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Thread: Cost of Running Hydronic System

  1. #1
    DIY Junior Member Oregon Heat Seeker's Avatar
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    Default Cost of Running Hydronic System

    We have a 1970s - built house in Southern Oregon, which is currently heated by a heat pump. I'm considering replacing this system with a hydronic system, but am curious about what the operating costs will be. For the sake of arguement, let's say that the house is ~1600 SF, and has 2x4 insulated walls. I've done some preliminary calcs, and sized hydronic radiators to deliver 30,000 btu/h. If I'm using a gas-fired micro-boiler at between 84 and 90% efficiency, how do i go about determining how many therms of gas it would take to operate?

    Since there are 100k BTU / Therm, if I needed 30,000 BTU/H continuously, that would equate to
    (30,000 BTU/H x 24H/Day x 30.5D/Mo)/ 100KBTU/Therm = 219.6 Therms / Month? I'm sure that's way too much.

    What percentage of the time does a boiler need to be boiling?

  2. #2
    In the trades Dana's Avatar
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    The boiler & radiation sizing is best matched for the heat loss in BTU/hour at the 1-3% coldest hours of the year, which will typically occur in 1-3 hour stretches in the pre-dawn hours of the coldest day of the year, never for 24 hour stretches.

    To come up with an approximate annual fuel use number you need to know the BTU/hr at design condition, the average efficiency of the boiler, and the total number of heating degree-days (base 65) that you would experience in a year based on the weather history. As an example, let's use your 30,000BTU/hr as your design condition heat load, 90% as the average boiler efficiency, and use nearby Medford OR for design temp and weather history data.

    According to table 1A on p17, your 99th percentile outside design temperature per ACCA Manual-J is +24F. Using 65F for an outside "balance" temperature base between heating & cooling loads, that's

    (65 - 24=) 41F heating degrees, at which your load would be 30,000 BTU/hr.

    But in a 90% boiler it would be burning (30,000/0.9 =) 33,333 BTU of source fuel or 0.33 therms per hour.

    If it were to stay that cold for 24 hours that would indeed be (24 x 0.33=) 8 therms.

    But that day would count for 41 heating degree days, so the boiler burns in fact (8/41) = 0.195 therms per heating degree-day.

    According to ClimateZone your annual HDD/65F is about 4600 HDD so the amount of heating fuel you would burn in a year is (4600 x 0.195=) 897 therms/year.

    The coldest average winter month is January, at 834 heating degree days, so a peak mid-winter monthly billing period would run about (834 x 0.195=) 163 therms/month, but less on average over the heating season.

    A design heat load of 30,000 @ 24F would be slightly to the high side for a tight 1600' 2x4 house with an average glazing fraction and double pane windows, but is realistic at higher air infiltration rates. With almost any 1970s vintage home it's worth air-sealing and spot-insulating, and either adding storm windows (low-E storm windows are even more cost-effective with a quicker payback but cost more up front), or if there's no other way around it, replacing some windows with a U0.30 or lower window.

    If yours is an open floor plan and you have lower than average electricity rates it may be slightly cheaper to heat with a ductless mini-split heat pump, which should have an average system coefficient of performance (COP) of 3.0-3.5 in your area. With an older R22 or R12 refrigerant heat pump with ducts the odds are you were only getting a COP of 2.0-2.5. Whether condensing gas v.s. split-system heat pump is cheaper depends on your local electricity and gas rates, and it may be a toss-up.

  3. #3
    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    We use a CAD program to perform a proper ACCA Manual 'J' 8 heat load and the estimated cost of operation for various fuels available is one of the many useful results.

  4. #4
    In the trades Dana's Avatar
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    CAD systems sure make it simpler and easier, eh? ;-)

    I'm not sure what "....prelminary calcs..." Oregon Heat Seeker performed to size the radiation for 30KBTU/hour, or at what average temperature it would deliver that, or if 30K was even the whole house heat load at design temp. But the math for getting to the fuel use at that location based on boiler efficiency assuming 30K load is pretty straightforward, if more complicated than Heat Seekers attempted methods.

    I suspect the true heat load & fuel use may be closer to half or 2/3 that, or could be with improvements to the building. It doesn't take superinsulation or super-tightness to get the heating loads of a 1600' house in the Pacific Northwest down to 20K. Even crusty old schoolers use 20BTU/ft or 25BTU/ft for their WAG overestimations. Manual-J on my mother's not-too-insulated none-too-tight place in WA comes in under 20 BTU/ft, and probably overshoots reality. A tightened up 1970s 2x4 framed house would almost certainly do better than her place unless there's a lot of U1 single-pane.

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    Master Hot Water Mpls,MN BadgerBoilerMN's Avatar
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    Most of my California radiant designs end up with tank-type water heaters serving domestic hot water and space heating.

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