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Community and Q&A

Minisplit indoor wall locations – 1.5 story Cape

Andris Skulte | Posted in Mechanicals on

Hi guys,

I’m ready to select my mini splits to heat/cool our typical 1600 sq ft 1.5 story cape in Connecticut. The CoolCalc.com Manual J heat loads are about 12k BTU/h upstairs and 26k downstairs with the 8°F design temperature. We have a 2500 W electric baseboard in the kitchen if it gets below the balance point (and a space heater for the upstairs bedroom).

With our layout, I’m thinking 4 indoor heads make the most sense.

I wish I could do a multi-zone, but they don’t modulate very well for the small bedrooms (we sleep with closed doors), so I’m leaning towards Mitsubishi MUZ-FH06NAH’s upstairs and FH12’s downstairs.

Downstairs would be a 12k head in the kitchen (cathedral ceiling w/ fan) and another 12k head on the fireplace wall in the living room.

Dining room, bathroom, and office would not be heated… Is there a way to estimate how much cooler they would be, based on circulation through open doorways?

Upstairs would have a 6k or 9k head in each bedroom. My upstairs concern is our bedroom has a kneewall and sloped ceiling, so no space above the window on the outside wall for the head. Next logical place is in the middle of the master closed (aka toddler’s room!) wall, aimed at the bed. I’d need a condensate pump too…

Can the vents be adjusted so we don’t have a constant blast of air at our heads? Or should I be looking at a floor mount or a ceiling cassette (that doesn’t modulate below 3kBTU).

The outdoor units would be wall mounted to stay out of the snow – two on the north side of the house, and two on the south side.

I need to upgrade my electrical panel now too (man, this is getting pricey!)… Wife will be thrilled to get “central” air for this summer, and I’m jazzed about ditching oil heat in the winter.

Anything else to be concerned about?

Thanks for the help!

 

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Replies

  1. User avatar
    Stephen Sheehy | | #1

    38kbtu seems pretty high for a new, small house. With decent windows, good insulation and air sealing, I'd expect a heat load maybe 20kbtu or less. My house is about the same size, on one floor though. With 0° F design temperature, heat load is less than half of your load.
    As for getting blasted by warm air, it really doesn't happen. With Fujitsu, you can orient the direction of air flow up or down and side to side. I assume the crane is true for Mitsubishi.

  2. Akos | | #2

    A right sized multi split with a ducted unit upstairs would work much better.

    You can put the ducted unit in the ceiling of the bathroom upstairs with a couple of short runs to the bedrooms. It would be much more comfortable and cheaper than the individual heads. I've attached a couple of pictures of a low pressure ducted unit for a 4 bed 1 bath floor.

    Installing one is not that hard and you get much better performance, comfort and cost.

    Depending on how your two floor line up, you can probably even run a duct down to the kitchen and bath downstairs and save an additional wall mount.

  3. FluxCapacitor | | #3

    If you go with ceiling cassettes there are distinct pros and cons.

    I have three Mitsubishi MLZ ceiling cassettes which fit between my joists. Ceiling cassettes punch huge wholes in your ceiling which need significant consideration regarding air leaks and insulation.

    On the plus side the ceiling cassettes are more aesthetic, have built in condensate pumps and can be installed in problem locations.

    Floor units have to be placed where you NEVER plan on putting furniture blocking the air flow.

    I recommend always prioritizing towards wall units, with floor and ceiling cassettes the “back-up” choice only if wall units won’t work.

    That said. I’m happy with my MLZ’s but I would definitely not recommend them in a small (<150sq feet) bedroom.

  4. Aedi | | #4

    I agree with Akos. A ducted minisplit system works better when dealing with small, closed-off bedrooms.

    While a 38k BTU heating load is a bit on the high side, it isn't out of line with older construction. You might want to look into the insulation and weatherization programs in your area, so see if you can bring down your heating load (and with it, your equipment cost).

