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Calculate BTU output for indoor unit of mini split?

Drps10 | Posted in General Questions on

first off, I want to thank this forum and the contributors to it! Been lurking for a short time and the info and links are amazing! This info is hard to come by at times and the people that take the time to give their knowledge is instrumental  

I recently installed myself a 4 zone Hyper Heat mini split system.

I want to calculate (I want to build a device, but for now paper calculations are fine) to figure out the BTU output each head is producing. I need to figure out the voltage and amps the unit is putting out in each fan mode and then multiply by 3.4 correct?

anyone know how to take the measurements I need on a msz-fs12na? 

The numbers don’t match per se for the ODU if calculated correctly. The odu unit submittal shows only going down to 22k btu and I read that they can really go down to 35% of the max (14,700 for me). The power I am pulling says I am pulling 6,700btu from the ODU correct?? 1.8kw x 3.4=6700 btu. 

backstory —-
42k outdoor unit with 6,6,12,18 indoor units. I have come to the conclusion I am oversized, but I don’t have a lot of options.  I have just replaced all the windows and been improving insulation, so my load demand is very low when calling for heat. The finished basement 12k has been the one that is running on the lowest setting (auto fan) and only pulling about 1.8 kw total system. Keeping the basement set at 69* and doing a great job at it. The heat works it’s way upstairs slowly. The outdoor unit is running almost 24/7 and the upstairs 18k turns on here and there (maybe 2-3 times per hr for a few min). I’m content with this operation because the ODU isn’t short cycling and in a very low state of use. When the 18k turns on, ODU will pull about 2.2-2.6kw for a short time then ramp down to the 1.8. The 6k units are oversized (I knew that going into it but no other option, realistically I should have not installed one, but I designed it for cooling not heating so I have to wait till summer to see). I have one mhk2 in the bedroom to operate when called on and then turn fan off when not needed and works great but maybe 2-3 a day with a Set point 67* in the bedroom. with the basement running 24/7 the ODU doesn’t short cycle. 

The basement has a mhk2 set to 69* with auto fan and the vanes set in a configuration I like (one side down the other directly out). 

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  1. Expert Member
    Akos | | #1

    To measure the BTU output of an indoor unit you need to know the temperature rise across the unit and the flow rate. The equation is:

    Flowrate CFM * 1.08 * delt T (F).

    So if you are running at 200CFM with a 40F delta the indoor unit is producing 8640BTU.

    The electrical draw of the indoor unit has very little to do with this as the heat is supplied through the refrigerant.

    You need the ODU power use to figure out the COP of your setup.

    So for example ODU is running at 2kW and the all your indoor units summed up are producing 12000BTU.

    COP=12000BTU / (2kW*3414)=1.76

    There is no easy way to measure the actual flow rate of an IDU. Your best bet is to set the fan to a fixed speed and use the matching flow rate from the data sheet.

  2. bfw577 | | #2

    Yes. It can be done by calculating the btus/cop at each head. See comment #46 from Akos in the link below. I did an example in the comment below it.

    The issue with your setup is the minimum capacity on those big Mitsubishi outdoor multi splits is massive. The minimum on your unit at 47 degrees is probably either 22500 or 24k btu. If your running one single 12k unit off it when its in the 40's out the other 12k is just being pumped and wasted to the other heads. A 1.8 kw load running a single 12k head is terrible efficiency. Its 47 right now here in New England and my single zone 12k mini split is running along at like 180 watts.

    You will be able to see this clearly when you start dividing the outdoor units electricity draw compared to what your getting at the heads. Akos noted he was getting a terrible cop of just 2 running just 1 9k head off his 24k multisplit in a comment further down.

    1. DC_Contrarian | | #3

      I wouldn't say the rest of the heat pump production is "wasted," it's still going to heating the rest of the house. It's just that those zones are being heated more than the thermostat is calling for. In effect the thermostat setting of the highest-demand zone is driving all of the other zones. In effect, it's like not having zones at all.

      1. bfw577 | | #6

        Its sounds like he was talking about only running the 12k unit and it drawing 1.8 kw. With only the 18k unit occasionally turning on. So it seems like that 12k unit might be spending a good portion running by itself off just the outdoor unit. If that's the case the cop could be a low as 1.76 as Akos showed in his post and is a pretty terrible cop at minimum capacity.

