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12092009, 01:25 PM  #1 
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Join Date: Dec 2009
Posts: 100

Sizing a room up
As far as figuring up btu's per room, how do I do that. example: 10' X 12' X 10' Ceiling. I Know that would equal my cubic feet, but how do I know what btu's are needed, is there a rule of thumb for this?

12092009, 02:39 PM  #2  
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Join Date: Jul 2008
Location: NW of D.C.
Posts: 5,990

Sizing a room upQuote:
For more precision there's always Manual J, for whole houses. Last edited by Yoyizit; 12092009 at 02:44 PM. 

12092009, 04:04 PM  #3 
Civil Engineer
Join Date: Mar 2009
Location: Boston
Posts: 4,141

Sizing a room up
If you want to make an accurate analysis, you need to calculated the heat loss through the six sides of your room, and size your heater to produce at least that amount of heat. To compute heat loss through a wall, you need to know the temperature difference between inside and outside, and the R value of the wall. For example.
You have a wall which is 8 feet high and 12 feet long, for an area of 96 square feet. The wall is insulated to R19. You plan to heat the inside of the house to 68 degrees F. The design outside temperature is 10 degrees F. Therefore the temperature delta is 58 degrees F. The loss through the wall is the reciprocal of the R value, or 1/19 BTU per degree per square foot per hour. So in this case, you lose 58/19 = approximately 3 BTU per square foot per hour, or about 300 BTU per hour total through the wall. You make the same computation for each of the six sides of the room (include ceiling and floor), but note that the temperature difference between the room and the attic is not the same as the temperature difference between the room and the outside. Similarly, each wall has different insulation. Do this for each room, add up the total, and you will know how much heat loss to expect under design conditions, so you can size your heater appropriately. Make sure you account for windows, they typically have lower R value than the walls they set in. Also, you need to adjust for air gaps, most houses are not very tight. So after all this analysis, you may simply want to ask a furnace supplier, they probably know more or less what works in your area. 
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