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  1. #1
    Join Date
    Oct 2007
    Location
    Pacific NW
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    7

    How Many BTU's Per Sq. FT?

    I am building a house in northern Oregon that is about 4,000 sq. ft, on an exposed hill and it has lots of windows. While we get snow, the average temps during the winter rarely go below 20 F.

    The house will have concrete floors with radiant heating. it will have five zones.

    My installer tells me to figure that I need a boiler that will have a capacity of 25 btu's per sq. ft. of house or approximately an output of 105,000 btus.

    I've heard from other sources that I should expect to have a capacity of less than half that .

    Which is correct?

  2. #2
    Join Date
    Jun 2005
    Location
    The Gray Northwest
    Posts
    661
    You cannot go by square footage alone. Rules of thumb are a shot in the dark. You must have a heat loss calculation done. It's worth the money and you won't regret it.

  3. #3
    Join Date
    Oct 2007
    Location
    Pacific NW
    Posts
    7
    This is something that is being done, it's just not finished at this point. The original system that was floated by me by the installer was for a boiler with a capacity of 150,000 btu's.

    I have the choice of going either electric or propane. Electricity costs are .045 cents per KW, versus $2.25 per gallon of propane, so I'm going electric.

    Having contacted Electro - they said that I should be figuring about 10 btu's per foot.

  4. #4
    Join Date
    Oct 2007
    Location
    Pacific NW
    Posts
    7
    Let me add that from what I've learned - 150,000 btu capacity puts me in the commercial range of needs. I would like to think that a 4,000 sq ft. house isn't close to needing commercial capacity.

  5. #5
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by oe2x View Post
    Let me add that from what I've learned - 150,000 btu capacity puts me in the commercial range of needs. I would like to think that a 4,000 sq ft. house isn't close to needing commercial capacity.
    With your low electric rates have you looked into a high efficiency heat pump? They produce more than what they consume. In other words, depending on the outside temperature it may produce 2 - 3 times as much as it consumes in energy. No other system will even produce 100% of what it consumes, much less multiples above what it consumes.

  6. #6
    Join Date
    Apr 2007
    Posts
    6,285
    HVAC Calc

    Don't guess what you need.....know...could be 10,000 btu's could be 150,000 btu's so many factors that need to be known to even remotely make a educated guess......and all of them have to do with being at your home and seeing what your specific needs are.

  7. #7
    Join Date
    Oct 2007
    Location
    Pacific NW
    Posts
    7
    Quote Originally Posted by mchild View Post
    With your low electric rates have you looked into a high efficiency heat pump? They produce more than what they consume. In other words, depending on the outside temperature it may produce 2 - 3 times as much as it consumes in energy. No other system will even produce 100% of what it consumes, much less multiples above what it consumes.
    I honestly hadn't thought of a heat pump for the radiant system...
    Please tell me more - models etc...

    I am doing one for the AC unit which will be able to be used as backup heating or to heat the house quickly after traveling and having the radiant hybernate.

    BigJon - many thanks for the link. This looks very helpful.

  8. #8
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    your thread title reminds me that I need help deciding how much horsepower I need for a newer car --
    should I use
    # of passengers?
    sq ft of 'foot-print'?
    cu ft of interior?
    cu ft of engine & trunk & interior?

    the sedans I am considering would have but slightly more glass than my '91 Camry -- which gets great mileage.

    ie: your question is kindred to my dilemma.
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

  9. #9
    Join Date
    Sep 2006
    Posts
    1,253
    Quote Originally Posted by oe2x View Post
    I honestly hadn't thought of a heat pump for the radiant system...
    Please tell me more - models etc...

    I am doing one for the AC unit which will be able to be used as backup heating or to heat the house quickly after traveling and having the radiant hybernate.

    BigJon - many thanks for the link. This looks very helpful.
    The heat pump would not be for the radiant, but rather, instead of. Sounds like you may already be going in that direction.

    I would try to find a way to have the heat pump provide most if not all of your required heat as it will be the cheapest way to heat the home and you will not have the cost of a secondary system. If 20* is your winter design temp, there are heat pumps that should provide sufficient heat at that temp that nominal if any supplemental would be required.

    Since the home is currently being built I would be looking to see what could be done to improve on the envelope of the building that will give you pay back everyday.

  10. #10
    Join Date
    Mar 2008
    Posts
    15
    have someone who knows what they are doing do a energy audit on your house, get it properly sized. Lots of things will determine that:

    how big, how many rooms, ceiling heigth, where you live, windows, doors, type of insoluation in your house, basement, crawlspace <-- is it conditioned or unconditioned, how many people live in the house, appliances, etc. Probably a few more factors in there but too lazy too look. If you want it done proberbly and want to know close to the actual amnt of btus you will need have someone do a heat calcution on your house

    youd be surprised on what you will need, a new home vs an old home that is the exact same size could have quite a bit in difference in btus. Guessing is not a big one in my book and the rule of thumb doesnt work well for me either, almost like 400sq ft is 12,000 btus (rule of thumb). Hope that helps

  11. #11
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,764
    In order to know how much pex to install, the installers need to know how much heat loss the house has.
    How can they know how much pex, but not how many BTU's the house needs.
    Even if your house only needs 10BTU's per sq ft for heating, you still have additional load from the ground.
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  12. #12
    Join Date
    Oct 2007
    Location
    Pacific NW
    Posts
    7
    While the heat pump may give me less expensive heating, it's not the kind of heat I want. I have minimum 10' high ceilings in all the rooms, so the idea of heating the lower half of the room is appealing.

  13. #13
    Join Date
    Mar 2008
    Posts
    15
    Quote Originally Posted by beenthere View Post
    In order to know how much pex to install, the installers need to know how much heat loss the house has.
    How can they know how much pex, but not how many BTU's the house needs.
    Even if your house only needs 10BTU's per sq ft for heating, you still have additional load from the ground.
    very correct, have someone do a heat loss/gain on the house.

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