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Mini split btu vs. electric baseboard btu

nick14z71 | Posted in General Questions on

if I have a room with a 2000 watt electric baseboard heater and 1kw=3412btu’s, then I would have about 6824 Btus in that room, correct? Now that would be if the baseboard unit ran for an entire hour straight? Which it never does, but I would assume I could heat that room with a 9k head easily as the Mitsubishi’s systems put out about 10,600 at 5F and fall off a little from there down to -13. I would be able to heat the room with a 6k head, but I think it would run out of heat around 0 or so. I’ve done a few manual J’s (or tried to at least) as I consistently keep getting different numbers from every HvAC tech I call. 

Also, if I put in a mxz3c30k hyper heat, which puts out 28600btus at 5F, then all 3 heads share that load, or if 2 heads are calling for heat and 1 is not, then do the two get a bigger share of what they could use? I’ve seen that if I marry three 12k heads to the 30k hyper heat, the submittal sheet shows they put out 9500btu’s. At what temp is that? 5F? Again, I think of two heads are calling for more heat, then they may get 11,000 and the other one could be 6500?

Nick

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Replies

  1. Expert Member
    Peter Engle | | #1

    Others can give you the specifics of the hyper heat unit, but I'll start with a bit of terminology and clarification.

    With heating and cooling appliances, we don't generally use "Btu's." We use "Btu/hr." So, your 1kw baseboard puts out 3412 Btu/hr. If that baseboard heater runs for 1 hour (using 1kw-hr of electricity), it will have added 3412 Btu of heat to the room.

    If the actual 2kw baseboard is able to heat the room at your design temperature, that means that your room needs no more than 6824Btu of installed heat. But, as you say, it could be rare for the electric baseboard to actually run flat-out. If it cycles on an off at your design temperature, then the room requires less than 6824 Btu/hr. The 9k head would probably be overkill, putting out 10,600 Btu/hr at 5F. How many hours do you spend below 5F each year?

    You talk about using a 6k head, but worry it will "run out of heat" around 0F. This is not true. The hyper heat systems have rated specifications far lower than that. Worst case, a heat pump system will "only" put out as much heat as a baseboard unit using the same amount of power. This equates to a COP of 1. Whether or not the 6k head will heat your room depends on the room's heat loss at your design temperature and the 6k head's output at that same temperature.

    As far as the heat sharing between the heads, this is a function of both the compressor size and performance, the outdoor temperature, and the actual heads installed. Each combination will have a different output at a given temperature. Perhaps Dana will weigh in with the data. He's much more fluent with this than I am.

    If you give us your location and some information about the size of the room, insulation levels, etc. we can give you better advice. Better yet, share some of the manual J results you've got for us to review and discuss.

  2. nick14z71 | | #2

    My location is NW, Ohio. Zip 43545. I believe my design temp is 4F (which seems high to me, as it feels like we regularly get minus degree weather.) I will attach my floor layout, and can give you either the Coolcalc loads, or the Load calc loads for the 1st floor for my home. One gave me a heat load of 22k and one gave me a heat load of 28k for the 1st floor.

    I realize that the 6k head will not "run out of heat", I chose a poor choice of words, sorry. But I meant what you stated, that it will not keep up with demand.

    I've had 3 HVAC tech.'s (who are ALL diamond certified Mitsubishi installers) quote me installs for the whole house and partial house separately. I was going to "start" with a system that would cover just the Living Room, Kitchen, and Upstairs Bedroom #3.

    1st installer quoted me MXZ-4c36NA2 non hyper heat, GL-12k in living Room, GL-12k in Kitchen, and GL-09k in upstairs bedroom. He used "HighSeer" to calculate the room loads.

    2nd installer quoted me MXZ-4C36NAHZ-U1 with Hyper heat, GL-15k in living Room, GL-12k in Kitchen, and GL-12k in Bedroom 3. I'm not sure of the program he uses, but he told me that he calculated heat loss for the kitchen and living room to be 24,365 at -5F.

    I'm trying to get the specs from the 3rd installer yet.