        1. Drps10 | | #7

          This is correct. My 12k runs almost 2/47 and the 18k turns on about 20 times a day just for about 5 minutes. I am working on doing a Manual J calculation to see what I actually need. Also, looking into maybe getting rid of this big unit and doing 2 smaller outdoor units.

  3. Drps10 | | #4

    Thank you everyone! I will see what I can come up with calculations. The extra heat is offloading upstairs living room which is helping with keep it at temp, but I know that’s not how it’s designed to work. I am now looking into downsizing the outdoor unit, but I don’t know how to maximize efficiency and operate the heads I have. The 12k basement and 18k(which should be a 15j) are the only ones that will really be operating, the 6k would be very infrequent and never high load.

    1. DC_Contrarian | | #5

      The starting point would be to calculate what your actual need is. If you have a heating history you can do that based on historical fuel usage. Or you can estimate it using a Manual J.

      1. Drps10 | | #8

        I have all electric baseboard heating. I do have accurate heating demands with the basement and upstairs. I will see where all this info gets me here shortly.

  4. Drps10 | | #9

    Looking over my numbers:
    I ran the avg use from 1/1-1/16 for my hourly avg in the basement and upstairs living area.
    The basement is a 8' baseboard that is 2500w. It turns on and off via thermostat. Average is 1.1kw/ hr. Looking at all the numbers, there are times its .5kw used for the hour up to 2.5. I don't know how low my FS12na will lower to for BTU, but the 1.1kw avg seems to transfer to 3700 BTU.

    The manual J calc for upstairs, I need your opinion . Since turning on the Mini Split, the upstairs heaters (baseboards) have never turned on, so that unit is heating the entire upstairs. So do I calculate to heat the entire upstairs (2 bedrooms,a bath, kitchen, living) or the intended area (kitchen, living room, hallway). If I do the intended area calcs say 15k, if I do everything (because it has been heating everything, but barely turning on ) is 18k.

    I myself feel I should have bought a 15k now having this run for just over a week.

    1. DC_Contrarian | | #10

      When doing a Manual J the assumption is that the insulation between rooms is insignificant compared to the insulation in the exterior walls, so heat will flow freely between rooms if there is a temperature difference. So not heating one room won't change the overall heat draw of the building, it just means the heat in the adjacent rooms will run longer.

      Also, you can't just translate kW into BTU for heat pumps. They have what's called a COP, coefficient of performance, and it's the ratio of energy in to heat out. Typically it's between 2 and 4. But it varies depending on conditions.

      Really the question you're trying to get to the bottom of is, "What is my actual COP?" You know what your electricity use is, you're trying to figure out how much heat is delivered. If it's not significantly over 1.0 then installing the minisplit was a mistake.

      Electric baseboard heaters have a COP of exactly 1.0. So if you have a usage history before installation and after installation, you can answer the question of whether the minisplits saved you money. The only caveat is you have to adjust for the weather outside. The standard assumption is that heat loss is directly proportional to the difference between outside temperature and inside temperature. If it's 70F inside, the heat loss will be twice as much at 0F as it will be at 35F. In this kind of work it's customary to use the average temperature for a day, given by averaging the recorded high and low. You should be able to get that for your location from any weather website.

  5. walta100 | | #11

    “I am now looking into downsizing the outdoor unit, but I don’t know how to maximize efficiency”

    Seems to my you have a working system yes it cycles more than it would ideally but in reality, the amount of dollars to be saved is pretty small compared to the costs of replacing the system.

    My guess is you could never recover the cost.

    If you decide the change out units consider going with the single head unit as they often will slow down to 10%.


  6. Drps10 | | #12

    Thank you guys! I am not at home so i don't have the information to calculate the numbers. I have been looking at submittals and NEEP website for data. I still want to get through the summer to see how the setup operates, but I was looking at doing 2 outdoor units. Being the basement 12k runs more than any other unit at a low setting ( not sure if that would change if I went to a 12k ODU), crunching numbers to do a Hyper 12k single unit for the basement. Then a 30K unit for the 18,6,6. I know its still large, but the model will drop down to 11k. Still to high for a low setting, but better than the 24k i'm at right now. I am using to much power for the demand we need (it's 34* ish the last few days), this setup will only get worse (i see a lot of short cycling happening) when we get into the 50-60.

