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Community and Q&A

ASHP efficiency at low temps

forcedexposure | Posted in Energy Efficiency and Durability on

Hello good folks,

I’m finally getting three ductless heat pumps  installed in my house to provide heat in the winter. The configuration is 3 indoor single zone Mitsubishi heat pumps, installed 1 to 1 with three outdoor units. They will be used to partially offset the amount of nat gas that my steam heat furnace uses to heat the house. The gas furnace is only 6 years old, and measured 82 AFUE efficiency. 

At what low outdoor temperature does an ASHP become inefficient, compared to using my gas steam heat furnace? 

I wonder if I can teach the “smart” thermostat for my gas furnace to automatically turn on when the outdoor temperature reaches that magic temperature. Or, alternatively, maybe I could get the ASHPs to somehow shut off when it gets to be just that cold outside.



thanks for your help! Kris 

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  1. Expert Member
    NICK KEENAN | | #1

    Need to know your prices for gas and electric.

    1. forcedexposure | | #2

      I burned 165 therms in January and my National Grid bill was $244.71.
      So, say $3.76 per therm.

      The electrical use in January was 264 kWh, which cost $64.77.
      Say .24/kWh


      1. Deleted | | #3


      2. Expert Member
        NICK KEENAN | | #5

        Does that include connection fees? The reason I ask is you need to look at marginal cost not average cost. If there is a fixed monthly service fee in there then the incremental cost of using more is less than the average cost.

        1. jwasilko | | #7

          I disagree about looking at marginal cost. I care more about 'all in' costs.

      3. jwasilko | | #6

        I had posted the output of a spreadsheet I had that calculated the Economic Balance point, but I don't trust what it's telling me right now.

        However, your therm price is wrong and should be $1.483/therm.

  2. Deleted | | #4


  3. jwasilko | | #8

    Here's the output of your fuel prices and mine. It looks like at your high electric prices, gas is always cheaper at all temperatures.

    We pay more for natural gas, but our electric is less than half yours, so we have an economic balance point of 15F.

    Note I used our MXZ hyper-heat unit for specs. If you let me know what model 1:1 units you have, I can re-run the sheet with the COPs for those units.

    1. forcedexposure | | #10

      Thanks for this. I believe the cost of my electricity is actually less than what my eversource bill says because I've got solar panels. unfortunately those solar panels do not generate enough electricity to cover the entire household electrical use.

  4. jwasilko | | #9

    Honeywell makes dual fuel thermostats that can switch between heat pumps and fossil fuels at set outdoor temperature. We use the Prestige IAQ for that.

    Mitsubishi's Kumo Station/Cloud can also switch between heat pumps and fossil fuels at set outdoor temperature.

  5. _jt | | #11

    Steam furnaces are hard to compare in efficiency since they have so many losses. It is probably worth trying to heat electrically for a day with resistance heaters and a watt meter to work out your actual BTU requirements and compare that to how much BTU your steam furnace delivers. That balance point could be quite a bit different in terms of delivered BTU. My furnace is about 80% efficient but fairly certain I lose about 50% more of that by the time the steam makes it into the 3rd floor.

    Add to that lack of zoning - and - unless you are in an extremely cool zip code it is unlikely that it is efficient to use that steam furnace except maybe when it's below zero F.

    Although given your cost ratio is so high per calculations above that may not be correct.

  6. walta100 | | #12

    A web search tells me
    What are National Grid rates?
    Customer Charge $20.00 per month Distribution Charge On-Peak 14.859¢ per kWh Off-Peak 4.328¢ per kWh Transition Charge 0.091¢ per kWh Transmission Charge 2.700¢ per kWh Energy Efficiency Charge 2.083¢ per kWh Renewable Energy Charge 0.050¢ per kWh Metering Charge If Applicable G-1 – General Service Small C&I Rate –
    For gas

    If for electric we back the $20 flat fee out of the 64.77 electric bill (64.77- 20.00)/ 264=.169 So it seem Kris is paying .17 a kWh for the next kWh they will use.