    I was hoping that a MXZ-3c30NAHZ-U1 with Hyper heat would cover these 3 rooms with a 9k head in each room. Is there a difference in installing three 12K heads on this unit vs. three 9k heads? besides cost. The Mitsubishi submittals show the each head operating at 9500btu no matter which configuration I choose. I assume that is because the compressor only puts out 28600btu at 5f. But there has to be a difference in running the two different configurations. I would assume that if only two heads are calling for heat, they would receive say 10,600 btu at max and the 3rd head could use the rest? The west side of the house is the kitchen and we get some high winds in the winter where i am at. I have no windblock. So if I keep the kitchen warm, the rest of the house does alright also. I was thinking of a 12k head in there and two 9k/s in the other two rooms.

  3. Expert Member
    AKOS TOTH | | #3

    Your house has a lot of rooms, which makes wall mounts a challenge. You need to do a room by room heat load calc to figure out if you can make it work.

    Multi splits have modulation limitation, it takes a lot of digging through the specs to figure out the limits, but generally you never want an over sized head connected to a multi split (over sized for the room load). For sure you DO NOT want a 12k head in a bedroom. In any reasonable construction, bedroom loads are between 1500 to 3000 btu, on a multi split even a 6k head is way over sized and will create comfort issues.

    You are in cold enough climate that you want a hyper heat unit. If you are trying to reduce a bit of the baseboard electricity, the best bang for your buck is probably a single unit (wall/celing/floor, whichever is easier) that can heat both the kitchen and living rooms through the opening between them.

    If you want to do the rest of the rooms, the best is a ducted unit in the ceiling of one of the bathrooms or hallways with ducting to each of the bedrooms.

    1. nick14z71 | | #4

      Well the upstairs bedroom I was referring to is 18x20, has three walls that are exposed to outside air (one side faces west, one east, and the south side is the garage attic.). So it does get chilly up there, not to mention it’s a cape cod so insulating is difficult. I’ve made it better, and tighter. But more can be done.

      What about the 12k heads for the kitchen and LR? I know you don’t have all the design of my house, but would you consider those oversized also at say 0-5f?

      Digging through the modulation limits of multi splits is tough if not impossible. I can even find how low of a modulation a mxz 30 or 36 go.

  4. Expert Member
    Dana Dorsett | | #5

    >"I’ve done a few manual J’s (or tried to at least) as I consistently keep getting different numbers from every HvAC tech I call. "

    That's because you're getting your numbers from an HVAC tech, not an engineer or an qualified architect/designer.

    A half ton FH06 head married to an MXZ multi-split compressor will deliver up to 8700 BTU/hr whenever there is enough compressor capacity behind it.

    http://meus1.mylinkdrive.com/files/MSZ-FH06NA_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf

    Almost NO bedroom would have a heat load anywhere near 8700 BTU/hr @ +3F (Toledo's 99% design temp) unless a window is open, but a fairly large 16' x 18' or 18' x 19' bedroom could have a design heat load more than half that, which if so, makes it not ridiculously oversized for the load. (An FH06 is usually overkill for the loads of 100-200' bedrooms.)

    It's highly unlikely that any single room in the house has a load that would call for a 1-ton head, but if there are big open archways to let it heat two rooms it can sometimes work.

    1. nick14z71 | | #6

      Dana, so where do I begin to look for an engineer or qualified designer?

      >” A half ton FH06 head married to an MXZ multi-split compressor will deliver up to 8700 BTU/hr whenever there is enough compressor capacity behind it.”

      That’s what I’m struggling with. Is what controls how much heat a head can out out. I would assume it’s the compressor. So a mxz3c30nahz-u1 has a specified heat of 28600 at 5f. When it gets to -3 outside, it’ll put out closer to...24,000? Is that enough to keep up with the heat loss of those 3 rooms? I know...run the numbers. Lol.

      But if in the -3 weather that unit can put out 24000, and the kitchen needs 9000 from a 9k head, it will still be able to deliver it of the living room only needs 6k and the upstairs bedroom is needing 8k? I guess what I’m asking is when the temp drops real low, do the heads suffer at all? Or it’s all about what the compressor can put out?

      I’ll google someone nearby for help, or check some listings. I don’t even know where to start.

  5. Expert Member
    AKOS TOTH | | #7

    Nick,

    I think the better first step would be to get an energy auditor in there and do some blower door directed air sealing. A couple cans of spray foam can probably knock your heat load down by 10% to 20%. It will also give you an air leakage value, which you can than use for closer sizing of your hvac.