  5. User avatar
    Dana Dorsett | | #5

    You understand that an FH12NA delivers fully 13,600 BTU/hr @ +5F, and is oversized for the rooms where you're placing them, right?

    https://ashp.neep.org/#!/product/25895

    The FH09 puts out 10,900 BTU/hr @ +5F, 12,200 BTU/hr @ +17F, has a wider modulation range and is more appropriately sized for the calculated loads.

    https://ashp.neep.org/#!/product/25894

    A pair of FH09s will deliver more than 22,000 BTU/hr @ +8F, vs. a calculated 25.5K load for that floor that I'm guessing is considerably higher than reality (but without more information it's hard to say.)

    To have any shot at heating adjacent rooms the heads need to be positioned where they can blow at an open door. It might work better if the one next to the huge heat leak (aka "fireplace") might work out better if it's on the outside corner blowing across the front door into the other front room. Similarly, the one on the other end of the first floor would have better heat distribution if it can be moved over a bit to be directed across the kitchen into the hall.

    Upstairs the FH06 blowing at the bed is actively a BAD idea. Having it blow directly on people makes it harder to sleep. Installing it on a partition wall also presents refrigerant line and drain line routing complications. Putting it next to the bed blowing toward the door to the other room would work better and is easier to implement (assuming it's not on a sloped ceiling and a very short wall.)

    I agree with Stephen Sheehy that 38K seems a more than just a bit on the high side for a heat load at +8F for newer construction or even for reasonably tight older construction.

    >"I’m jazzed about ditching oil heat in the winter."

    Have you verified the CoolCalc calculated whole house load against (wintertime only) fuel use on the fossil burner? See:

    https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new

    Unlike CoolCalc fuel use is a direct measurement, and includes all of the standby & distribution losses along the way. If the fuel use number is lower than the CoolCalc the latter is still useful for estimating the room loads- just scale them by the ratio of the fuel-use calculated total and the CoolCalc whole house total.

    As a reference, the measured heat load of my circa 1923 1.5 story 2400' bungalow + 1600' of insulated (but unfinished) basement is about 33,000 BTU/hr @ +8F now that I've tightened it up some & addressed the most egregious insulation deficiencies. (It still has plenty of know deficiencies still on the "to do" list. Houses are never really "done".)

    A dumb rule of thumb for insulated older 2 story houses with clear glass storms over the windows and doors that shut would be around BTU/hr per square foot of conditioned space @ 0F ( less at +8F) if it's reasonably tight. When it's over 20 BTU/hr per square foot of space as measured by the oil-burner (ignore the CoolCalc for now) there's probably still ample low hanging fruit to be had on the air sealing & insulation front, and it's worth figuring out where that is.

  6. Andris Skulte | | #6

    Thanks for the great replies! You're making me think :)

    Our cape was built in the 1960's, and I'd say it's average construction. We had a home energy audit done 10 years ago, and after addressing the very low hanging air sealing fruit, the blower door was 1610 cfm @ PA50. I forget if that's before or after we had the windows replaced - Harvey double pane vinyl frame argon fill, I think, in the mid-2000's. Next on the list is buying a thermal camera and doing more air sealing - sill plate and band joist in the basement (spray foam likely), basement wall insulation (after backyard drainage fixed), and then knee wall / upstairs air sealing... I've done the spray foam can on all the obvious outdoor penetrations, and will do the basement/inside floor holes and plumbing holes this year.

    Oil use last 5 years averaged 650-720 gallons a year for heat and hot water. Summer time oil use seems to be 11 gal/mo (separate direct fired hot water heater) so backing that out, heating oil use is about 570 gal/year. The HDD65 ranges from 5000-6400 per year, giving a K factor of 9.4 (oil bill has 8.04). 65° - 8°HDD = 57 HDD / 9.4 = 6.06 gallons / day * 137400 BTU/gal / 24 hrs/day * 85% boiler efficiency = 29,500 BTU's when back calculating using my oil bill. 22% less than the CoolCalc Manual J of 38k... If I re-rate the downstairs of 26k * 78% = that gives 20.3k load, pretty much perfect with Dana's recommendation to go with a pair of FH09's.

