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3600 kwh per month?

DanET1983 | Posted in Energy Efficiency and Durability on

We installed a Mitsubishi ductless mini-split system last fall for an 1880’s house, 2 story, R-13 insulation in the walls, likely R-19 in the attic.  

The MassSAVE energy audit recommended this as a replacement system for the house as it was being remodeled (formerly had heating oil.

There are 4 inside units; 3 wall mounted Model MSZ-FH09NA, one floor model that is larger for the entire first floor (open concept).  The outside condenser unit is Model MXZ-5C42NAHZ connected to a 50 amp circuit with a separate 120 volt circuit for the controls.

The first month’s heating bill was $920 and the second month was nearly $1,100 with the thermostat turned down to 67.  

The installer is coming back out to look, but has already done so once before and claims that the system is running fine.  Any suggestions on why the spike in energy usage?  These are supposed to be efficient and we have had a relatively mild winter in central MA.

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  1. maheatpumpguy81 | | #1

    Hi, I'm currently dealing with a similar troubleshoot. I'd recommend reading through my thread I started back in December: "High December electric bill with Mitsubishi heat pumps." I converted from oil to a whole-house heat pump system.

    Hopefully this gives you a good lead on troubleshooting your system.

    Did you do a heat load calculation (Manual J) on your home? If not, we can approximate it for your entire home (not room-by-room) from your oil use over the course of last winter. This will help to determine if it is appropriately sized for your home.

    Also, did the installer check the refrigerant charge yet?

    Good ways to evaluate it is getting a power monitor and attaching to the circuit the outdoor unit is on. Also, do you think you have some major air leaks in your house that need correcting? Maybe rent a FLIR device and check. My sense is that fossil fuel heat sources deal with air leaks better than these heat pumps, but that's just a hypothesis.

  2. MAinspector | | #2

    It would help to include a bit more information about your home's construction components. (Insulation envelope, window U factors, etc) as well as conditioned area. Are the floors insulated? Are you sure there is R13 in the walls, some walls may not even have insulation in a house of that age. You listed R19 with a ?...I would double check, and than I would plan on upgrading. An 1880's house is certainly not going to be energy efficient without major upgrades.

    Also, what was your electric bill before you installed the mini splits and did you install any other electric equipment that would add to your monthly costs (new water heater or other appliance).

    A little context would help. It could be possible that your home is hard to heat and your mini split system is working its butt off to keep up.

  3. DanET1983 | | #3

    Thanks for the feedback - have not done an audit, but windows are likely circa 1980s double pane as they have the fogging effect in the middle of some indicating lost seal. There is insulation between the basement and the first living floor (house is on a hill, so half of the "basement" is living space with one mini-split and that portion is over a crawl space.

    Last winter the house did not have central heat (being renovated) and had a woodstove in the basement with 5 electric space heaters - the highest electric bill last winter was $600 when using the electric heaters to keep up.

    The attic over the original portion of the house has visible pink insulation that I assume to be R-19 but can check. The addition has roof trusses with insulation above the ceiling, but not accessible to see what thickness.

    We put on a home energy monitoring system in mid-December which seems to confirm the whole house demand at around 3600 kwh, and I would have to check on the circuit to the new heating system (we have 8 zones being monitored).

    Luckily, the hot water heater is a Rheem Performance Platinum that seems to be very efficient.

    I have been considering checking to see if there is a short to ground happening by using my non-contact voltage sensor, but can't get to the heating circuit EGC without being next to live circuits. Will probably just check the house ground.

  4. jwasilko | | #4

    Is your water heater gas or electric? If electric, is it a heat pump water heater?

    It would be helpful to post your kWh rate when you're posting pricing, since it's hard to compare $ and usage that way.

    We're in the Boston area, and we just recently switched to heat pumps for the main part of our house. We have 2 Mitsu air handlers and an MXZ-8C48NAHZ. The main part of the house was remodeled in 2009.

    We've got a 32x38 room over the garage that is still heated by natural gas, as is our water.

    As a point of comparison, Nov 17-Dec 16 we used 3079kWh. We've got an electric steam humidifier that draws quite a bit when it runs, and I'm a technology geek, so I've got a bunch of computers in the basement.