    The online resources do a decent job of getting you in the ballpark, if you have heating costs you can do an estimation based on past data to double check:

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

    Once you have the heat loads, you can then look at properly sizing both outdoor and indoor units.

    P.S. For multi splits, generally the only time the indoor units de-rate on capacity is if the connected indoor capacity is above the capacity of the outdoor unit. In this case, if one of the zones is not calling for heat, the rest can then put out more heat (up to the max capacity of the indoor unit).

    1. nick14z71 | | #9

      >” I think the better first step would be to get an energy auditor in there and do some blower door directed air sealing. A couple cans of spray foam can probably knock your heat load down by 10% to 20%. It will also give you an air leakage value, which you can than use for closer sizing of your hvac.”

      I have had a “free” energy audit done on my home last year provided by my electric utility in which they did a blower door test. The only data he gave me was “965cfm for your 2800sq ft. House.” My house isn’t that large though unless he’s including the basement.

      I think I’ve bought stock in spray foam and have done one basement sill in the west side along with a few rooms at the base of the wall to the floor. Also outlets and a few can lights. I don’t have many penetrations. More can be done, and I’ll keep working on it as we re-do rooms.

  6. Expert Member
    Dana Dorsett | | #8

    >"Dana, so where do I begin to look for an engineer or qualified designer?"

    https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations

    I strongly recommend using a computer spreadsheet tool to set up and maintain a room by room IBR type load calculation to sanity-check any third party's load calculations. For ductless sizing a careful IBR load calculation is almost always good enough for sizing the heads for heating load, but it doesn't address cooling loads at all.

    https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1

    https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-2

    https://www.greenbuildingadvisor.com/article/calculating-cooling-loads

    Even when done carefully these methods will overstate reality compared to an aggressive rigorous Manual-J.

    A freebie online tool such as LoadCalc.net will also oversize it by double-digit percentages most of the time, even if one assumes a hermetically sealed (zero infiltration) house, but that's still way better than the typical triple-digit oversizing of many (or even most) HVAC installation companies. The cheapie/freebie Coolcalc.com tool is a true ACCA registered Manual-J tool that for most construction will deliver reasonable numbers if you take the time to learn how to use it correctly, and avoid the temptation to be conservative rather than aggressive on R-values/U-factors & air leakage.

    >"So a mxz3c30nahz-u1 has a specified heat of 28600 at 5f. When it gets to -3 outside, it’ll put out closer to...24,000?"

    If the load is 20,000BTU/hr @ 70F indoors, +5F outdoors (a 65 F temperature difference) , the load at -5F (a 75F temperature difference) would be roughly 20,000BTU/hr x 75/65= 23,077 BTU/hr. With any of the MXZ compressors the capacity at -13F will be at least 70% of it's capacity at +5F. So if it loses no more than 30% over 18F of temperature drop, about 0.6% drop per degree-F. So at only a 10F temperature drop would be a less than a 12% hit or about 0.88 x 28,600 = 25,168 BTU/hr

    >" Is that enough to keep up with the heat loss of those 3 rooms? I know...run the numbers. Lol."

    You've got it!

    A really crude rule of thumb (a rule of thumb that's usually meaningless on a room by room basis) would be that tight construction insulated 2x4 framing with clear glass (no low-E) double panes or clear storms over single panes usually comes in between 15-20 BTU/hr per square foot at 70F indoors, 0F outdoors (for the whole house load), sometimes a bit less for 2-story houses with efficient shapes, but the upper floor would still probably be in that range. Even if falling in to the higher end range that would put the load of your ~650 square foot second floor at about 20 x 650 = 13,000 BTU/hr. The ~1100' downstairs load would usually come in at the lower end of that range, or no more than about 15BTU/hr x 1100' = 16,500 BTU/hr @ 0F outdoors, 70F indoors. So for the whole house you'd be at no more than 29,500 BTU/hr for the whole house.

    With 2x6 framing & low E windows it'll come in about 20-25% lower, or about 23K, give or take.

    But the error bars on dumb rules of thumb like that are large (on both sides of the range), even bigger on an individual room basis, which is why it's important to do the math on it.