    The layout of the south bedroom doesn't leave much "good" wall space. It's a Cape, with a knee wall, so the sloped ceiling takes over half of the room, and no wall space on the outside wall to mount it. Blowing at the bed seems like the only place it can go, if I don't go for a ducted system, or a large Fujitsu that could be ceiling or floor mounted. I really like how low the FH06's modulate for that bedroom.

    Dana - With our 8°F design temp, and ASHRAE recommending 1.4x oversize, it seemed like going with a pair of the FH12's downstairs wouldn't have any penalty. The Mitsubishi install manual has the same circuit ampacity for the FH06-09-12, and the indoor/outdoor units look the same, so I'm guessing it's just a larger compressor with the heat exchangers staying the same. Add'l cost is minimal, and efficiency is close if you normalize to BTU's... The biggest difference would be the 1600 vs 3700 min BTU's at 47°F. With both units in large open rooms, it didn't seem like an issue (but I could be wrong). I was afraid of under-sizing on those few days a year that drop below 8°HDD (or a cold snap several days in a row). I can be talked out of going with an FH12 to a FH9, though... But the house will be better as I get around to the honey do's!

    1. User avatar
      Dana Dorsett | | #8

      >"With our 8°F design temp, and ASHRAE recommending 1.4x oversize, it seemed like going with a pair of the FH12's downstairs wouldn't have any penalty."

      ASHRAE's 1.4x oversize factor isn't particularly relevant to modulating systems set at a constant temperature, such as mini-splits or modulating condensing boilers. That recommendation is primarily about the comfort vs. efficiency vs. capacity compromises with fixed output systems. At 1.4x the recovery ramps from overnight setbacks aren't tediously long, and the duty cycle is still high enough during cold climates to not be too annoying/uncomfortable.

      With modulating ductless mini-splits 1.4x oversizing can end up with comfort problems during the shoulder season due to the cooler output temp and cycling. The minimum output of an FH12 @ 47F is 3700 BTU/hr, more than TWICE the minimum output of an FH09. The whole-house load @ +47 is going to be 10-11K, but it's not evenly distributed. With FH09s they will still be running pretty much continuously even when it's in the 50sF, but FH12s may start cycling even in the low 40sF. A ductless head that isn't blowing won't be throwing any air into the adjacent spaces no matter how well places, and room to room temperature differences can rise when cycling. When in doubt go smaller, even if you think it's marginal.

      Check out Nate Adams' comments on sizing vs comfort in his short instructional videos:

      http://www.natethehousewhisperer.com/home-comfort-101.html

      http://www.natethehousewhisperer.com/hvac-101.html

      http://www.natethehousewhisperer.com/hvac-102.html

      Some of the oil-burner's output heat went to jacket & distribution losses into the basement and "The Great Outdoors" from the room to room pressure differences generated while running. With typical ducted air system those losses account for ~10% of the fuel use, often more than 15%. Without needing to support those parasitic losses the heat that needs to be supplied by the ductless is lower than the ~29.5K derived from the fuel use numbers. The basement will run somewhat cooler (= less loss from the basement), and there won't be air handler driven air infiltration. If it was only a 10% parasitic loss that means the whole-house load is about 26.5K. The ratio of adjusted-measured 26.5K to the CoolCalculated 38K is then about 0.7x.

      The CoolCalc load for the first floor came in at about 25.5K, so the real first floor load is probably about 0.7 x 25.5K = ~18,000 BTU/hr. At a 70F-8F= 62F delta-T that means the first floor load grows by about 18,000/62F= 290 BTU/hr per degree F. So at 0F outside (a 70F delta-T) the load of the first floor is about 290 x 70F= 20,300 BTU/hr. Without even looking up the capacity tables a pair of FH09s should be able to deliver that, given that they deliver 21,800 BTU/hr @+5F. The crossover point at which they might begin to lose ground against a 70F indoor temp is probably about -2F, and by -5F they might "only" be able to hold the line at 68F indoors if it dwells at -5F for half a day. When was the last time the high for the day was below 0F? When was the last time it was in negative double-digits?