    For gas we back out the 24.77 minimum charge on the first 3.3 therms
    (244.71-24.77)/ (165-3.3)=1.36 So it seem Kris is paying 1.36 a therm for the next therm they will use.

    Jeff would you please post a worksheet with .17 electric and a 82% furnace 1.36 a therm


    1. jwasilko | | #14

      You posted National Grid rates for NY that are quite different for us in MA. Not sure where the OP is located, but in MA our monthly fee of $12 (.40 per day) doesn't include any gas. Per therm we pay $1.3958 (during the winter, as rates drop in the summer).

      Here's the sheet at the rates you asked for. It puts the economic balance point at 37F.

  7. Expert Member
    Dana Dorsett | | #13

    It 's important to figure out the therms per degree-hour ratio of the steam system at a couple of different weather ranges to nail this. Steam boilers (which are boilers, not furnaces) are notoriously lossy, with big standby & distribution losses- you can't use the AFUE or steady state efficiency of the boiler's nameplate ratings to determine the efficiency.

    Very few existing steam systems are anywhere near right-sized for the space heating load, and operate at a low, inefficient duty cycle even at the 99% outside design temperature.

    1. johns3km | | #15


      I'm curious how "inefficient" my steam system is when compared to either hotwater or forced air alternatives. My 2500sf of conditioned space, with 1000sf of uninsulated, leaky basement, used 685 BTUs/HDD (base 71, 71 indoor temp, no setbacks) last month with 1049 heating degree days, 216 therms heating. Roughly 6.5 BTU/(HDD*SF) for a 50 year old, 200k/153k steam boiler. Considering practically zero electricity usage, that doesn't seem horrible considering there is a lot of low hanging fruit to be done in the basement. Monthly average temp was 29F outside of Boston. Wouldn't I still have similar basement/duct losses with an airhandler in the basement? Where do my numbers stack up compared to MA area BTU/HDD or btu/hdd/sf retrofits? Or monthly heating bills per month? I've seen some posts on here with $700 monthly electric bills so a $300 gas bill doesn't seem too bad given I heat to 71F, 24/7.


      1. Expert Member
        Dana Dorsett | | #18

        >"My 2500sf of conditioned space, with 1000sf of uninsulated, leaky basement, used 685 BTUs/HDD (base 71, 71 indoor temp, no setbacks) last month with 1049 heating degree days, 216 therms heating."

        The base temperature isn't the indoor temperature, but rather the heating/cooling balance point temperature. The reason the indoor temperature isn't the balance point is that occupied homes have internal heat sources, such as 24/7 plug loads, water heater standby losses, mammalian occupants all giving off heat, which reduces the load on the heating system, and adds load to the cooling.

        Most 2x4 framed houses will have a balance point within a degree or two of 65F, tighter 2x4 framed house and most 2x6 framed houses will have a balance point closer to 60F.

        Your house uses WAY more than 685 BTU/HDD which would be 28.5 BTU per degree-hour. That would mean when it's a new record-low -30F (100 degrees colder than the base temp you used in the calculation) the heat load would be 100F x 28.5 BTU per degree-hour= 2850 BTU/hr. That's the heat load of a typical BEDROOM, at Boston's +10F outside design temp.

        So, how many base 65F HDD was covered by that 216 therms?

        We can take a guess at it, assuming it was 30 days @ 29F average temp.

        The 29F average temp is 31F below the presumptive 65F balance point, so:

        ... 31F x 30 days would be 930 HDD(65)

        ... 216 therms/930 HDD= 23,226 BTU/HDD(65)

        ... 23,226 BTU/HDD / 24 hours/day = 968 BTU per degree-hour.

        ... at 80% combustion efficiency the net is 0.8 x 968 BTU per degree-hour= 774 BTU/degree-hour.

        ... assuming a design temp of +10F (typ for most locations inside of Rt 128) that's 55F below the presumptive balance point, and thus an implied heat load of...