    1. nick14z71 | | #10

      >” I strongly recommend using a computer spreadsheet tool to set up and maintain a room by room IBR type load calculation to sanity-check any third party's load calculations.”

      The 2nd installer used the Wright soft software. But he says it only will let him do the whole house load and he breaks it down room by room from there. I’m assuming he means he just back calculates the footage and distributes the load evenly.

      I’ve used coolcalc and load calc. Coolcalc estimated 28k for the Entire first floor heating at -5, but gave me some ridiculous 56k for the upstairs. I’ve gone over the numbers but can’t figure why it is so high.

      Loadcalc estimated my 1st floor around 22k. It just seems I don’t trust myself with the loads. If I error wrong and install too small of a system, it’s on me. If the hvac contractor goes too big and it’s uncomfortable it’s on him. But I’ll pay for too much of a system and My efficiency will....suck.

      Would it be advantageous to oversize (slightly) to help pick up the laundry/bath a little from the kitchen (they’ll still have they’re own electric heat).? Or the upstairs bedrooms in the hopes it picks up that bath a little?

      I have more reading to do tonight on some of those links you sent. I really wanted a system installed this winter to see how it performs but I don’t want to rush into an expensive mistake.

      FYI, I haven’t paid for the coolcalc reports yet, but can and post if someone wants to look it over.

      1. this_page_left_blank | | #15

        "If the hvac contractor goes too big and it’s uncomfortable it’s on him."

        And good luck getting them to a)admit the issue due to the load calc, and b)do what's necessary to actually address the problem, which would include almost a complete replacement of the entire system.

  7. Expert Member
    Dana Dorsett | | #11

    >"The 2nd installer used the Wright soft software. But he says it only will let him do the whole house load and he breaks it down room by room from there. I’m assuming he means he just back calculates the footage and distributes the load evenly."

    ...as if the load were distributed evenly... (NOT!!)

    For what it's worth, where did his whole house cooling and heating numbers come out?

    And what indoor & outdoor design temperatures did he use?

    >"Coolcalc estimated 28k for the Entire first floor heating at -5, but gave me some ridiculous 56k for the upstairs."

    Why were you using -5F as an outside design temperature (a ridiculous number to be using in ZIP code 43545 )? Fort Wayne Indiana's +2F or Toledo's +3F are the right numbers to use, not 0F, not -5F, even though it hit's negative double-digits there sometimes. Sizing the equipment to between 1.2x - 1.5x (absolute max) at the 99% design temp would right size it. If you start using arbitrarily cooler design temps "just in case" the risk of oversizing to the point of low efficiency and low comfort goes up.

    To hit a 56K load for a 650' second floor would require leaving a couple of windows open, even at -5F.

    >"Loadcalc estimated my 1st floor around 22k."

    That tool typically overshoots reality by 25-35". Discounting by 20% to (0.8 x 22K=) 17,600 BTU/hr would probably be a bit closer to the real numbers. Using LoadCalc room-by-room for sizing the heads would usually be an OK oversize factor, and apply no further upsizing unless absolutely necessary. When it's on the edge, go with the smaller ductless head. eg: Let's say LoadCalc comes up with 11,400 BTU/hr for a room/zone load. The real load is probably more like (o.8 x 11,400= ) 9120 BTU/hr.

    The FH06 is good for 8700 BTU/hr, which if LoadCalc's number is a 35% overshoot the load would be (11.4K/1.25=) 8,444 BTU/hr, so iit might just make it on design day with out much margin, but if it's a 25% overshot it's not going to cut it.

    The FH09 is good for 10,900 BTU/hr, which would be a 10,900/9120 = 1.2x oversize factor, which is just what the doctor ordered for optimal efficiency & comfort.

    The FH12 is good for 13,600 BTU/hr which would be a (13,600/9120)= 1.5x oversize factor- the absolute maximum that should ever be considered. Going beyond that would be slipping down the efficiency and comfort scale.

    But if one assumed the 11.4K number was real (usually isn't, if it's from LoadCalc) the FH12 or even the FH15 is what a conservative minded designer might choose, but it's almost always the wrong joist.

    But go ahead and use it, room by room. You'll find that most rooms will come in well under 8700 BTU/hr if subtracting out excessive infiltration & ventilation load. If LoadCalc is coming up with 5000 BTU/hr and the real load is 4000 BTU/hr even the FH06 is going to be sub-optimally oversized.

