Four-unit apartment heat pump
I’m looking to install heat pumps in a four-unit apartment, 2,600 sq feet, 1900’s era building we recently acquired. The floor plan is a one-bedroom unit and a two-bedroom unit on the ground floor and the second floor is an exact replicate. I’ve received conflicting advice from different contractors ranging from a 12k to a 25k btu compressor. Given the cost of the install, I’d like to get it right. Manual J calculations on each unit and the entire building have been done, assuming we start and complete 2 inches of foam insulation in the basement.
Here are the btu needs based on a 99% design temperature of -4 F for Waterville, ME heating to 70 F. Maine law requires the heating system must be able to maintain 68 F down to -20 F so we’ll have to add electric resistance for those 1% of heating days.
1 bedroom downstairs 17,500 btu
1 bedroom upstairs 16,543 btu
2 bedroom downstairs 18,242 btu
2 bedroom upstairs 17,191 btu
Total building 71,809 btu
Based on rebates (1k per unit) available in the state, each unit needs a single head heat pump that has a HSPF 12.5 or greater for a total of 4 heat pumps. Both contractors have bid on fujitsu 15k aou15rls3 compressor and wall mount indoor model asu15rls3y, but that is only good down to -5 F. Does it turn off at -5F?
One of the contractors prefers the smaller 12k unit and thinks the larger 15k is overkill. He like the Mitsubishi muzfh12na compressor tied to a wall mount mszfh12na. The nice thing with this is it’s rated down to -13F.
High electric bills will make for unhappy tenants that move out. Are there better heat pump options that meet my needs?
I really would like to try out these heat pumps as natural gas is not an option, only oil or propane, both of which I can buy at bulk discount. Over the last year I paid about $2 a gallon for fuel oil. The home is currently setup with 4 zones of oil heat, but there was a freeze up a few years ago, which will require substantial repair (at least $2000 to repair a 6 year old system). If you think heat pumps are a bad idea in this home, please also feel free to share why. Thank you in advance for your help.
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Replies
With single mini split in the living space, you will need electric baseboards in the bedrooms to get even heat. You can size these slightly larger to make up for any short fall of the main unit during polar vortex.
I would double check the heat loss for the units, I'm a bit warmer than you with 2F design temp and a mostly insulated 2x4 construction 3 unit uses 24kBTU for the whole place.
For the Fujitsu units, I would go with the AOU XX RLS3H version as these are rated down to -15F and come with a base pan heater. Pan heater is important in cold snowy climates.
Most, if not all, mini splits continue to operate bellow their minim rated temperature, they just start delivering less heat there.
With dedicated outdoor units for each apartment, you can get away with a bit of over-sizing. The over-sizing only hurts if you have a long very humid shoulder season and need a lot of moisture removal.
Both 09 and 15 units have similar minimum capacity, but the 15 will run at much higher COP when delivering the 6kBTU to 10kBTU that your apartments probably need.
Thank you for the reply. I have read about the Fujitsu's with pan heaters and worried a bit about the vampire loss of the pan heaters, but I guess it's a necessary evil if the heat pumps are to act as the primary heat source.
That's interesting about your 24k heat loss. That's amazing. How many square feet is it? Are you tenants' pleased with the heating of their home?
Our building is also 2x4 construction, but has 34 windows, and 6 exterior doors, which adds a lot the the load. The walls have some cellulose, but I would not call it dense pack-it's settled and you can easily pole your finger in it. We may go drill holes and pack it more densely. With doing that and and only designing to state minimum 69 degrees we can get the numbers down to about 56k, but four 12k units won't meet this need at -4 F. The problem with up-sizing to the 15k is more cycling which will wear out the compressor some, but we do need 13-18k per unit on those really cold nights. Is there any argument to undersize to a 12k and install more backup heating? On the surface that seems like it would be more expensive to have the backup heat kick on.
The place is 1800sqft, not that large and not a lot of windows. The tenants are quite happy with the heat there, in the dead of winter there was never an issue with keeping it around 74F.
If you do have oil fill up data, you can double check your heat loss calcs by following here:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
If your load is indeed 72k, you can still meet it with four AOU12RLS3H units and a bit of supplemental heat in the bedrooms. These are good for 15k at -5F, with each bedroom add in a 1.5kW baseboard (5Kbtu each) and each bathroom a 500W (1700btu) , should bring you well above the 72k at design temperature with plenty of capacity to spare for a cold snap.
Either way, I don't think you'll loose that much by over sizing the units. The minimum output on most of the Halcyon low temp units is low enough that you'll only get short cycling in the warmer part of the shoulder season.