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Thread: Metered gas use

  1. #1
    DIY Junior Member
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    Default Metered gas use

    Is it typical for an average gas hot water heater, a gas range and a gas dryer to use appx 1 therm per day with a typical family?

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    Moderator & Master Plumber hj's Avatar
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    That is about 100 cf of gas, and a water heater uses almost half of that per hour, then add in a gas dryer, furnace, cooktop, and oven, it could be stretching it to just use one therm per day.
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    Plumber jimbo's Avatar
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    As hj said, a water heater figure about 1/2 therm per hour that the burner runs. Each cook top burner will use anywhere between 0.05 and almost 0.2 therms per hour. A gas oven...probably 0.2 to 0.3 therms per hour.

    All in all, it is hard to see how you would use less than 1 therm per day.

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    In the trades Dana's Avatar
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    The majority of non-space heating fuel use would be for hot water.

    Four 8 minute 2gpm showers/day with an annualized 60F temperature rise is 24 minutes @ 60KBTU/hr, or (24/60)hours x 60KBTU/hr= 0.24 therms output. With a 0.60EF hot water heater that takes 0.24/0.6= 0.4 therms of fuel input.

    If your bathing is the typical 40-50% of all hot water use you're looking at around 0.75 therms/day or more for just the hot water, or 275therms/year, which would be consistent with mid-efficiency tank type HW heaters at standardized estimated volumes of hot water use for a "typical" family.



    (^^ it's 238 therms/year for this pretty-good EnergyStar model ^^)

    The rest of the fuel use adds up to peanuts by comparison, but a therm/day isn't unreasonable. Just one "endless shower" teenager who regularly stays in the shower until the hot water runs out could easily double that.

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    Plumber jimbo's Avatar
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    Well, I looked up my bill. We live in a very small condo, just me and wifey. Gas WH and gas stove. She does all the laundry in cold, and we cook largely with the convection nuker. Occasionally fry an egg or a pancake on the stove. No gas heat. Electric dryer. Our bill for January is 15 therms, or about 1/2 a therm per day.

    I could see that a gas dryer would add a lot for us, because she washes a lot! More us of the oven would add some. More than 2 people would be more hot water. Is your goal to be at 1 therm or what is the question exactly?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I used 55 therms for December. Gas heat, stove, dryer, water, and barbeque (yes, I use it in the winter here, too!). Now, I typically fill a large soaking tub once a day, and do my clothes in a warm wash with cold rinse. Summertime, it's closer to 15-20 therms.
    Jim DeBruycker
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    In the trades Dana's Avatar
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    Quote Originally Posted by jimbo View Post
    Well, I looked up my bill. We live in a very small condo, just me and wifey. Gas WH and gas stove. She does all the laundry in cold, and we cook largely with the convection nuker. Occasionally fry an egg or a pancake on the stove. No gas heat. Electric dryer. Our bill for January is 15 therms, or about 1/2 a therm per day.

    I could see that a gas dryer would add a lot for us, because she washes a lot! More us of the oven would add some. More than 2 people would be more hot water. Is your goal to be at 1 therm or what is the question exactly?

    Two people couldn't possible generate enough laundry to make the dryer a significant part of the gas bill. It's still going to be at least 75% hot water use.

    And 15 therms/month would be about right for a 2 person household with tank HW heater with primarily showering/bathing use. A typical standing pilot ignition burns through ~0.20-0.25 therms day or ~7therms/month. Some of that pilot heat ends up in the tank, but most is lost as standby. The total standby loss of a gas-fired tank is going to be somewhat more than that for standing pilot units.

    But for any tank units you can estimate the fuel used to support the standby loss this way: A gas fired tank burns at ~80% efficiency. The DOE EF test assumes a use of 64.3 gallons (536lbs) of water/day with a temperature rise of 77F, so the total BTUs that went into heating the water is (536 x 77=)~41,275. With an 80% burner takes (41275/0.8 =) 51,594 in source-fuel BTUs to generate that temperature rise. But if the unit tests with an EF of 0.60, it means it actually burned (41275/0.6=) 68,792BTUs of source fuel. The difference (68792- 51594) is 17,198 BTU/day, and x 30 days is 515,940 BTU (or about 5 therms/month.)

    Mind you, the EF test assumes an ambient room temp of 67.5F, so if the tank is in a room dramatically warmer or colder than that the standby losses will vary.

    So for two people 8-10 therms/month above standby use would be a modest bathing budget.

    Most gas dryers aren't going to use more than ~80 therms/year (~7 therms/month) even for a family of 4, or about the same order of magnitude of the standby loss on a tank heater. For a two person household it's about half that- not exactly "in the noise" of your hot water use, but not a big adder.

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