Minisplit Head Locations for PGH Renovation Project
I’m planning the HVAC layout for my ‘Pretty Good House’ renovation project. I will be using minisplits for heating and cooling. The recommendation is to install 36,000 – 48,000 BTU of capacity for my very well insulated and sealed 1600 sqft Zone 5 house. The house has a lot of well shaded windows (22). I don’t expect a lot of heating thru the windows in the summer but won’t be surprised if I’m losing heat thru the windows in the winter.
The house has a very open layout with one large dog legged living area that includes the kitchen, dining and living room areas at ~850 sq ft, an ~250 sqft master bedroom, ~180 sqft guest bedroom and the balance of space in 2 small baths, a small office and small mudroom/laundry area. The guest room will be closed off when not in use.
My initial thought was to install one small minisplit (~6,000 BTU) in the guest room, another smaller unit (~9,000 BTU) in the master bedroom and the third (~24,000 BTU) at approximately the center point of the large open space. The open space extends ~50 feet from one end to the other. I’m hoping to use primarily the 24,000 BTU unit for heating and cooling with the other units running only when it is very cold or very hot out.
My questions are as follows:
1) Does the sizing of the individual units make sense?
2) Should the large unit be mounted in the middle of the space to be heated or cooled, or is another location better?
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Replies
Hi!
1) No, this sizing makes no sense because it doesn’t seem related to a heat loss calculation, just guessing by contractors. You can try to find someone to perform a manual J calculation. Another method would be examine the pre-renovation fuel usage to determine the “before” heat loss. It wouldn’t surprise me if the before heat loss was already below the 36kbtu lower end. Just based on problematic rules of thumb, 9000btu /250 sqft is extremely high.
2). Is there a basement/attic? If possible, adding ductwork will give you the best distribution and adds heat/AC to the bedrooms better than two additional heads would.
Thanks for the comments.
The sizing calculation was from an on line calculator. There isn't any pre-renovation fuel usage available and if there was it would be worthless as pre-renovation the house was a poorly insulated '70's ranch with R11 at best in the walls and poorly blown in cellulose in the ceiling with heating from a coal stove and electric baseboard units.
The renovation has been a gutting to outside four walls, extension of exterior stud walls from 2X4 to 2x6 equivalent and replacement of every exterior window and door along with the addition of a lot of windows. The house now has 2.5" polyiso on the outside, and will have R23 in the inside walls with better than R60 blown in insulation in the attic. I'm sizing the system based on 24,000 BTU for 1,600 sqft conditioned first floor space with addl heat/cooling (the other two minisplits) available on an as needed basis.
The house is a ranch, so no usable attic. The basement is open and will eventually be finished space. It has ~2.5" foam on the inside walls, but nothing under the slab. I'd prefer not to F-around with ducting and an airhandler, so am looking for a simpler DIY solution, hence the minisplits and relatively open floor plan.
Wow!!! That's quite the renovation. 24,000 for the whole house seems more like it, but be extremely warily of manual J calculations, they're very error prone. It could still be significantly less. Among many potential problems, be suspicious of infiltration inputs if you don't have a blower door number.
The ductless or ducted (minisplits can be ducted too) decision is challenging. I like ducted because it's easier to distribute, better at filtering, can usually match the load better (what's the point of modulation if minimum output is still short cycling?) and easier to replace. It may or may not be cheaper upfront, but moving forward, replacing one system will be cheaper than replacing 3. I think you'll find that adding those two bedroom minisplits will cost thousands more than extending the ducts from the rest of the house. Based on what you've already done to the house, it shouldn't be too difficult. All that said, it's not always possible: I myself have both a ducted and ductless system for one room.
I'm with Paul. Go for a ducted unit in the basement. Wall mounts are not worth it simply for the extra maintaince down the road plus the comfort issues with oversizing in your bedrooms.
If you really don't want ducts, with that level of insulation, you can probably heat the whole place without any ducting by simply over heating the basement. Think of it as low cost heated floor :) You can hang a slim ducted unit form the basement ceiling and let it do its thing.
Did you do a Manual J takeoff with accurate inputs? For a house that size with the details you list, assuming it's reasonably airtight, the load looks larger than I would expect.
Edit: I see you answered this already. The problem with oversizing is that sometimes the outdoor unit can't ramp down enough for efficient operation, and it won't dehumidify as well as a unit that's properly sized.
Are you suggesting that a 24,000BTU unit is oversized for a well insulated and sealed 1600 sqft space in Zone 5? I am looking at three units - 6,000, 9,000 and 24,000 with the 24,000 being the primary and the others only turning on when the 24,000 can't meet the demand.
I expect the 24,000 to probably always be large enough, or extremely close. But if the bedroom doors are closed, distribution can be an issue. For a bedroom, 6000 is oversized. 9000 is extremely oversized. So the decision becomes - do you spend money on adding oversized ductless heads to distribute the warm/cool air or take another approach?
Yes, here in CZ6 I design well-insulated and air-sealed homes, usually between 1200 sf and 3200 sf in size. The larger homes have loads in the range of yours, and the smaller homes have about half that load. But there are a lot of factors that affect the calculations so I can't really say what you would need.
I've seen this tool mentioned a few times on GBA: https://betterbuiltnw.com/hvac-sizing-tool
You could try making an account and seeing if you can run your numbers there and cross-reference what you get with the numbers you've been given. I haven't used it myself, but I've been meaning to check it out.