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Minisplit setup for New Jersey, 750 sq. ft. house over crawl. Bouncing my HVAC contractor’s plan off you guys.

sayn3ver | Posted in Mechanicals on

The short. I have a 25×30 rancher over a currently vented crawl. It sets almost perfectly south facing on the 30ft long side. No tree shading on south or west. Deciduous tree shading in the east and some on the north side. It’s two bedrooms one bath, kitchen and living room. Standard stick frame 2×4 construction and vinyl siding. House was built in 1963 in a post WWII development.

Bedroom1: 10×12
Bath: 5×9 rough estimate
Kitchen: 9×11
Living: 14×14 rough.
Short hallway off living room with bedrooms at the end: 4×7

Living and kitchen I would treat as open concept/one room as there is no physical barrier or wall between them.

Short term goals: roxul comfortbatt r15 walls, roxul comfort batt r45 ceiling (r30 between joists and r15 ran across tops of ceiling joists in a vented attic). Then address the crawl space grading the yard the weeping tile, sump and converting to a closed crawl with foundation insulation.

Long term goals. If we stay put we plan to rip the roof off and go up for a master suite and out one side for an eat in kitchen. If we to this route I will be leaning towards doing a remote wall or persist treatment to the home along with upgrading the windows and doors.

Anyways I used a free online manual j and come up with roughly 30,000-34,000 btu/hr whole house requirement. But I’m an electrician not an hvac guy by trade.

My hvac contractor is suggesting a multi split unit for the bedrooms…two 6000btu heads on the smallest mitsubishi multi compressor they make which I think is 20k btu. He is also leaning towards a 15,000btu single zone for the living roomand kitchen area.

My concern is with the bedrooms. Mostly. I feel that the 20k compressor for two 6k btu heads is excessively oversized and the minimum demand for that outdoor unit is roughly 7,800btus. Which means those heads will only be able to throttle to roughly 65% to meet minimum demand.

I can post a floor plan later if it will make this a more productive conversation.

I was hoping to use a higher seer single zone 9 or 12k btu wall or floor model in the tiny hallway which connects the bedrooms and bathroom.

The current AC compressor is a 2 ton unit that is quite vintage. I don’t see why hitting somewhere around that number with an equivalent mini split offerings wouldn’t work. The heater is like a 90,000btu gas air handler from like 1970/80 which. Even with the ductwork removed from the bedrooms now ( in the middle of renovating) the three remaining registers in the bathroom, livingroom and kitchen keep the bedrooms comfortable.

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  1. Expert Member
    Dana Dorsett | | #1

    30-34,000 BTU/hr @ +10-15F for a 750' house with R15 2x4 and R45 attic means you have zero foundation or floor insulation in that vented crawlspace, and sleep with the single-pane aluminum framed windows cracked or something (?).

    That's 40-45 BTU/ft^2-hour , which might make sense if this house was Fairbanks AK or something, but not for New Jersey. With clear-glass double panes or clear glass storm windows a more typical BTU/ft^2 ratio would be about 20BTU/ft^2-hr, assuming no foundation insulation. With an insulted foundation and some air sealing it could be under 15 BTU/ft^2-hr. I suspect the Manual-J has some gia-normous air infiltration number skewing the result. But if you have a gazillion yards of single pane glass, maybe it's that high.

    In your location I'd strongly consider insulating & air sealing the crawlspace, and installing an appropriately sized Mitsubishi SUZ/SEZ dedicated mini-ducted system to manage the bedroom zones with the mini-duct cassette located in provided the duct lengths could be made appropriately short. The -GE06 heads have a heating capacity of 7200 BTU/hr, which is below the min-loading of the 20K compressor- it would not modulate.

    But get the Manual-J right first, (for the "after" picture of your upgrades) using realistic 99% outdoor temps (not the low during the polar vortex events.) 30K is nuts- even 20K is probably too high. A 3/4-1 ton mini-duct cassette with a dedicated compressor would probably handle the bedroom zones just fine, and a 3/4-1-ton wall-coil mini-split like the -FH09NA or -FH12NA would cover the bigger open zone.