    I am slightly better at energy use right now compared to my baseboard heaters, but I could be so much better if I had the right equipment. I still need to calculate the cost to run some new linesets and electrical to the IDU. I don't know what a used unit would sell for, but to recoup some money would help too!

    1. bfw577 | | #16

      How much electricity is it using on an average day? Do you have high electricity rates?

      1. Drps10 | | #17

        With taxes we are about $.13 a kw.

  7. walta100 | | #13

    “I am slightly better at energy use right now compared to my baseboard heaters”

    Something is off about this statement your HP should have a COP over 3 even thou it is cycling so your heating bills should be down 66%. The cycling losses should amount to less than minute of run time for each cycle.


    1. bfw577 | | #14

      He stated he is able to heat his entire place with just his 12k unit downstairs, the upstairs 18k unit only cycles on for 5 minutes 20 times a day. He uaed his baseboard heat to calculate his heat loss at 3700 btu/hr so he is massively oversized. His outdoor units minimum capacity at 47 degrees is 24k and he is running just 1 single 12k head off it. His outdoor unit is drawing 1.8 kw running just that 12k unit and in Akos post the cop would be around 1.76. That is pretty terrible low modulation cop compared to single zones that can hit 6.

      If the poster is in New England where electricity costs are approaching .40 kwh in some states a more efficient setup might make financial sense.

    2. Drps10 | | #18

      There isn’t much cycling with the outdoor unit. The basement keeps a low demand so the outdoor unit runs almost 24/7. the 18k upstairs cycles on and off (oversized for the room, but I designed for summer a/c), but 99% of the time the outdoor unit is already running

  8. walta100 | | #15

    Let’s do the math.
    The system uses 1.6kW when running on low.
    The system cycles on 20 times a day.
    The electric rate is 0.40 per kWh
    The new outdoor unit will cost $4000.00
    The outdoor temperature range for excessive cycling lasts 90 days each year.
    Getting the equipment started and the refringent flowing normally takes one minute.

    This gives us 20 minutes of start-up penalty per day. Every 3 days is one hour of start-up penalty.
    Each hour of start-up penalty uses 1.6kWh times .40 cents = 0.64 for 3-day times 30 $19.20 per year. $4000 divided by 19.2= 208 years to pay for the replacement unit.

    My guess is his field calculated COP is way off.

    How sure are we that the unit is charged correctly? If the charge is wrong the COP will fall off quickly.


    1. Drps10 | | #21

      I do appreciate the break down! It is nice seeing those numbers, but also need to calculate the cost savings while the unit is running for the correct demand. Compared to the person above, at the same temp with the correct ODU it would be a savings of 90% per hour to heat.

      That could be a difference of $5.60 to heat for 24hr (with my setup) or $.56. Or $150 for an avg 47* month of $150

  9. Drps10 | | #19

    The calculated amount that was need was correct (from the program from Mitsubishi) and I watched the installer put it in with his scale. We did not pull refrigerant out and weight the outdoor unit. If that was off, then there is some of our issue. I did the math from that other Poster about how much his 12k was pulling. If mine was pulling that instead of the 1.8kw. It could be almost $100+/- depending on temps savings per month. Based on 47* temps. Which happens more than the teens temps.

  10. Drps10 | | #20

    I did the math for the last 8 days and the avg was 43kw per day which breaks down to about 1.7kw/hr the heat the entire house. Which is about the same if not a slight savings from baseboard.

    The hyper heat 12k will pull at max 1.15kw. The basement never runs that high, (always on a whisper or low fan in auto mode). Down to 2250 btu.

    I do imagine that since the ODU is pushing warmer refrigerate since it’s running at 22kbtu which allows the indoor 12k to be blowing warmer air, so I think if I were to get a single zone 12k it may run a bit harder but still in a great efficient band!

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