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Calculating Heat Load for Small Home

PeterCape | Posted in General Questions on

Hi GBAs,

I am new to here and I am overwhelmed by the information there. This is such a good website!

Long story short, I am a first time homeowner on Cape Cod, MA (Zone 5) with a ~1500 sqft 2 stories house. I recently want to install a mini-split ductless heat pump because my current electric baseboard heating just give me CRAZY bill (I have been getting ~800 dollar/month for three months now).

I have reached out to three HVAC companies, they quoted me different machines but their outdoor units are ~36000 btu to 42000 btu range. Yet when I try to do Manual J calculation (which I admit is more complicated than I thought), it only gives me about ~20000 btu for heating. I am bombed by the difference here and I don’t know what to trust… So my questions are:

1. On ballpark, what is the rough estimate of heating BTU for my house? I have a finished basement with no heat source. Just finished Mass Save recommended insulation which blows R38 up to 12 inch in the attic with R60 rigid board and install 2 inch knee wall board as well. Blow Door Values at CFM50 is 1770 (previous 1920, said that 1770 is the lowest that they can go). THE PROBLEM is that I don’t know the insulation material in the sidings since previous homeowner didn’t tell me, but this is a 1980ish Cape…

2. I noticed that some people say if the system is oversized, the energy bill will go up with short cycling and all that stuff, but some others may say since the current system has variable speed motor, the oversize issue will not be that much. I am wondering that whether this claim is true?

3. A Mitsubishi specific question…one company quote me a MXZ-SM42 series and the another quote a MXZ-5C42 series. Both with H2I tech but it seems that SM is the newer generation? I am wondering that anyone knows the difference of the two?

Thank you so much!


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  1. paul_wiedefeld | | #1
    1. PeterCape | | #3

      Hi Paul,

      Thank you for your input! I have read the article but unfortunately I don't know the BTU rating of my electric baseboard... they were installed by previous owner and I didn't get any manuals of these...Is there any other way to calculate load without knowing the current system's BTU rating?



      1. Expert Member
        Akos | | #5

        You don't need to know the size of your baseboards. The reason the link talks about equipment size is to figure out the conversion efficiency. Baseboards are 100% efficient.

        The calculation Paul linked to is based on energy use only. The calculation seems to be missing an all electric baseboard setup, luckily it is relatively easy to convert.

        Take your winter electrical usage for a cold month and subtract the house baseload electrical from a month with no heating and cooling (May/June is good).

        For example, say in Jan you use 1150kWh. In June it is 150kWh.

        This means you used for heat 1000kWh. Baseboards are 100% efficient, so you supplied 1000kWh as heat. To converter to BTU, muliply by 3412.

        So 1000kWh*3412=3414000 BTU of heat.

        The rest follow the link.

  2. Deleted | | #2


  3. PBP1 | | #4

    Hi Peter, electric baseboard may be assumed to be 1:1 to get you started. 1 kWh is 3412 BTU (many calculators online). But you can also get an estimate without having to even consider BTU until the very end (please double check everything for yourself).

    Worst case?
    If you are paying $0.14 per kWh and your bill is about $800, then you may be using about 5,700 kWh per month. If you divide that by 2.5 COP of a heat pump, then it's 2,285 kWh. A heat pump may do better or worse as to COP.

    Not so worse case?
    I imagine your bill at $800 has non-kWh fees in it, I checked a couple online, guessing about $70 non-kWh. So, that puts you at about $730 or about 5,200 kWh per month. Divide by 2.5 COP of a heat pump, and you're at 2,085 kWh.

    Possible case?
    Now, if that $800 bill has $70 in non-kWh fees and about 300 kWh/month that are not electric baseboard heat, you are down to 5,200 kWh/month - 300 kWh/month = 4,900 kWh/month. Divide by 2.5 COP of a heat pump, and you're at 1,960 kWh.

    Tailored case?
    Maybe take some of those factors into account and post what you get?

    kBTU/h Estimate?
    To get kBTU/h estimate, take month heating estimate for electric baseboards in kWh, divide by 30 days, then divide by 24 hours to get kWh/h, then convert to BTU. For example, 4,900 kWh per month/30 days/24 hours = 6.8 kWh/h, which is 23.2 kBTU/h.

    Good post re Storrs, CT, with advice from expert Dana:

    Look at this for -13 F, pretty good COP: "The MXZ-4C36NAHZ has a COP of between 2.83 to 2.12 (min to max output) at -13 F, as per the ratings listed above."

    And: "In Storrs CT the number you really care about is the COP at +25F" and "Nobody in Storrs CT should care about what the efficiency is at -13F at any modulation level, since over the lifecycle of a heat pump it's not likely to see more than an hour or three (if any) at that temperature, and it's certainly not worth designing the heating system to cover that temperature."

    1. paul_wiedefeld | | #6

      “ kBTU/h Estimate?
      To get kBTU/h estimate, take month heating estimate for electric baseboards in kWh, divide by 30 days, then divide by 24 hours to get kWh/h, then convert to BTU. For example, 4,900 kWh per month/30 days/24 hours = 6.8 kWh/h, which is 23,202 kBTU/h.”

      This is average heat loss, but you need to know design heat loss (the 99th percentile), so heating degree days must be incorporated.

      1. Deleted | | #8


      2. PBP1 | | #9

        Thanks Paul, and, yes, I know that's average. And, my units messed up, 23.2 kBTU/h, not 23,202 kBTU/h. If the average is 23.2 kBTU/h, then the design heat loss is certainly greater by perhaps 6k to 8kBTU/h.

        For a point of reference, with 20 F average monthly temp (including sub-zero), my monthly ave is around 21 kBTU/h and design at -1F is around 28 kBTU/h.

        To me, the average is high for 1500 sq ft on Cape Cod, thus some air sealing and insulation may be in order. Along with a detailed Manual J.

  4. Deleted | | #7


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