    The SEZ/SUZ series 1-ton puts out about 10K @ +17F:

    The 1.25 tonner puts out about 13.5K:

    The output can be split between 2-3 rooms, as long as the duct runs are very short.

    The floor plan, along with the (re-) calculate room loads would be useful, as would a ZIP code, to assess the outside design temp requirements.

    When building a high-performance addition onto a house like that the whole house heat load can even go DOWN, even though you're increasing the size of the house, since you're replacing some lower-performance window & wall area with considerably higher performance window & wall area.

  2. mackstann | | #2

    I'm in Portland OR, perhaps a tiny bit warmer than NJ but not a lot warmer. Same climate zone (4) I believe.

    I have a 900sqft house, uninsulated/unheated basement (stays about 50F in winter), and before we had some improvements done, we had uninsulated walls and about R-25 of fiberglass in the attic... we heated the upstairs with a single 18,000 BTU Mitsubishi Hyper Heat mini split.

    30,000+ does seem like way too much. Multiple units, while perhaps nice for achieving more consistent temperatures between rooms, are not strictly necessary. Leaving doors open at night helps.

  3. sayn3ver | | #3

    I forgot. The zipcode is 08052.

    Ill draw up a floor plan tomorrow after work and post it.

  4. sayn3ver | | #4

    Thanks for the replies. The manual j I did was done as the house stands now, which was double pane vinyl replacement windows that were done before we bought the place.
    . The manual j was also done with no floor insulation. It's a vented crawl, with only 3/4" tongue and groove oak over the floor joists. No sub floor and no insulation. The current walls before my renovations were a poor attempt at blown in loose pack? cellulose.

    The window and door areas are average. There is a total of seven 2x4' windows total and one original wood frame double pane picture window which is roughly 5x4 with a 1.5x4 double hung on either side. I'm not at the house now to verify dimensions. Ill provide accurate dimensions on the floor plan. The front and rear doors are both 3x80" solid core wood doors with poor weather stripping/sealing with I'd say 20-30% single pane glass windows. The front door has a brass mail slot that has been taped up by the previous owner for what i imagine is draft stopping.

    Eventually the windows and doors will get upgraded but I cannot give an accurate time line as to when.

    I'm tearing walls open to inspect and repair any termite damage. While I do that I'm upgrading electric (as I am an electrician) and insulating. I am opting for air tight drywall because its my understanding that it would be vapor open enough if we ever do an addition and we do a deep energy retro with a remote or persist envelope.

  5. user-2890856 | | #5

    Lance ,

    Don't proceed with this HVAC contractor , he obviously has not a clue . Your home requires no more than one compressor . The added fact that your heating and cooling loads are more than likely 150% too high is concerning . What area of Jersey are you located in ? If you would like , contact me ( [email protected]) and I can share the name of a quality contractor whom knows enough not to be a detriment to your home and wallet .
    I honestly cannot see your house having a higher heating load than 12,000 BTUh , probably lower in all reality . You would probably be better served with a ducted system properly sized and designed by someone other than whom you have been speaking with .

  6. sayn3ver | | #6

    Quick layout. Almost to scale.

  7. sayn3ver | | #7

    Does anyone else have any recommendations or ideas?

  8. exeric | | #8

    As others have said the HVAC guy you have may not be up to the challenge. Even if he is competent it may be too early to use him. I would advise doing all the insulation and air sealing work before doing an HVAC retrofit. You'll then get a much better fit to your actual BTU needs.

    Another thing to think about is the fact that it is a fairly small house. To me it seems a waste to install a ducted forced air system for such a house. Ducts always imply large friction losses just because of the restricted diameter of the ducts. Add to that the fact that ducts going to all locations usually requires at least some ducts in unconditioned air spaces so you'll have losses their also.

