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Community and Q&A

Propane furnace with low kwh usage

Yoshibear | Posted in General Questions on

I am building a roughly 1000 sq/ft house that is all offgrid (solar). I am required by code to have a thermostatically controlled heater so going with a propane furnace – however the blower on these seems to use a very large amount of electricity – and its also hard to determine the actual kwh use as the specs only list Amp regardless of fan settings – for ex this 9 speed furnace here  – the specs say min circuit ampacity = 8.7 but that doesn’t really tell me much in terms of solar/battery needs.  

Does someone have either a chart/table that shows better data for popular furnaces or a recommendation for a low power direct vent ducted furnace (I will probably be required to use a 60K btu – climate zone 5 @ 7000ft – regardless of how efficient the house may be).

Edit:  I see potentially good info on the AHRI directory but not sure what the acronym means?  eae including eso and PE?  Sounds like the eae refers to the average kwh/year usage I may be looking for though that doesnt take seasons/zone in consideration. 

Thank you

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  1. gbcif | | #1

    What about a radiant system, hydronic system with high efficiency pumps? Perhaps less Amps.


  2. Yoshibear | | #2

    Cost would be an issue there for me. I think there are efficient 50-60K BTU furnaces but I can;'t tell how much they draw nor how often that fan would kick in. I see mentions on some posts about a 300-600W usage on average but that doesnt help much in getting a picture of what I might actually need to heat the house over 24 hours. House is super well insulated (i.e. passivehaus style) - so it should not need to run a whole bunch - winters get usually 15-20F lows. But translating that into kwh/day for my solar load calculations has me stumped.

  3. Expert Member
    Akos | | #3

    Even the most efficient furnace will be an issue off grid. These types of long runtime loads add up quickly both in needed battery capacity and PV array to supply them during winter conditions. A typical furnace ECM blower will use about 150W running and about another 50W for the draft inducer.

    Even if sized correctly, batteries inevitably run out and the generator fails to start at which point you risk freezing your cabin.

    Usually off grid heating is done by through the wall vented propane heater. These are pilot operated, don't have any fans and require no power. If this is mostly for backup heat/freeze protection, you can install one into your mechanical room or basement and set it at a low enough temperature to not run when you are heating with a wood stove.

    1. Yoshibear | | #4

      Well I use a wood stove to actually heat so no concerns there really but code requires me regardless to have a thermostatically controlled 50-60K BTU heater which eliminates any of the fanless wall mounted ones. Im mostly wanting to get an idea if my solar/battery system will handle it if I decided to not use wood stove for whatever reason.

      1. Expert Member
        Akos | | #6

        Code requires a properly sized heat source for the house, there is no requirement anywere for a 60000btu unit. Unless you forget to install windows, there is no way your cabin will be at 60btu/sqft, so you won't need that big of a fuel burner.

        Figure out what your actual heat loss will be and size from there.

        A well sealed and insulated 1000sqft in zone 5 should be around 10btu/sqft. Even if 2x that, it is still well within the wall units.

        As for solar battery handling a furnace, it is highly unlikely unless you have a very large array and battery. I have a 10kW array up north which during a week of snow storms will produce about 0.6kWh/day.

        Say the furnace was 100W, that would need 13kWh battery to run the week. And that doesn't include power for lights/fridge/tv/internet.

        1. Yoshibear | | #8

          Well I go by the online calcs - the plan examiners dont care too much as long as it close - 40000 might work for them.

          1. Danan_S | | #11

            Those calculators are for typical, leaky houses with average insulation. A passive house should use 1.4kWh/sqft/year for heating.

            You should model the house's actual heat loss.

          2. Expert Member
            Akos | | #13

            There is so many things wrong with that calculator that there is not even a point in addressing them.

            Run a Manual J on your place and see what it will come up with. Coolcalc is a reasonable on-line tool and also produces a Man J for a couple of dollars that you can submit to your building department. If you just want to see where you are at, you can try a pretty decent free tool (they want your email address though) here:


            The only thing plan examiners want to see is a Man J and equipment that meets the calculated load.

      2. Expert Member
        DCcontrarian | | #14

        Here code is that you do a Manual J and install a heat source according to it.

      3. Expert Member
        DCcontrarian | | #15

        There are thermostat-controlled propane heaters that don't use electricity.

        What I don't know is whether you can get one that is meant for permanent installation. Usually what they really want to see is that the house can be left unattended without freezing up. Insurance companies like to see that too.

        1. Expert Member
          Akos | | #16

          The through the wall units can only be permanently installed, they can be primary heating source. They are often used in cottages as backup heat with un-reliable local power.

  4. walta100 | | #5

    You may want to consider a Monitor brand Kerosene heater it should satisfy the inspector/ insurance requirements. It will take up very little floor space and only needs 130 Watts for ignition and 80 Watts when running.


    1. Yoshibear | | #7

      Oh thats very interesting. Thanks!

      1. StuSid | | #10

        MPI is out of business, but rinnai makes similar products. Low electric usage. the monitors only need a propane hookup and one hole in the wall for exhaust/intake. It will satisfy code and your insurance company.

        Also search for a used MPI422 kerosene monitor. It has an internal kerosene tank, 1gal. If its just for backup/emergency it is a good option, finding them is the challenge.

        1. Tom_K | | #17

          Yeah, used a 16k BTU Rinnai propane heater (similar to current model EX17DTP) for a 600sf cabin in zone 8, no problem keeping the place warm, very low power consumption. Quick look at the spec sheet says 0.7W standby. Less than most consumer devices in sleep mode. If you're really hard up you can run it off a pedal generator.

  5. Deleted | | #9


  6. Danan_S | | #12

    Another option is to run a radiant loop attached to a tankless propane water heater. If the idea is to keep the pipes from freezing, add a sensor for the pipe temperature attached to a pump that runs when the temp drops too close to freezing. A Chilipepper recirculating pump would work for this purpose ( This also has the benefit of keeping all combustion outside of your presumably very tightly sealed house.

  7. nrosdal | | #18

    the more efficient you go the longer planned runtimes they have which is great for air circulation and comfort but very bad for your power use as the fans can be alot for off grid setups even though they don't draw a ton when running 18 hrs a day.

    Have you thought of looking at something similar to a rv setup? they would vent through wall and need a small blower that runs on dc already and are not designed to run constantly like newer more efficient home furnace. In our 5th wheel we can go a few days in cold weather with just 2 6v batteries and no solar. So with more batteries and some panels it would look like a great option (if they make them large enough for your sized place.

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