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Thread: Electric Baseboard Heat Calculation

  1. #1
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    Electric Baseboard Heat Calculation

    I have a customer that wants electric heat in their basement. I never really learned how to calculate this...just always guessed and the rooms were always finished rooms with insulated walls. This situation is for a basement that is not finished or even insulated, in fact the heated space on the first floor above will be insulated from the basement. The basement ceiling is going to have fiberglass insulation placed between the joists.

    It sounds like a wacky situation but this home is about to be converted from a summer cottage to a year round home. The owners goal is to keep the pipes in the basement from freezing. In previous winters the place was just shut down and the pipes were drained to prevent frozen water pipes. His other goal is to keep the basement warm enough to be able to run a washer and dryer. I can't imagine doing laundry in a room that is only 40 degrees.

    The house has cement block foundation walls that rise about three feet out of the ground. No insulation on the block walls. The space height to the bottom of the ceiling joists is five feet. The space is about 30' by 33' and the outside temp goes as low as zero Fahrenheit. He is right on the coast of Massachusetts so below zero pretty much never happens because the ocean moderates the temps.

    Any ideas on how to calculate the wattage this will need?

  2. #2
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    here's a thread on it:

    http://forums.mikeholt.com/showthread.php?t=108071

    1W per cubic foot is 8W per sq ft in most places (8' ceilings).

    The wildcard is how much outside air gets into the basement. This should be close to zero if he wants to heat it.

    You've got one room that is almost 1,000ft2? I dont have a good calculation for you, tho I'd start with 1 8' heater (2000-2500W ea, 4 total) on each wall. 8' is generally good up to 350sq ft, tho I'd go half that for uninsulated space.

    He could probably maintain 40*F with one heater tho if it fails he's got frozen pipes... unless he heat traces them.
    Electricians do it until it Hertz!

  3. #3
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    That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  4. #4
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    Quote Originally Posted by Dennis Alwon View Post
    That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO
    I agree, tho if the pipes are (presumably) at ceiling level, and the ceiling is insulated, it shouldn't take much to keep it above freezing. Personally, I'd heat trace the pipes and put in a portable for doing laundry, but that's me. Our old house, the dryer was in the carport, and if you wanted dry clothes, you go outside year round... tho it never gets to 0* here.
    Electricians do it until it Hertz!

  5. #5
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    Quote Originally Posted by Dennis Alwon View Post
    That's ridiculous and an outrageous waste of energy. You will need about 10kw of heat min, IMO
    I agree but I'm not sure what to suggest to the customer.

  6. #6
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    Quote Originally Posted by mkgrady View Post
    I agree but I'm not sure what to suggest to the customer.
    Tell her that energy codes will not allow you to wire in the BB heaters unless the room is insulated to the energy code. This is true around here
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



  7. #7
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    Quote Originally Posted by Dennis Alwon View Post
    Tell her that energy codes will not allow you to wire in the BB heaters unless the room is insulated to the energy code. This is true around here
    I wish I knew a lot more about heating and insulating. I'm wondering how I would respond to the following hypothetical questions from a customer (either in KWH or .20 per KWH):

    If I do not insulate the basement walls what will it cost to heat this basement to 40 degrees during the winter?
    If I do insulate the basement walls whatever will it cost?

  8. #8
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    How to do the calculation, with a bunch of assumptions along the way:

    If it's 0F outside and 40F inside, that's a delta T of 40 degrees. Assume the area losing heat is 4 feet by 125 feet = 500 square feet. That includes 1 foot of buried wall, but none of the rest of the buried wall or the slab. Ignore the heat gain from the presumably conditioned space above.

    Assume the concrete block walls, uninsulated, are R-2, and ignore any contribution from "still air" layers. That means the heat loss is 40 F * 500 ft^2 / 2 = 10,000 BTUs/hr. That's about 2900 watts per Google.

    As to the cost, you would need to know how often the outdoor temperature is below 40 degrees, and by how much. The instantaneous rate of heat loss will be proportional to the temperature difference.

    Cheers, Wayne

  9. #9
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    Quote Originally Posted by wwhitney View Post
    How to do the calculation, with a bunch of assumptions along the way:

    If it's 0F outside and 40F inside, that's a delta T of 40 degrees. Assume the area losing heat is 4 feet by 125 feet = 500 square feet. That includes 1 foot of buried wall, but none of the rest of the buried wall or the slab. Ignore the heat gain from the presumably conditioned space above.

    Assume the concrete block walls, uninsulated, are R-2, and ignore any contribution from "still air" layers. That means the heat loss is 40 F * 500 ft^2 / 2 = 10,000 BTUs/hr. That's about 2900 watts per Google.

    As to the cost, you would need to know how often the outdoor temperature is below 40 degrees, and by how much. The instantaneous rate of heat loss will be proportional to the temperature difference.

    Cheers, Wayne
    Thank you Wayne. Good to know there is a way to figure it out but it is really over my head. Is it safe to assume insulating the block walls will make a big difference in operating cost?

  10. #10
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    Quote Originally Posted by mkgrady View Post
    Thank you Wayne. Good to know there is a way to figure it out but it is really over my head.
    The idea is to size your heat source to just keep up with the heat loss during the coldest design day of the year, so that under those conditions the heater will run continuously. (Assuming you've calculated the heat loss correctly.) The basic heat loss formula is

    Heat Loss (in BTUs/hr) = Area (in square feet) * Temperature Difference (in Fahrenheit) / R value (in American units)

    Google will do unit conversions, so I just ask it to convert BTUs/hr to watts.

    The tricky part is modeling the boundary area properly. In my sample calculations I just considered the concrete block walls (including 1 foot below grade), and I ignored the heat loss through the slab below and the heat gain through the floor from the presumably conditioned space above. That made the calculation easy and meant I only had to use one R-value, for the concrete block walls.

    Quote Originally Posted by mkgrady View Post
    Is it safe to assume insulating the block walls will make a big difference in operating cost?
    You can definitely say that the heat loss though the walls (and thus operating cost attributable to that) is inversely proportional to the R-value. It seems concrete block walls have an R value of about 2. If one added 1 inch of EPS foam board insulation to the inside of them, with an R value of 4, to get a total R-value of 6, then the heat loss through the walls would 1/3 as much. [But if the block walls were R 3 to start, then the new R value would be 7, and the heat loss would now be 3/7 as much.]

    Cheers, Wayne

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