        ...774 BTU/degree-hour x 55F heating degrees= 42,570 BTU/hr @ +10F

        Since you were looking for a comparison, I live in a sub-code 90 year old 1.5 story house with a bit more than 2400' of fully conditioned above grade space, plus 1600' of insulated but not directly heated basement (that coasts along at ~65F all winter on the distribution losses of the heating system). The conditioned space is normally kept at 70F, no setbacks. The largest zone (out of 5) uses a low temp hydro-air handler running off a buffer tank served by the boiler, with ~110F - 115F air, covering a bit more than half the heat load of the house, with 90 year old somewhat leaky and completely un-insulated ducts.

        My net-BTU/degree-hour based on base 65F weather data with a non-condensing boiler comes out to 565-570 BTU/degree-hour. Moving my house to Boston with design temp of 55F would result in a heat load of no more than 55F x 570 BTU/degree-hour= 31,350 BTU/hr, or about 25% less than your house.

        It's a comparable sized house and should have a comparable load. The insulated basement is still meaningful, since the distribution losses mostly accrue to the house, and are not completely lost, whereas in your house most of the losses to the basement are truly lost.

        The dead of winter is the best-case for your heating system efficiency, since at your ~4-5x oversize factor the duty cycle of the burner is quite low during the shoulder seasons- it's mostly standby loss. Running the base 65F numbers for October/November or March/April will likely show a higher BTU/degree-hour constant than on December through February periods.

        >"Wouldn't I still have similar basement/duct losses with an airhandler in the basement?"

        Yes, if the ducts were leaky and uninsulated. What is the temperature in your basement? Assuming a basement temp of 50F the difference in room temp to steam temp is about 155F, the difference between ducted hot air temp to room temp would be about half that, but there's more surface area to the ducts than the steam pipes.

        If your standby & distribution losses are so high that your basement is WARMER than 50F during cold weather you'll get a very SERIOUS benefit out of air sealing and insulating the foundation to the current R15 continuous insulation IRC code minimum.

        1. johns3km | | #20

          Thanks Dana, guess I was off in a couple places. I misspoke and should have said 685 BTU/degree hour, not HDD, my fault, and still incorrect!

          I thought if you use Base 65, the internal loads would get you to an internal home temp of 68. With a 71F set point, shouldn't I use Base 68?

          For the 29 day period 1/16-2/13, Norwood Airport gets me 875HDDs Base 65F (35F average). My ecobee 5 min interval of outdoor temps from "an algorithm of nearby weather stations", gets me an outdoor average of 32 F. So roughly 32-35F outdoor temp. That would put me closer to 822 BTU/degree hour Base 65 (750BTU/degree hour Base 68).

          So while about 30% higher than your home, I've calculated my basement losses to over 30% of my heating load with 50% of the foundation walls above grade. Lots of gains to be had there.

          So with an insulated basement wouldn't I be pretty close to your set up, and you still need to account for electricity? I don't even have a stack damper which might shave off another 5-10% of standby losses.

          So compared to a run of the mill furnace, or 95% condensing furnace that isn't sized correctly and doesn't spend most of the season modulating, I'd say my steam setup is pretty comparable to the average household, and certainly more comfortable, and silent. It's not like it is 4-5x less efficient and using 2500BTU/degree hour.


    2. Expert Member
      NICK KEENAN | | #16

      I would think in a residential setting most of those distribution losses would go right into the building envelop and not actually be lost. Same with standby losses.

      1. Expert Member
        Dana Dorsett | | #19

        Given the "...1000sf of uninsulated, leaky basement uninsulated basement..." the majority of the standby & distribution losses ARE truly lost.

        Very often with 4x - 5x oversized steam systems the boiler room is the warmest room in the house in winter. At R1-R1.5 an uninsulated foundation is usually well into double-digits as a percentage of the whole house heat load, even if the basement is only running 50F.

  8. _jt | | #17

    If you are perfectly insulated sure, but in reality you are losing it to the outside - and even in the best case the lack of zoning and stack effect means your top floor will be too hot.

    In my case I found that that my 140000 BTU steam boiler produces 113000 BTU. But delivered capacity (if I swap in 6 electric space heaters in it's place) - is around 30000 BTU.

    It's about the same price to run the boiler versus space heaters for me! But not a huge difference.

    Try it yourself and see. Buy some $40 space heaters from Walmart and see how much many it takes to maintain temperature.

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