    Dec 17 to yesterday we're at 3000kWh.

    Do you keep the thermostat at 67 24x7, or do you set it back overnight?

    It might be useful to compare to your previous oil bills (if you have them from a previous heating season). It might also be helpful to compare your no-heating/no-cooling electric usage (may/september) to now.

    1. DanET1983 | | #5

      Thanks - the water heater is a heat pump style that uses little electricity.

      I should have clarified, the total kwh usage for the past two months has been around 3600 kwh for the entire house.

      The house was purchased less than 2 years ago - unoccupied and had been gutted of all copper pipes and the oil fired furnace had been removed. Not sure on prior oil usage for the house. We do keep the house at 67 degrees 24x7 based on the advice of the installer. He indicated that the energy cost to ramp up the temperature and heat the contents of the house would likely be more than any realistic cost savings.

      Sounds like we are using similar kwh per month, which seems excessive based on what we were told about the efficiency of this heat. Even the local oil company would not provide a quote to install an oil heating system, they said the mini splits are so efficient and cost effective they priced on that system.

      Guess it's time to order up some more firewood and get used to feeding a woodstove 24x7 again.

  5. Expert Member
    Dana Dorsett | | #6

    Not all months are the same. I noticed last June seemed a lot warmer than last December.(Maybe you experienced that too? :-) )

    The better way to compare monthly usage it to note the exact meter reading dates, and download a spreadsheet of base 65F heating degree-day data between those dates from a nearby weather station from

    Unless it's sub-metered it's likely that only about 3000 kwh went into the multi-split, the rest of the went into running the popcorn maker and binge-watching K-dramas on NetFlix etc. Sum the HDD for those periods (throw away either the first or last day from the periods to avoid double-counting them). To get a baseline estimate of "other" electricity use, subtract out the kwh usage of your lowest month from the heating season periods, then divide the total kwh/HDD.

    There will be some amount of statistical noise related to monthly differences of other power use as well the average efficiency of the heat pumps at different outdoor temperatures and modulation levels, but the kwh per HDD should generally fall within a range.

    At the rated ~HSPF 11 of the 5C42NAHZ the ~3000 kwh would be equivalent to about 11,000 x 3000= 33,000,000 BTU, which is equivalent to what you get out of 275 - 280 gallons of #2 oil in an 85% burner (not counting distribution & standby losses.)

    Some of the heat that went into the heat pump water heater was also sourced by the 5C42NAHZ- (the fine print of the second law of thermodynamics has something about "no free lunch"), but it's a small fraction of the total wintertime bill in most houses.

  6. tommay | | #7

    I love these stories. Your refrigerators and and air conditioners are two of the biggest electrical users in your home, why would you want to use such a device to heat your home? Not only that, but leave it outside to operate. These units may be good for moderate temperature zones but not for areas where there are much colder temperatures. I just looked at my electrical meter which was installed three years ago next month and it read just over 2800 Kwh. I feel for you folks who have this much usage in one month. The only real way to cut down on energy use is to not use it and find free, alternative methods such as sunshine for free light and heat and moderating temperatures in your homes to lower heat loss.

    1. jwasilko | | #9

      Tom: I disagree with you about heat pumps. I do agree with you that cutting energy use is important (via insulation or renewables, for example).

      In the end, heating comes down to $/BTU. It's nothing more than that.

      For us, heat pumps are cheaper than natural gas when it's above 15F outside.

      It's our first heating season with the heat pumps. Our combined natural gas/electric bill for Dec was $100 less than last year, with more HDD this year. We've gotten our January gas bill, but not yet our electric bill. I can estimate from our solar production/consumption meter, and it looks like we will save $150-300 in January compared to last year.

      We've only had 4 days so far this heating season where we've used natural gas (because it was under 15F).

      Cold weather heat pumps are a proven technologym and it just comes down to $/BTU.

      1. tommay | | #11

        Sure cost per BTU's is a factor but how many of these BTU's you use is an even bigger one. Insulation and renewables are a contributor, but not everyone has the choice of installing these options so the next best thing is to learn how to use less, knowing what heat loss is and how it works and what you can do to lower it without investing. Your monthly savings is still much higher than what can be achieved by using other methods of saving.