    A better plan is to use the house itself as the ducts. In such a situation you would replace long ducts with 4 inch thick vented openings between rooms. Also this "duct" would be within the conditioned air space of the house so you'll have less losses there also. From your floor plan you might be able to get away with a vent in the wall between the living room and Bedroom 1. In turn you will have a small 3 or 4 ft long generous diameter duct in the smallest square (whatever that is) dividing Bed 1 and Bed 2. Finally, put a vent in the wall between the 2nd bedroom and the bathroom. Then use a bath fan in low volume ventilation mode to pull air from the living room into Bed 1 then into Bed 2 and finally out through the ceiling in the bathroom.

    The advantage of a system like that is that you can get away with a highly efficient low volume fan because of the mostly room size "ducts" and also having all air moving in conditioned space. There is one caveat to this: you have to have fairly air tight construction, which it sounds like you are attempting. That means no undercut doors in both bedrooms and bathroom, and keeping them mostly closed. Of course the obvious goal of a plan such as this is only needing one heating/cooling source and still keeping the bedrooms warm.

    EDIT: I think I should add that this is an experimental plan as I haven't seen it suggested elsewhere. But I can't see why it wouldn't work and I'm putting my money where my keyboard is because I'm in the process of attempting it on my own house.

  9. iLikeDirt | | #9

    I see a perfect place for a ductless mini-split cassette: right between the two west windows on the exterior wall. It will be looking right down the hallway leading to the two bedrooms, so air thrown that way should make it in there, but it will be blowing into the kitchen and living room as well. Everyone else is right about focusing on your insulation and air sealing before doing the HVAC project. Get that load down as low as you can!

  10. exeric | | #10

    Go with Nethaniel's plan for the placement of the heat pump. Then just see if you get enough heat to the bedroom's without any additional construction. If it doesn't work well enough for the bedrooms then just make sure you have made provisions for my idea, i.e. no electrical wiring etc. where inter-room vents would likely be placed in conjunction with a bath fan.

  11. sayn3ver | | #11

    In order to finish the rest of the project I need to remove the rest of the supply duct that is coming in below the ceiling joists above the tub and in the hallway ceiling (7ft ceilings).

    I don't want to do that until I have the new system installed.

    The space between bed1 and bed2 are closets. I forgot to label them.

    I was given advice off like from a member here suggesting a small forced hot air system in the crawl and add mini splits later if I do an addition.

    My problem is here is my build timeline and budgetary thinking:

    Finish bedrooms and bathroom and begin moving in while I gut and finish livingroom and kitchen.
    Actually start living here.
    Convert open to closed crawl.

    I do not have the budget, time or space to do it all at once. The use of two mini splits was going to allow me to compartmentalize the build into phases. Or so I thought. I was figuring on my relatively small footprint to working in favor of the mini's but I am learning that's not the case.

    I was given advice off the board from

  12. Expert Member
    Dana Dorsett | | #12

    If you have a copy of the Manual-J, you'll probably see a huge infiltration/ventilation load as well as a huge conducted heat load related to your vented uninsulated unsealed crawl space. It would not surprise me that between the infiltration and R1 plank floor the floor was literally HALF of the total (calculated and real) heat load of that house.

    Size the equipment for the "after" picture of the heat load, which I would hope includes a sealed-insulated crawl space, or at the very least putting some Tyvek on the underside of the floor joists (with cross-furring 16" o.c. to support the housewrap) and packing the joists full of fluffy stuff. At that point Richard McGrath is right- there's no way it could be more than about 12,000BTU/hr total @ +15F. (That's 16 BTU/hr per square foot of conditioned space). It might be under 10K.

