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Gas usage--Fireplace vs. Furnace

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  • Gas usage--Fireplace vs. Furnace

    I'm trying to do some rough cost analysis:

    We have a gas fireplace on our main level, about 12 feet from our programmable thermostat. It keeps our family room plenty warm. We have a vaulted ceiling and a balcony overlooking the family room, and the bedrooms right upstairs. The heat rises and heats the master bedroom very nicely as well. The rest of the house stays pretty cool, but since we're not in those other rooms, that's fine with us.

    When the fireplace is on, it cranks out enough heat such that our gas heater never turns on (themostat set to 70), even when it's 20 degrees outside. Both our family room and bedroom get to around 72 degrees.

    So my big question is if this is really saving me money? Is leaving our fireplace on from 5pm until 11:30pm use less gas than letting the furnace cycle on and off?

    I guess I also have a more specific question: how does the gas usage per hour compare between a typical gas fireplace vs. a typical gas furnace? Undoubtedly a furnace is much more efficient in extracting heat from the fuel than a fireplace, but the tradeoff is that we're only heating a portion of the house.

    Any thoughts or anecdotal evidence in other homes appreciated!

  • #2
    Re: Gas usage--Fireplace vs. Furnace

    To be able to tell you exactly, we'd need to know the btu input ratings and efficiency ratings of your furnace and fireplace.

    Chances are your furnace has atleast twice the input rating of your fireplace. You might be wasting the same amount of gas out of each of the appliance flues due to the high efficiency of your furnace and low efficiency of the fireplace, but if you are heating everything you need while using half of the gas, I guarantee you are saving money.

    A 70,000 btuh furnace which is 92% efficient loses close to 5600 btuh out of the flue. A 33,000 btuh fireplace with an efficiency of 78% will lose about 7260 btuh. So in this scenario even though you are losing an extra 1660 btuh per cubic foot of gas while heating with the fireplace, you are saving money by burning less gas initially.