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Mini-split to carry me through the winter

jonny_h | Posted in Mechanicals on

I’ve got an HVAC contractor who seems very solid, and I’m trying not to drive him crazy before I even get a quote on my project, so I’ll drive y’all crazy instead 😉

Here’s the situation:  My house has an 80% efficient gas furnace.  Following the “out with the old, in with the new” procedure with my gas use and heating degree days from mid-December to mid-March this year, the load was around 680BTU/degree-hour, or a house load of around 40k BTU/h.

The above load was under the conditions of: mediocre quality double pane windows; pretty decent cellulose in the walls and attic; leaving the 2nd floor closed off from the 1st floor; several substantial holes and areas of missing insulation due to ongoing renovations; running the thermostat at around 68-70

Over the next couple of months, the renovations should have me at what I’d hope to be substantially better conditions:  New triple-pane windows; 3″ of exterior polyiso on the walls and 6″ on the roof; substantially better air sealing (but maybe a few holes still); still able to leave the 2nd floor closed off; needing to keep the house at a temperature amenable to working on the interior & also avoid freezing pipes (but not actually living there yet).  I also may be getting rid of the chimney, which would mean the old furnace wouldn’t be usable as a bridge anymore.

EnergyVanguard did a Manual J for me, based on the anticipated / estimated “finished conditions” of the house; they put the whole house load at 35k BTU/h or about 583 BTU/degree-hour

I had been primarily looking at Fujitsu mini-splits (and that’s what the installer I like so far carries) (probably ducted, but maybe ductless) — but apparently they’re mostly backordered by several months.  BUT, I may be able to get an 12LZAH1.  That leads me to the following train of thought:

– At my design temperature, that unit can allegedly provide ~22k BTU/h

-At my predicted 583BTU/degree-hour, allowing an internal temperature of 50F instead of 70F, I’d need about 23k BTU/h

– My gut also says that the 583 can probably be lower under the desired conditions I’m going for — leaving the 2nd floor mostly closed off, and not really considering the basement fully conditioned (EV’s manual J considered the unfinished basement as being heated to the same temperature as the rest of the house)

So, my temptation is to install the Fujitsu 12LZAH1 on the 1st floor.  It seems that it’ll be sufficient to keep the house from freezing this winter, and potentially carry most of the load during more-moderate temperature conditions.  Ultimately, the 2nd floor will likely get its own mini-ducted unit; if that were done the 12LZAH1 would be sized appropriately for the 1st floor.  At that point, I might just leave it, or if doesn’t provide adequate comfort then perhaps I could give it to [someone else in my family who currently heats with wood and doesn’t have A/C, but has pondered mini-splits] and install a ducted unit to serve the 1st floor and basement.

So, am I certifiably insane?  Or is my gut and my rough estimates correct, and I should go ahead and get the Fujitsu 12k unit?

EDIT for posterity: My numbers above on the 12LZAH1 were are wrong.
Key point: In the Fujitsu catalog, when it says “Max Htg % of Nominal Capacity at [temperature]”, the “Nominal Capactiy” that percentage is based off of is the nominal / nameplate capacity of the unit, ie 12k BTU for the 12LZAH1, NOT the “Nominal Heating” Capacity in the same table (16k for that unit).

Thanks to @Kyle R in another thread for the link to, which has links to the design & technical manuals with detailed capacity tables for Fujitsu equipment — search by the indoor or outdoor unit part number and look for “Datasheets” -> “Technical Manual”. 

Another interesting point from those tables — the capacity *does* increase as indoor temperature decreases — but it’s generally a matter of 1k BTU/h or less — helpful, but the limitation is more how much heat can the outdoor unit pull from outdoor air, rather than how effectively the indoor unit can dump it to indoor air.

So, to answer my own question:  It’s not a super crazy idea, but I would’ve needed the 15LZAH1, and there are less expensive methods to provide temporary heat that don’t risk destroying a mini-split head with construction debris.  And, it doesn’t matter anyway because the 12LZAH1 was out of stock before I had enough time to think it through anyway!  Loving the supply chain issues these days!

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  1. kyle_r | | #1

    What about construction debris? I wouldn’t want to run a ductless head with a bunch of drywall dust, etc. The wire mesh screen filters on these is not going to stop that fine of dust and you will have dirty coils in no time.

    From a heating point it should work though, but I would try to find a 15k unit.

  2. charlie_sullivan | | #2

    At 50 F interior temperature, the output and COP of the minisplit will be higher than the specs for 70 F inside, so it might do even better than you predict.

    The concern about drywall dust is a real one--might trash the fan bearings as well as clog the heat exchanger. But any of that can be fixed.

    In theory this could be a scenario in which an air-to-water heat pump would help. If the fan coil it feeds inside gets trashed, it's cheap and easy to replaces, or disconnect it, take it outside, hose it off, and bring it back in.

  3. kyle_r | | #3

    I would see if there is a way to run a flexible gas line to a through the wall heater.

  4. jonny_h | | #4

    Dust is definitely a thing, though it may be an issue with any form of heating -- I have seen some people make wooden filter boxes to add on high-filtration disposable pleated filters to mini-split heads in workshops / woodshops, and I could put together something like that pretty quickly. Google "mini-split filter box" for some example images of this type of setup.

    Charlie, good point on the performance at lower indoor temperatures -- I wasn't able to find extended performance charts on Fujitsu stuff, so I was only going off of what I found in their brochure.

    The "in theory" caveat on air-to-water is key -- the technology of those is pretty neat, but all that I've seen are at least double the price of a complete mini-split for just the outdoor unit!

  5. paul_wiedefeld | | #5

    If you're installing electric resistance with the final ducted install, how about install baseboard heaters for the near term? Expensive to run, but it'll be motivational to finish the renovations on time. After you can keep them for redundancy.

    1. jonny_h | | #7

      I hadn't planned on any electric resistance / baseboard units, save for some bathroom in-floor heat, but that is a fair point -- those little oil-filled radiant space heaters are pretty cheap (to buy, not to operate), and generally pretty safe to leave unattended -- a couple of those in strategic locations would probably be enough. Though, those would still be buying something that wouldn't ultimately be used, while (in theory / hopefully) the mini-split would be suitable for most of the first-floor load after everything's done. Still -- something to think about, that is a quick & easy solution that I hadn't thought of previously.

  6. Expert Member
    Akos | | #6

    I have a 7500W electric shop heater. It can be wired to a standard 40A stove feed and provides 26000BTU, enough heat to keep about 1000 sqft of uninsulated but air sealed place comfortable when it dips down to the single digits.

    As long as you are not keeping the place at 70F all the time, cost to run it is not that much, very simple to install and don't have to worry about breaking it as it was under $200 new.

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