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Sizing a pellet stove vs a traditional furnace

Post in 'The Pellet Mill - Pellet and Multifuel Stoves' started by EatenByLimestone, Mar 22, 2008.

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  1. EatenByLimestone

    EatenByLimestone Minister of Fire

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    I was thinking, which normally leads to trouble.

    My house has a 100K btu NG furnace. The furnace turns itself on and off as needed to keep the temperature what the thermostat tells it to. Since it turns itself on and off, it is only using the 100K btu every once in a while.

    If a pellet stove could be located centrally, could it be rated at a significantly lower btu output since it would be running constantly?

    What got me thinking of this was my little 25K btu woodstove. It's located in the back corner of an addition and I blow a bit of heat into the house. It's not in an ideal spot to heat the house, yet a couple times this year I turned the furnace off (and forgot to turn it back on) and the stove was able to keep the house warm for days. A good part of that probably has to do with thermal mass and plaster walls, etc. But I also wonder about the woodstove cranking ~20Kbtu constantly.

    Would a smaller heater (like a pellet stove) cranking out a constant heat, do the job of a much larger furnace that cycles on and off.

    Matt

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  2. rap69ri

    rap69ri New Member

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    Good question. I have a pellet stove that is rated at 38k btu in my kitchen that has heated my house since November. I had to leave my furnace on, oil fired, for hot water all winter. My furnace is rated at 120k btu, and I used it a handful of times this winter on the coldest night to make sure my pipes didn't freeze. For the most part my little 38k btu rated pellet stove kept my house very comfortable all winter.

    I know I didn't answer your question, but your thoughts seem to be correct in my case anyway.
  3. sylvestermcmonkey

    sylvestermcmonkey Member

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    Once the inside of your house is at a desired temperature, the heat output of whatever appliance you're using simply has to equal the heat loss to the outside. You don't need a 100,000 BTU furnace for that.
    Here's a quick link to cure you of that trouble:

    http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/heatloss.html
  4. rap69ri

    rap69ri New Member

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    Thanks for the college physics flashback sylvestermcmonkey, and the reminder that I can still do the math ;-).

    In the end it all comes down to dollars and cents. If it's cheaper to feed the pellet stove running continuously than the oil/gas furnace which runs only on demand, then run the pellet stove.
  5. sylvestermcmonkey

    sylvestermcmonkey Member

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    Pretty much. I did a cost justification recently, and compared my cost per delivered million BTU for various fuels:

    propane $42.27
    heating oil $30.32
    natural gas $11.76
    wood pellets $20.20
    electric $36.25
    coal $12.50

    I used recent local prices and reasonable estimates of efficiencies for modern heating appliances. The real eye-opener for me was propane. Even electric heat would be cheaper. YMMV.
  6. rap69ri

    rap69ri New Member

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    I was always against NG because of the possibility of blowing up, but the price is starting to outweigh my fears. If oil continues to climb at its present rate I'll replace my 3 year old oil furnace with NG next year.
  7. webbie

    webbie Seasoned Moderator Staff Member

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    My advice for sizing most pellet stoves calls for consideration of the amount of pellets that are to be used!

    What that means is that most people are NOT going to haul "what ever is needed" into their house, but are going to run from 1 to 2 bags in a 24 hour period. In cold weather, that means about 80 lbs per 24 hours, or a ton every 25 days. As you can imagine, that could be 4-5 tons in a long heating season, and few people use this much....

    So working backwards........80 pounds in 24 hours = a little over 3 pounds per hour - which equals about 17,000 BTU output and 22,000 input. As you can see by these calcs, sizing a pellet stove to something over 40K BTU is only for those who can imagine hauling 5 tons or more of pellets into their living area.

    Pellet stoves are SPACE heaters and should be sized as such. Most gas or kerosene space heaters are from 15K BTU to 40K. Not much reason for a pellet to be larger except in rare cases.

    NOTE: Due to differences in pellets, corn, etc - I would still size a pellet stove up by about 35% - in other words, if I needed 30,000 input, I would buy one that could do 40,000 input.
  8. Ken45

    Ken45 Minister of Fire

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    We have been disappointed with ours and I would not disagree with Craig's observation. But then they are expensive space heaters!

    One thing they are good for is in warmer weather when a wood stove is too much heat or has to be run inefficiently. A pellet stove is also nice in that it has a thermostat control and can even out the temperature variations from a wood stove. It's also good when we are away for the day. Bottom line, we are concluding that our pellet stove is a supplement to a wood stove, not an alternative.

    OTOH, I know a lot of people are happy with pellet stoves alone but we are not in that group.

    Ken

    Harman Advance (pellet)
    Hotblast 1400
    PE Summit (awaiting installation for next year)
  9. Shooter

    Shooter New Member

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    Then there is the heat loss factor. Our house is 3yrs old and well insulated (1800sq ranch, walkout basement). Just installed a Quad CB1200i and have been burning 1 bag of pellets per 24hrs. We have had nights in the teens and days in the 20's. This morning it's 5f. The furnace hasnt come on since I started the stove up on saturday. If the winds start blowing/howling to the point of rattling the siding here up on a hill then things will change. But for now, it's up to the insulation and floor plan to disperse the heat. And we have an open floor plan.

    Too many variables to trust entirely on BTU rating for real life comfort.

    My wife loves this thing. :)
  10. yak651

    yak651 New Member

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    Shooter, do you have your stove in the walkout basement or is it on the main level of the ranch? I have a ranch about the size of yours and trying to get a feeling for how well a pellet stove will work if I install in the basement for providing supplemental heat in the main level
  11. Shooter

    Shooter New Member

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    We have a Redman bocca/state code modular that has an open floor plan from the den (on one end) through the atrium, kitchen and then the dining and living room area. On the end (in the den) we had a direct vent/zero clearance propane fireplace that put out 18,000btu that I used to do a rough estimate on how the insert would do in that area. It works great. So far, so good. We are still on the lowest setting with 10deg temps outside. Temps are 70deg at the end bedroom. House is 28x64.

    For us, putting the stove downstairs (freestanding) would be counter productive due to the occasional high winds. And we love the cozy warmth of having the stove up here with us. The insert was the only way to go since we didnt want a stove in the living room or dining room......and that propane fireplace was sitting doing nothing but looking pretty.
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