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what size boiler do I need?

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Curt-Lennox- Winslow, Oct 13, 2013.

  1. Curt-Lennox- Winslow

    Curt-Lennox- Winslow New Member

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    I have a 2700 sq. colinial with an unfinished/unheated basement. I currently use oil for my hot water and baseboard heating (forced hot water). How do I determine what size pellet boiler to look at for heating my house and domestic hot water? Is that something to leave up to the pellet boiler salesman or is there another way to figure it out? I have a pellet stove on the 1st floor but it doesn't heat the whole house evenly.
    Curt

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  2. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    To give you some first ball park number on heat loss, you can use these guidelines for "moderate" New England conditions:
    - old house not insulated or poorly insulated: SF of house x 37 BTU/sf
    - house in 2x4 stick construction fairly well insulated: SF of house x 30 BTU/sf
    - house in 2x6 stick construction very well insulated and tight: SF of house x 25 BTU/sf
    - house in 2x6 stick construction super insulated and very tight: SF of house x 15 BTU/sf
    - Passive House: SF of house x 8 BTU/sf
    Example:
    a 2x6 stick build house in 2012 of 2,500 SF should have a max heat loss of 2,500 x 25 = 62,500 BTU/hr
    The same square footage for an old house would result in 2,500SF x 37 = 92,500 BTU/hr
    The above guidelines include for DHW for 2 adults and 2 kids.
    These guidelines are up for lots of discussions and other trade professionals will have their say on them, but it gives you a ballpark.
    If you apply them you will not end up with a boiler that is way over-sized.
    stayfitz and Bster13 like this.
  3. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Bster13 likes this.
  4. R Mannino

    R Mannino Member

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    Smaller is better with boilers unless you have storage.
    hobbyheater likes this.
  5. heaterman

    heaterman Minister of Fire

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    Or a boiler that can modulate as some pellet boilers are able to do. That feature allows a fair amount of margin for error.
  6. Curt-Lennox- Winslow

    Curt-Lennox- Winslow New Member

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    heaterman, what do you mean when you say a boiler that can modulate. I am looking at a windhager biowin 21.
  7. maple1

    maple1 Minister of Fire

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    Speed up & slow down. By themselves.
  8. PassionForFire&Water

    PassionForFire&Water Minister of Fire

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    Modulating in case of the BioWIN = heat output BioWIN matches the heat loss of the house (heat demand)
    From max heat output to 30% of max.
    Last edited: Oct 15, 2013
  9. heaterman

    heaterman Minister of Fire

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    The 21kw unit you are looking at will go from a maximum firing rate of about 72,000 btu down to about 23,000 btu.plus run anywhere in between in as little as 1% increments.
    This enable the boiler to match what is going on in the home whether you have one heating zone calling or all of them. (provided you have a multi zone system of course)
    Last edited: Oct 17, 2013
  10. Dana B

    Dana B Feeling the Heat

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    Curt:

    As Marc pointed out earlier the BTUs needed for your home are largely determined by how well insulated it is. I recently purchased a Biowin pellet boiler but I'm also undertaking insulation projects in both my basement and attic.

    Like me I'm assuming you're considering the pellet boiler as a way to save money on energy costs. If you're seriously considering a pellet boiler it would behoove you to take a holistic approach to the energy efficiency of your home to see if there are ways that you can improve upon your home's insulation and air flow. There is a wealth of information at your fingertips on the internet about insulating your home properly.

    .
    Last edited: Oct 18, 2013
  11. Nofossil

    Nofossil Moderator Emeritus

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    Do you know what oil consumption was for the last heating season? That can help dial in your heat usage. Rule of thumb: Peak heat load = twice average heat load.
  12. Curt-Lennox- Winslow

    Curt-Lennox- Winslow New Member

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    Nofossil, my oil consumption for last winter was about 300 gal. for heating hot water, as I heat with a lennox pellet stove. My last year on oil only was about 800 gal.
    Curt
  13. Nofossil

    Nofossil Moderator Emeritus

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    25 gallons a month for hot water is a bit higher than average, I think.

    A gallon of oil in a typical boiler provides around 100,000 to 120,000 BTU of usable heat. Back of the envelope calculation:

    Assume a 4 month heating season.

    25 gallons a month for hot water, and 125 gallons a month for heat (your extra 500 gallons from your 800 gallon year divided by 4 month heating season).

    That's about 3800 btu/hr average for hot water and 19,000 btu/hr average for heat. Your peak heat demand would be about twice as high, or 38,000 btu/hr.

    That means that a 45,000 btu/hr pellet boiler would be completely maxed out on a really cold day. You'd want some headroom, but 60,000 btu/hr seems like it would be in the right range - maybe a bit higher to allow for quicker recovery.

    Of course, your mileage may vary. I'd try calculating it a couple of different ways.

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