    From the floor plan it looks like a single wall-coil head could manage the kitchen & living room load, with a min-duct cassette located in the top of the hall closet splitting the output between the bedrooms (and maybe the bathroom too, though it can probably "coast" just fine, with maybe a 300 watt cove heater or something if it can't. Without doing the math I suspect it's a 3/4 ton head for the main open space (which has enough capacity for the whole house), and a 3/4 ton mini-duct cassette for the rest. Either way, you're looking at a 1.5 ton 2-cassette mini-split, max..

    But run the heat load calculation before committing to an approach. The above is a WAG.

    For heat load calculation purposes, using any design temp lower than +15F (Philly's 99% bin) would be a mistake, given your proximity.

    If you want to try it with a single head, a 1-ton in the open space, with a modest amount of cove heater or electric baseboard to back up the bedrooms would work. As long as you're not oversized on capacity by a factor of more than 1.5x the oversizing would deliver somewhat higher efficiency with most mini-splits. With a pretty good 3/4 ton wall-coil you're probably covered, but with a 1-ton like the MSZ/MUZ FH15NA , which delivers over 13K @ +17F or Fujitsu AOU 12RLS2 you would have some margin:

    If going with the 2-head approach the SUZ mini-duct cassette will deliver 6700 BTU/hr @ +15Foutdoors, 70F indoors, which split down the middle probably covers the bedroom loads nicely. See the capacity tables on p.26:

    The ~ 1.5 ton MXZ-2B20NA dual zone compressor will support a KA09 mini-duct + FE09 head (or FE12) combination, and you'd probably be good down to negative single digits for outdoor temps.

    But do the numbers with a pretty good calculator, not all online heat load calculators are up to snuff. Worst case, build an I=B=R spreadsheet (or download the Slantfin or somebody else's IBR tool), which will overshoot reality by 15-25%, but not by 150%.

  13. sayn3ver | | #13

    I'm back.

    Is it worth putting up the $50 for hvac-calc ( for a single home calc?
    I tried using the fast-calc spreadsheet and got similar numbers to the ones ran by rich.

    I know this seems redundant and cycling back to the beginning but would a single zone mitsubishi hyper heat 9,000 unit be way to big for each bedroom? They have a minimum heating loads of 1600btus and 1700btus cooling. If this route I would probably go with 3x mitsubishi MSZ-FH09NA units, one in each bedroom and one for the living space.

    If not I'm still leaning towards a 9000btu floor model by fujistu (9rlffh) for the hall and a 9-12k btu wall unit for the living space.

    I'd prefer mitsubishi to fujitsu. In this area from the contractors i spoke with say mitsubishi has better support and parts. The ability to use the redlink thermostats with the mitsubishi's and integrate the supplemental resistive heat with redlink as well is nice. (The honeywell econnect thermostat for the resistive supplemental and the mhk1 units for the splits).

  14. exeric | | #14

    Lance, you are overthinking this. For a 750 sf home you only need 1 minisplit. Get GBA prime and read this article:

    Place the mini where it will shoot down that short hallway. It would be extremely silly to buy two extra minis that would always be short cycling just for a temporary situation. Why are you still thinking like this? If you need temporary heat in the bedrooms during construction then get plug in heaters at your local Kmart for that.

  15. sayn3ver | | #15

    I read the article. The article says single point heating with mini's is possible and had some 1100sqft/mini split suggestion.

    My concern is I am an electrician and not an hvac contractor. The hvac contractors I have spoken with locally either have said that mini's won't work at all or they grossly oversize them. I find that hard to believe since NJ, while being a northern climate is still rather tame in terms of average winter temps.

    My concern also stems from the uncertainty of the home long term. Right now I am fixing up the existing footprint so we can move in snd get on our feet and started with our new life. 5 years from now if we decided to stay we may do an addition doubling the square footage and doing a deep energy retro to it. Possible pv solar too if the numbers work out 5 years from now.

    I'm sure a single 12-15k btu mini would work as the house stands as is (2x4 walls with r15 batt, 2x8 ceiking with r45 batt and r10-r15 crawl foundation insulation) for all but maybe the rare super cold day. But I'd like the possible redundancy of a second unit in case of failures. I'm also concerned with cooling during the summer. My wife likes to sleep with the door closed.