      If the parasitic losses of the furnace are more than 10% (pretty common) it means you have even more margin. But worst-case it's not a crime to plug in a small space space heater to cover the shortfall in those rare instances when it's not going to reach as high as 0F outdoors for a couple of days, or to hang out and binge-watch K-dramas upstairs in the bedrooms where the mini-splits are oversized for the zone loads. And if you continue to work on the building envelope efficiency the load will drop, and the margin will increase.

      If the heat was delivered by a 3x+ oversized hydronic boiler without a heat-purging controller the parasitic jacket losses and flue losses will be more than 15%. See the annual efficiency column in Table 3, p.9 (p14 in PDF pagination) of this document:

      https://www.bnl.gov/isd/documents/41399.pdf

  7. Andris Skulte | | #7

    Akos & Aedi - The upstairs heating load of 12k * 78% = 9360 BTU. The cape style house has a flat roof dormer for 1/2 of the second story, and a very small attic (no access currently) in the centerline of the house. I could vertical mount something and steal some bathroom space for a utility nook, or put an access hole into the ceiling - Not sure if that's worth it for air sealing and insulation. Literally no room to move around the equipment, so all access would be from an opening next to it. We only use the south bedroom & large closet (toddler room), and the north bedroom is for guests a few times a year, so we liked the idea of leaving that room unconditioned unless needed. I wonder what the minimum modulating ranges on a slim duct system would be, so we're not getting blasted in the shoulder season. I don't have enough load for in-ceiling units for each room, but a ducted system could split the load.

    I'm not keen on the thought of adding ducts and ceiling penetrations into an unconditioned attic, though...

    On edit - Thanks for the pictures, Akos! I see the ducts are inside the conditioned space. Our upstairs ceilings are 90" high, and I'm 6'3", so losing headroom would have to be minimized. Losing 6" would be acceptable... Not sure if it'd be weird walking under a <7' ceiling when going into the bathroom - that's the logical place to run the duct, N-S axis of the house (and avoid it being in the hallway...)

  8. Akos | | #9

    The turndown on ducted unit really depends on the outdoor unit they are connected to. With a multi-split, the turndown is not that great but if sized properly for the load, they work quite well. I would try to get some of the downstairs rooms onto the ducted unit though. A slightly larger unit costs little extra and would save you an extra head downstairs.

    In condo land here with 8' ceiling height it is common to drop the bathroom ceiling by 6"-8" to accommodate plumbing/ducting. I'm 6.4", I notice the ceiling is lower but I never find it annoying. With your 90" celing it will be a bit worst though, so maybe try to mock it up and see what it feels like. If the bath doesn't work, try either a horizontal or vertical mount in the closet.

    What I find best is to keep the ducts/hvac above the door as you enter. Because of the height of the door header, your eyes miss the loss in height, most people don't even notice the ducts unless specifically pointed out to them.

    I like to run either oval or 4x10 / 4x12 ducts when height is an issue with no framing bellow the duct (just drywall across) . If the ducts are inside the conditioned envelope, there is no need to insulate them, saves you a couple of extra inches as well.

    Getting a ducted unit into an old house is like a jigsaw puzzle, you have to do a bit of figurin' to get it tucked away enough while still keeping it accessible and minimizing duct bends.

  9. Andris Skulte | | #10

    I had a response typed up, then accidentally closed the tab, and lost it all. Doh!

    Dana - Thank you. Didn't know the 1.4 oversizing came from... I've downloaded Nate's 101 guide to read. The oil burner is for hydronic heat, and is an unknown vintage (before 2005 when we bought out house) New Yorker CL3 w/ a 1gph jet. Their current catalog specs show 84.7% efficiency w/ a purge. Not sure if mine has it, or not. With a separate oil fired hot water heater, the boiler is shut off in the summer. Seems like it is close to Unit #3 and Unit #5 in the link you posted (thanks, neat read!), so <1% standby loss. If we keep it for emergency backup, I can have a .6 gph jet swapped in at the next tune up, so it's just be very oversized instead of grossly oversized!