    2. bfw577 | | #10

      A modern fridge uses very little power. My fridge draws 75 watts when running. The yellow energy guide sticker on it said it cost less than $75 a year to run. Maybe if your running a fridge from the 70s would it be even close to your largest electrical consumption.

      "These units may be good for moderate temperature zones but not for areas where there are much colder temperatures"

      This isnt 1980. Heat pumps have come a long way. Both my mini splits work down to -22 and can provide their full rated heat to -5. I have no issues heating my entire home with them. They cost me nothing to run as I have solar panels, but If I did have to buy the electricty they would have cost me around $80 a month the last 2.

      1. tommay | | #12

        How much did you pay for that fridge? Does that factor into your savings? That little yellow sticker is just like a suggested retail price sticker on an item. How you use it and the conditions it is subjected to, will determine it's real cost.

    3. CollieGuy | | #15

      One hundred and nine days into the heating season, we've spent $305.00 to heat our fifty-two year old, 2,900 sq. ft. Cape Cod with two mid-efficiency ductless mini-splits (1,927 kWh at 15.805-cents per kWh). Our cost to supply this same amount of heat with our oil-fired boiler would have been $790.00 (841 litres at 93.9-cents per litre, at an 82% AFUE).

      [Currently, it's -12°C and the winds are gusting at 81 kph.]

  7. joshdurston | | #8

    I agree there is no free lunch. Once thing that can happen with zoned systems, is that you pour more heat into otherwise undeserved areas. When I switched from a furnace to panel rads, my basement went from 16C to 21C since I had a rad down there but no wall insulation to speak off.

    So I gained comfort but spent more on gas since I was heating spaces I wasn't before.

    If you have a head in the basement (and your basement isn't insulated that well) you may want to turn on the occupancy feature or schedule to only run when someone is down there.

    I would play with setbacks as well, it can be tricky to optimize with time of use rates. I'm sure I use less KWH setting back my FH09, but I pay a lot more for electricity between 7am and 7pm then at night so I don't want to recover on peak rates which are almost double off peak.

    Maybe your utility has a web portal to analyze your usage. (mine gives me hourly smart meter data). Attached is a heavy usage day for me.

  8. bfw577 | | #13

    Its 18 degrees out right now, my house is 70 and my solar panels are powering both my mini splits and sending most of the power back to the grid. No fossil fuels being burned. Tonight I will run them off the credited power I sent back to the grid today. I am a year away from breaking even on my panels and then they will be essentially free to operate for both heating and cooling.

  9. Expert Member
    RICHARD EVANS | | #14

    It was -1F this morning here in the mountains of NH. My single Fujitsu 1 ton unit was still blowing 85+ degree air and keeping us warm and cozy. Granted our home (30,000 cubic feet including basement) is super insulated and 'wicked' tight. Its all electric and we are averaging just less than 400 kWh per month for everything. Its awesome!

    HOWEVER, I cannot recommend mini-split heat pumps as there are just too many horror stories of over-sized multi-splits or inadequately charged lines, etc. The manufacturers- especially Mitsubishi- share the blame for this as they encourage 'a head in every room'.

    Ductless mini splits seem to be either the best solution or the worst solution depending upon the contractor.

  10. DanET1983 | | #16

    Well, we discovered that one of the units was not cycling off, had a stuck solenoid. That unit was on 24x7 since starting and could never get the coil temperature down to shut off the fan. Fingers crossed that makes the difference! We had set the thermostat at 63 and the first floor was not going below 67 the whole time.

    Adding more insulation and sealing leaks as we go. And burning wood again - which only costs me some effort and the use of my father's land!

    1. jwasilko | | #17

      Good news!

    2. Expert Member
      RICHARD EVANS | | #18


      Fingers crossed! My faith is being restored already... 😁

  11. twoodson | | #19

    Every contractor in St. Louis wanted to sell me a split in every room. Awful and ugly.

    I eventually did the design myself and told a contract exactly what I wanted (new Fujitsu mid static) and it’s been awesome so far.

    Things are going to have to change substantially before these become mainstream in natural gas dominated heating/humid climates.

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