Single Mini Split for Whole Home (600 sq ft)
TL;DR: Is a single 18k BTU Mitsubishi Mini split enough to maintain comfort in a 600 sq ft home with the attached layout.
I have read several posts about this same topic and believe this will work for my purposes, but wanted to see if there is any newer knowledge changes anything.
I own a 600 sq ft. 2 bed/1 bath cottage that I intend to rent out as a long term rental. It is located in zone 5 (WNY). The outside walls are insulated to r-13, with r-19 above the ceiling under an attic floor. I intend to insulate better over time where possible using exterior insulation and adding cellulose to the attic floor.
The house is currently heated by a very oversized floor furnace that is in the center hallway of the house. It’s a bit unsightly and an opening for air and pest intrusion from the dirt crawlspace below the house so we have resolved to get rid of it as part of electrifying as much of the house as possible. I received an estimate for a single head Mitsubishi 18k BTU unit, mounted centrally (kitchen wall next to smaller bedroom in attached floorplan). The installer’s manual J indicated a present day heating load of just under 18k BTU and a 12k BTU cooling load. As this home will be tenant occupied, I want to make sure that a single air handler will keep them comfortable assuming the doors are kept open.
Is a single head 18k mini split enough to keep comfort in a whole small home with multiple bedrooms?
Are there other better options or suggested improvements for comfort that don’t involve a huge budget?
Thank you,
Tom
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Replies
I would suggest a ducted split with takeoffs for several rooms
Most who have one minisplit are in a new energy efficient house.
Different levels of heatloss[IE air leaks] will mean if you do not have good airflow to a room it will get cold.
Tenants will get grumpy.
The R-13 walls and especially the R-19 ceiling is going to be a huge energy penalty in a Zone 5 climate. If the house is "leaky" with a lot of air leaks, that will account for around 30% of your energy loss. Air leaks are bad. The air leaks will also cause some rooms to run hot/cold based on how far your minisplit head unit is located at.
Ductless minis work best in air tight, high R-Value homes. The far away rooms might see 2-3 degree temp variables in such an energy efficient home but in an older, leaky home. You might see 10-15 degree temp variables, which is not best for comfort. One can strategically place fans around the hallways to help circulate the air from the air handler into the far reaching rooms. Not ideal but it can help.
The house is largely gutted. I am doing air sealing work in all the open walls and from above the attic and in the crawl where they are not open. I’ve already removed the window trim to seal around them. I am also installing insulated skirting to better encapsulate the crawlspace. Some of the vinyl siding is cracked so in the near future I will be replacing that and with it adding a layer of foam to the exterior for additional insulation.
I know this won’t be perfect or even pretty good house levels of insulation and sealing but I figured if I could cut drafts successfully as I’ve done in other properties it would go a long way. If that’s not enough I guess it’s back to the drawing board or a ducted mini handler instead.
Even in a super-insulated house, that layout wouldn't work well with a single head, and certainly not in a poorly insulated, drafty house. I would consider using one-way ceiling cassettes in the smaller rooms, and small high-wall units in the larger rooms, on a single outdoor unit. There will be a performance penalty to using that type of multi-split but it will keep the house warm and comfortable.
First step is to follow the process in this article to use your past heating bills to get a handle on the total heating load for the house:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
That is not a great floor plan for point source heat. You can make it work with a single wall mount if you install electric baseboard heaters in the rest of the rooms. With doors open, part of your heat load would still come from the wall mount so won't cost as much to run as an all baseboard setup.
Not a fan of multi splits generally, but you next best setup is a small two zone unit with heads in the living room and kitchen. This would get the bulk of your heat from heat pumps but would still need baseboard heat in the bedrooms and bathroom.
Instead of skirting, if you can build a proper sealed an insulated crawlspace underneath, the best bet would be a single ducted unit there with duct runs to each room.
Another option is to find some space near the center of the house (say that jog after the hallway to the kitchen) and install a slim ducted unit in vertical orientation and have a couple of short duct runs to each one of the rooms.
Thanks for the feedback. I’ve been looking at ducted units but the duct design is a bit daunting. There is space in the crawl and I will be encapsulating/insulating down there so it would work for distribution, it is just hard to figure out supply and return plenum sizing and filter sizing.
Seem unlikely that this will be a DIY install. Since a pro is doing the install, they will also do the design work.
If it is a DIY then you do have more than a little studding to do and duct design is the least difficult part.
Walta