    Also, we have the 2500W electric baseboard in the kitchen my wife uses every morning (no hydronic in there) to take the chill off, and an electric oil filled space heater in the toddler's room when our night time set back from 67 goes to 61... This would make the annual fuel use estimate low.

    Akos,
    If I did go with a compact ducted or air handler mini split setup, creating a soffit behind the bathroom door would work. I'd have grates high on the wall, but then that would be aimed at the bed, same as the wall mounted mini split which would (guessing) have better air mixing.

    I found this https://www.nrel.gov/docs/fy12osti/53352.pdf, and it looks like the page 6 high sidewall example is what I'd have, aimed directly at my head in the bed with the ducted setup (if the duct is in the logical soffited chase in the middle of the house, since I'd like to avoid the attic). I'll see what duct sizes I'll need for each room's heat load, and figure out how to lay that out, along with the returns...

    Thinking out loud - I'm strongly leaning towards the wall mounted mini split, which seems a lot simpler than a ducted setup, even with the low-ish room loads and worse distribution. Cost seems like it would be more expensive if I go 12k ducted with an air handler instead of two FH6 single zone wall mounts. Ducting both floors with one air handler would be a pain in our house, and the FH09's/12's downstairs should allow better modulation than a single zone air handler with a hot kitchen and cool bedroom scenario...

    Stephen's reply about not feeling the draft makes me more comfortable with this idea.

    Dana - You mentioned in the first reply to not do this. Seems like vent louvers can be adjusted left/right to have a center dead spot where our bed would be (which duct louvers could do too).

    I think I need to visit someone with an FH wall mount to see what it's like. And mock up the ducts to see the impact... 4x10 with bare drywall wouldn't be bad. For the temp differences, I could always do a transfer vent if it's extreme, but I'm thinking with doors open, heat rises, and upstairs would be fine if we did not set the thermostats back....

    Time to get off the pot (wife is getting impatient w/ my deliberations!) and make a decision. :)

    Thanks a lot for the help, guys! I'm learning a lot. And questioning more, the more I learn!

    1. Akos | | #12

      I have a wall mount 9k unit in my bedroom and to get it comfortable I had to do a some DIY mods on the flow guides. The minimum flow on mine is 200cfm which is much higher than the FH6, so maybe the FH6 would work much better.

      If I could go back in time instead of a 3 head multi split, I would install a single ducted unit. It would have been way cheaper, more comfortable and more efficient.

      Not sure what costs you are looking at, up here in the great white north, the cost of a wall mount is about the same as a slim duct. Each zone is usually $1.5k in materials alone, so eliminating a wall mount should save money.

      People are always afraid to open walls, thus the tendency to go with wall mounts. With drywall, I can open a wall, run duct close the wall and put on 2 coats of mud before the end of the day, paint the next day. Key is to use an oscillating saw and cut the drywall right in the middle of the stud. This way the piece you cut out can be put right back in when you are done. Bit messy, but so is all construction work.

  10. User avatar
    Dana Dorsett | | #11

    A 1 gph jet is about 115- 117, 000 BTU/hr clearly 3x oversized or more for your actual loads, so a good chunk of the fuel BTUs is being dumped into the basement as standby loss. A heat purging boiler control usually has LED or LCD displays and programmable parameters- if you don't see that on your boiler you don't have heat purge, but the steady state efficiency is still in the 84-85% range when freshly tuned. (The burner tech may have written the combustion efficiency on a tag during the last tune up.) Not all boiler can be down-fired to 0.6 gph- read the specs carefully, and usually the steady state efficiency drops a bit when down-fired but average efficiency rises due to the higher duty cycle.

    Unless you're running the space heaters at a high duty cycle it's not going to affect the fuel use load calculation. If it's an hour or two in the morning to take the chill off it's not a lot of heat. The somewhat higher duty cycle of the heater in the toddlers bedroom may have some affect, but it's still a very small relative to the oil used by the water heater.

