4 tonner isn't likely to be screamingly oversized for cooling ~900' of space at the top of the house, even if there isn't a lot of window area, even if 250-300' of that space is storage space behind kneewalls.
Floor units usually cost a few hundred more than high-wall units, if only because the manufacturing volumes are so much smaller. If the floor units seem too expensive you can still mount a high-wall unit in a wall-nook cut into a kneewall as long as it meets the top and side clearance requirements. The LG Art Cool Premier still runs a decent COPs at it's minimum output levels, which 1023 BTU/hr (heating or cooling at AHRI conditions), and is slightly cheaper than a Mitsubishi FH06 that has a minimum output of 1600/1700 BTU/hr, but there are others.
The LG head needs 8" clearance to the top, 4" to the sides- I seem to remember Mitsubishis & Fujitsus can be a bit tighter that that. You can pretty much ignore clearance requirements to the bottom, which are mostly about head-banging or blowing air in your face, but make it at least a foot for reasonable mixing.
This guy looks pretty comfortable and happy with his l0-mounted high wall unit with a foot of clearance under the bottom:
That could just as easily be an inset nook into the wall, as long as the top and side clearances meet the manufacturer's spec.
Right, my philosophy is if a simplistic worst-case analysis returns a load smaller than the smallest mini-split, then my calculations are done. Loadcalc coincidentally agrees pretty well, so that's a nice sanity check.
Haha if a low wall-mount is good enough for a dog, then it's good enough for me! (I probably won't go with that, though...). Thanks for the help!