  16. Dana1 | | #16

    An FH09 has a max output more than 5x your likely heat load for a bedroom, which is just plain ridiculous, and your AVERAGE winter load would be barely above the minimum. The air volumes moved by an FH09 head even at it's lowest speed are pretty high for a 120 square foot bedroom.

    Even a 3/4 ton mini-duct cassette splitting the output between the bedrooms would be overkill, but at least it would modulate.

    If you're looking at it from 5-year plan point of view muliple mini-splits would be ridiculaous. A single head mini-split head in the main area, and some cove heaters on a wall thermostat (or $50 oil filled electric radiator type space heaters) in the bedrooms should it not quite keep up during cold snaps is cheap backup.

    Then if/when you build out the expansion do the heat load calc on it before committing to a solution. If you are doing the additional space as air-tight high-R roof and wall assemblies and using decent windows the heat load can even be SMALLER after the build than the current heat load, which would make having multiple FH09s even more silly.

  17. sayn3ver | | #17

    I understand now.

    For me to size the main mini split, I would base my decision on a manual j on the house as a whole and not a room by room?

    For me to size supplemental/back up heat sources like some hardwired radiant electric panels, do I just throw 400-500w per room randomly? Or do I need to install strip base/ electric cove heaters sized to carry the full heating load?

    Is there a software solution that isn't $500 that is worth purchasing for me to acquire an accurate manual j?

  18. iLikeDirt | | #18

    I also didn't want to spend the $500, so I started with and then slowly made my own Manual-J-ish spreadsheet that I idiot-checked using the above and various other sources. It may not be perfect, but it's perfectly suitable for my needs.

    The better your envelope, and the smaller your house, the less any of this really matters at 750 square feet and shooting for a good envelope, you're going to have nothing to worry about. A 500w electric heater will put out 1,700 BTUs, which is just about perfect for supplemental heating in a bedroom that's only indirectly conditioned by the main unit. And heck, you may discover that you don't even need it at all. Personally I'm fine sleeping in a cold-ish bedroom; that's what all the covers are for, right?!

  19. Expert Member
    Dana Dorsett | | #19

    There are reasonable freebie online tools like the one Nat.G recommends, or you could download Slantfin's"Hydronic Exlorer" I=B=R methods tool (which will overshoot reality by at least 15% but it's not terrible.)

    If you tell us the construction type and R values so we can calculate or look up the relevant U-factors, it's easy to build your own I=B=R spreadsheet using standard spreadsheet tools. With a +15F outside design temp and an 70F indoor design temp, that's a 55F delta-T. Without resorting to optimistic interior & exterior air film performance, the U-factor of 2x4 16"o.c. R11 construction is about U0.10 BTU hour per square foot per degree F. With it's R15 it's more like U0.08 or slightly lower. So if you measure up the total area of the exterior walls the wall losses are:

    Square feet x 55F x U-factor= BTU/hr

    The U-factor of clear glass double panes, or clear storms over wood sashed double hungs is about U0.5. A code-max replacment window is U0.34, but U0.28 is not an insanely expensive upgrade. Solid doors are also about U0.5. Add up the square footage of windows & doors on a room by room basis and list the window & door losses too.

    An R19 attic has a U-factor of about U0.055, but with R45 you're looking at about U0.0.025

    So let's take a WAG on how Bedroom 1 pencils out, assuming 9' tall walls, and with what appears to be about 20 square feet of window at U0.34, and an R45 attic:

    Wall area: (10 +12) x 9' = ~200 square feet, less 20' of window leaves 180 square feet. Wall losses are then:

    180' x U0.08 x 55F= 792 BTU/hr.

    20 square feet of window at U0.34 is:

    20' x U0.34 x 55F= 374 BTU/hr.