    1. nick14z71 | | #13

      >” For what it's worth, where did his whole house cooling and heating numbers come out?”

      “ 1st Floor at 5°F = 30039 BTU Heat Loss

      1st Floor at -5°F = 34523 BTU Heat Loss

      2nd Floor at 5°F = 17775 BTU Heat Loss

      2nd Floor at -5°F = 20429 BTU Heat Loss”

      That was his email to me.

      >” Why were you using -5F as an outside design temperature ”

      I know the design temp in my area is 4F. But my wife was looking at possibly getting rid of some of the electric baseboard units. Not all. But some for appearance reasons. Trying to slightly....slightly oversized to keep us going at -5. Below that we will use in-wall fan forced electric resistance heaters.

      1. Expert Member
        Dana Dorsett | | #16

        >"I know the design temp in my area is 4F. But my wife was looking at possibly getting rid of some of the electric baseboard units. Not all. But some for appearance reasons. Trying to slightly....slightly oversized to keep us going at -5."

        Changing up the outdoor design temp is a screwy way of approaching oversizing. To keep it consistent, calculate the load at the 99% temperature bin, and only THEN apply an oversize factor when specifying the equipment. Cooking the load higher by using illegitimate inputs, then upsizing from the already higher number is a common way to get to crazy-suboptimal oversizing factors with heat pumps.

        It looks like the HVAC contractor probabaly had a fat thumb on the scale too, since their 34.5K number is more than 20% higher than your CoolCalc's 28K for the first floor load @ -5F.

        1. nick14z71 | | #17

          Ok. Thank you for the time and information Dana. I will do that and see where I’m at.

          I’m pretty sure I made the hvac contractor mad today (one of the diamond certified ones, lol, what a joke).
          He told me after he put the MXZ4C36 compressor outside with 3 indoor 9k heads, I would be able to add 1 or 2 more heads later.

          I told him the max number of units he could add is 1 for a total of 4. His response was “yeah your right, I’m not really sure why that is, but when I add it to the Mitsubishi design tool it kicks it back at me”

          Oh boy...

          It’s beyond frustrating that I can’t call a HVAC tech and have a paid “professional” figure this out. I read forums on here of people who have great hvac techs help them out and are open to new ideas and working though the problem with them to find the best solution. I wish I could find someone local like that. But it seems most are still stuck in the “put ductwork and a 80k furnace in” mode.

  8. nick14z71 | | #12

    Does Mitsubishi run any load numbers themselves? Seems like they’d offer assistance in this area to avoid issues.

    Also, does anyone have specs for minimum btu’s a mxz3c30 or mxz4c 36 hyper heat can put out?

  9. nick14z71 | | #14

    Dana, or Akos....back on post #2 I listed my house design layout. Akos, you commented that designing a system for my house with so many rooms can be challenging. This is what I’m running into. Even if running the numbers or calculating the heat loss for these rooms and coming up with 4K-9k heat loss on rooms, it is virtually impossible to heat the whole house unless I get into ducting.

    The “one head per room” approach, is oversized because I can’t hook up five 6k heads to a smaller compressor. I would need to use at least the mxz5c42 and have more compressor output than I do head input which lowers my efficiency.

    Conversely, if I have 2 multi splits outside, I still need a mxz3c30 and possibly a mxz2c18. I’ve been quoted systems like this and it is definitely NOT cheaper going with 2 separate systems. Ducting on the first floor may work because I have basement access. But the ducting on the 2nd floor would be difficult because of the cape cod issues.

    I have submitted documents to energyvanguard and they said they would send a proposal along with their recommended heat loss calcs. Anyone used their service before and recommend?

    Nick

  10. Expert Member
    AKOS TOTH | | #18

    Once you get the heat load you'll probably be in the range of the mxz3c30 or mxz4c36.

    You can do a PEADA18AA7 head in the basement for the main floor and two 6k wall or ceiling mount units for the upstairs bedrooms. Better setup if you have the ceiling height in the bathroom is a SEZ-KD12NA or SEZ-KD9NA there ducted to the two bedrooms. With a straight shot, the low static head should work and you'll need very little ducting/bulkheads.

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