    If you mount the FH06 opposite the bed you'll probably have to figure out empirically which direction delivers the best comfort. Continuous side to side sweeping or the iSee function modes aren't likely to be the most comfortable for sleeping, even though they're better for mixing and distribution. In the typical Cape bedroom there isn't much stratification and mixing problems anyway, so just keeping it from blowing on your face is probably going to be enough. Setting it to only run the blower at low speed would help too, even if that lowers the as-used efficiency a bit at the outdoor temperature extremes. (Even set up to modulate the blower it will likely be at LOW 95% of the time anyway.)

  11. FluxCapacitor | | #13

    FYI: If I recall right the FH06 only allows ALL up/down & side/side vane settings in cool mode, while in heating mode the vane settings flexibility is dramatically reduced to just a partial control. For example, in heat I don’t think you can aim one side down and the other side up.

    You might want to confirm with Mitsubishi what vane settings are available in heat mode since this sounds like it could be critical in you application.

    I noticed the same with the MLZ cassettes...only partial vane control in heat mode.

  12. Andris Skulte | | #14

    It's been a month since my HVAC contractor friend and I installed the four single zone mini splits (Mitsubishi FH09's downstairs in the kitchen and living room, and FH06's in the 2 upstairs bedrooms). Install went smoothly, and only one bad flare from the premade line-set (as well as the warehouse delivering the wrong lineset sizes which were unlabeled, and installed before we noticed when making the connections!). I ran the linesets for the two first floor units, fished inside the walls. My contractor buddy was hesitant, so I did it while he was away. Install looks much tidier on the outside, since there is only one set of vertical line hide's on the wall (Note, if anyone uses the much cheaper Pioneer line hide, replace all the exterior screws with stainless, so you don't get rust 2 months later... I think it was #8 sheetmetal 3/4" I used...)

    I've been thrilled - The wall mounted opposite our bed hasn't been blasting cold air at us with the vanes adjusted to avoid the bed, and looking at the IotaWatt power monitor, you can see it cycling from 20W to 150W for a minute once it hits the cooling set point at night. It's very comfortable. Running the downstairs mini splits for the living room and kitchen have kept the house super comfortable, even during this summer's heat wave. Even the rooms that don't have the units have been comfortable when the door is left open during the day.

    Cooling is a huge improvement over the larger window units that we replaced. We'll see what winter brings!

    For anyone on the fence, I wish we did this sooner.

  13. User avatar
    Dana Dorsett | | #15

    Thanks for the update, Andris- have any pictures to share!?

  14. Andris Skulte | | #16

    Dana,

    Here ya go! I think it came out nicely, and the outdoor units with two on either side of the house blend in well enough. They are barely visible from the street, and the wall mount should mitigate the ground snow issue. Minimal vibration into the house, and the indoor units are very quiet (the little electric eye moving is the loudest part!).

    The units were mistakenly ordered without pan heaters, so I'll see how that goes in New England. Worst case is I'll be out there in 0 degrees swapping them in, and swearing at myself for not doing it this summer!

    Last step this weekend was slicing the rubber insulation on the lineset, pulling it apart 2", and spray foaming the penetrations into the house. Too bad the spray foam can's nozzle broke off before the first use! Amazingly, I didn't get too messy, other than the latex gloves.

    I was thinking about writing an article about this too, if there's any interest (from a DIY homeowner's perspective).

    Thanks again for your help and guidance.

    1. User avatar
      Dana Dorsett | | #18

      It's good that the outdoor units are mounted above the snow depth line- it's amazing how many installers treat it as if it were just an air conditioner.

      How are the sound levels coming through the wall during the spin-up for those bracket mounted compressors?

      The head-banger interference isn't too bad upstairs. Somehow I had envisioned lower ceilings that that.

  15. Jaccen | | #17

    I would certainly be interested in a homeowner's DIY article.

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