    The attic is 120 square feet of U0.025, for

    120' x U0.025 x 55F= 165 BTU/hr

    If you insulate the foundation wall or the floor, add 20% to the wall loss number to get the floor losses in the right ball-park, so for now multiply x 1.2 and you're at 950 BTU/hr. If you don't insulate the floor and/or the foundation, the floor losses will be as high or higher than the rest combined.

    So the total room losses are looking like 950 (walls) + 374 (windows) + 165 (ceiling) for a whopping ~1500 BTU/hr.

    Add a 15% multiplier fudge factor for infiltration/ventilation losses and you're at 1725 BTU/hr.

    From that subtract 250 BTU/hr for one sleeping adult human and you're back at ~1500 BTU/hr. A 500 watt space heater/cove heater/ electric baseboard/ panel radiator would more than cover the full load, as Nathanial suggests, but if you just leave the door open to the common area with the mini-split and use an electric blanket when it drops below 15F you'll be fine. If you need to demonstrate to the code inspector that the room can be heated to at least 68F at the 99% outside design condition, a hard-wired 500 watt heater with line voltage thermostat is pretty cheap to install, but that's not to say you have to actually use it.

    That's 1500 BTU/hr for a 120 square foot space, or about 12.5 BTU/hr-ft^2

    Do similar calculations with the real window U-factors and wall/window areas and see what you come up with. The 2 bedrooms combined, along the closet spaces add up to maybe 3500 BTU/hr if you insulate the crawlspace. The rest of the space has more window area for a higher BTU/hr-ft^2. Assuming it's 15BTU/hr-ft^ for the whole 750' you're at 11,250 BTU/hr for the whole shebang.

    And this is even using pessimistic U-factors for the walls & ceiling. The average performance in calm weather will be at least 10% better, and your plug loads like refrigerators and DVRs will also peel off some call it 10,000 BTU/hr.

    An -FH09NA can actually cover that load, but it's not overkill to drop in an -FH12NA, which is slightly oversized, and would be in the sweet-zone range of oversizing where it always modulates at part-load efficiency, but doesn't spend much time doing on/off cycling.

    Note that at +15F the FH09 would have to run near it's max speed, which delivers 12,200 BTU/hr while burning 1440 watts. Normalizing BTU/hr to watts, that's 12,200/3.412=3576 watts of output, for 1440 watts of in put, a coefficient of performance of 3576/1440= 2.48, which is OK, but not great. That would bring the seasonal average COP down into the low 3s. With your load the FH12 would be modulating at a COP in the high 2s to nearly 3 at +15F, and deliver an even higher seasonal average COP.

    The -FH15 would also be in the sweet-zone, and would give you substantial margin for polar vortex events:

    Note, that at +17F at it's "rated capacity" (which is different from it's max capacity) it's basically covering your entire load, but using only 1020 watts to deliver the 11,000 BTU/hr. Normalizing BTU/hr to watts, that's 11,000/3.412= 3224 watts of output, for 1020 watts of power use, which is a coefficient of performance of 3.16. That means your average COP efficiency would be over 3.5 with that unit, whereas with the FH09 it would be closer to 3.0, since it nearly maxes out during cold weather, at COP of about 2. With the FH12 you'd be running a COP a bit less than 3.0 at +15F, but 3.5 or better during your average winter weather, so it too would average

    Run your own numbers, sanity check it against Nathanial's favorite online Manual-J calculator if you like, but it's looking like a the 1-ton or 1.25 ton unit would deliver the best overall efficiency and would still cover the load of an expanded house, provided the floor plan was sufficiently open. If the crude heat load done as above is under 10K, the FH12 might be a better fit than the FH15, but neither would be criminally oversized the way a 6000 BTU/hr head would be for the 1500 BTU/hr peak load of a bedroom. It's also not a crime to add a second mini-split or mini-duct type mini-split if that's a better fit when you expand the house.

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