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Btu calculator
- Thread starter BengaliK
- Start date
I assume you mean heat loss calculator. If so, the free ones are either crap or hard to use (Heatpunk is supposedly one of the best, but it's not well set up for simple calculations for one room). It's easier to do it yourself using the relevant equations for heat loss and what you know about the construction. The real problems come in estimating heat loss from floors and air exchanges (which depends on how leaky your room is, which is typically a complete unknown).
36000 btus is about 10.5 KW, a typical heat loss for a whole 3 bedroom semi built in the 1960s or 70s with little or no wall or floor insulation. Unless your lounge is about 10 metres square with single skin (4") walls and single glazed windows, I would go with 15000 as more likely!
36000 btus is about 10.5 KW, a typical heat loss for a whole 3 bedroom semi built in the 1960s or 70s with little or no wall or floor insulation. Unless your lounge is about 10 metres square with single skin (4") walls and single glazed windows, I would go with 15000 as more likely!
You don't need a calculator. Just a pen and paper. Or ideally excel.
Get your room areas (A)
Discount floor or roof if the space above or below is heated. Ignore internal walls.
Look up the heatloss for your window type, wall type, floor and ceiling (last two if relevant) - (W/m²·K) - (U) - see list below.
Inside target temp 20 degrees (pick a number).
Outside temp -4 is typical (depends where you live)
Delta of above is 24 degrees (DT) - yours may vary.
Then multiply them together
Q = A . U . DT
This will give you watts (W)
Repeat for all surfaces with heatloss.
Ventilation. The bane of my life. If you don;t have it measured you need to assume air changes per hour.
Really old leaky houses with draughting chimneys you can assume 3 to 4.
2 for an old house with minimal effort to make it leak proof.
Modern building will be about 0.6 to modern regs. And everywhere in-between (ACH).
Calculate volume of your room (V)
Air heat capacity = 0.33 (C)
Q = C . V . ACH . DT
Add up all your Q's to get your total (W)
Multiply by 3.412 to go from Watts to BTU.
BTU = W x 3.412
Only use this BTU value to size the radiators for temps quoted by the radiator company (usually DeltaT50).
If you are on a low temp system, you need to adjust the BTU.
For reference here are some typical U values that might be useful (hopefully they are all correct, just pinched them form my spreadsheet)
Walls
stone - 450mm - 2.23
stone - 300mm - 2.78
brick - 228mm - 2.11
brick - 450mm - 1.40
Floor
suspended - no insulation - 1.75
concrete - no insulation - 1.20
brick - 228mm - 2.11
celotex - 100mm - 0.22
foam glass - 200mm - 0.25
rockwool - 200mm - 0.2
windows
single - 5.8
90's double - 3.1
modern double - 1.4
secondary glazing - 2.7
ceiling
non insulated - 1.5
50mm glass - 0.68
100mm glass - 0.3
200mm glass - 0.2
modern regs - 0.13
Get your room areas (A)
- floor area.
- External wall area
- Window area
Discount floor or roof if the space above or below is heated. Ignore internal walls.
Look up the heatloss for your window type, wall type, floor and ceiling (last two if relevant) - (W/m²·K) - (U) - see list below.
Inside target temp 20 degrees (pick a number).
Outside temp -4 is typical (depends where you live)
Delta of above is 24 degrees (DT) - yours may vary.
Then multiply them together
Q = A . U . DT
This will give you watts (W)
Repeat for all surfaces with heatloss.
Ventilation. The bane of my life. If you don;t have it measured you need to assume air changes per hour.
Really old leaky houses with draughting chimneys you can assume 3 to 4.
2 for an old house with minimal effort to make it leak proof.
Modern building will be about 0.6 to modern regs. And everywhere in-between (ACH).
Calculate volume of your room (V)
Air heat capacity = 0.33 (C)
Q = C . V . ACH . DT
Add up all your Q's to get your total (W)
Multiply by 3.412 to go from Watts to BTU.
BTU = W x 3.412
Only use this BTU value to size the radiators for temps quoted by the radiator company (usually DeltaT50).
If you are on a low temp system, you need to adjust the BTU.
For reference here are some typical U values that might be useful (hopefully they are all correct, just pinched them form my spreadsheet)
Walls
stone - 450mm - 2.23
stone - 300mm - 2.78
brick - 228mm - 2.11
brick - 450mm - 1.40
Floor
suspended - no insulation - 1.75
concrete - no insulation - 1.20
brick - 228mm - 2.11
celotex - 100mm - 0.22
foam glass - 200mm - 0.25
rockwool - 200mm - 0.2
windows
single - 5.8
90's double - 3.1
modern double - 1.4
secondary glazing - 2.7
ceiling
non insulated - 1.5
50mm glass - 0.68
100mm glass - 0.3
200mm glass - 0.2
modern regs - 0.13
Ha! I see that as I was writing so was MJ, who has kindly provided information for doing your calculations. I would add that solid floors are more difficult than simply multiplying a U value by the area and the delta T. Heat loss occurs primarily at the base of the external walls and the ground below the bulk of the floor will not typically drop much below 10 degrees and can even act like a heat battery.
In a useful paper entitled 'Heat loss from a solid ground floor' in Building & Environment (volume 28, no 3), M.G. Davies recommends that the U value of an uninsulated concrete floor in a rectangular room be calculated as
U = [lamda/W][2/pi.x]ln[(1+x)(1+1/x)^x]
Where, ln is the natural logarithm, lambda is the thermal conductivity of the ground material, W is the thickness of the walls/footings, and x is a function of the length (L) and breadth (B) of the room (in metres), given by
x = LB/[W(L+B)]
Good luck!
In a useful paper entitled 'Heat loss from a solid ground floor' in Building & Environment (volume 28, no 3), M.G. Davies recommends that the U value of an uninsulated concrete floor in a rectangular room be calculated as
U = [lamda/W][2/pi.x]ln[(1+x)(1+1/x)^x]
Where, ln is the natural logarithm, lambda is the thermal conductivity of the ground material, W is the thickness of the walls/footings, and x is a function of the length (L) and breadth (B) of the room (in metres), given by
x = LB/[W(L+B)]
Good luck!
Good spot.I would add that solid floors are more difficult than simply multiplying a U value by the area and the delta T. Heat loss occurs primarily at the base of the external walls and the ground below the bulk of the floor will not typically drop much below 10 degrees and can even act like a heat battery.
I missed this when writing up.
I've used 9.2 degrees in my calculations for ground temp.
I think I got this form MSC.
You then take the values and reduce by your (elevation * 0.006) to get your local air and ground temperature.
| design ambient temperatures | air temp | gnd temp | altitude m |
| Plymouth | -0.20 | 11 | 27 |
| Belfast | -1.20 | 10.5 | 68 |
| Cardiff | -1.60 | 9.9 | 67 |
| London | -1.80 | 10.2 | 25 |
| Manchester | -2.20 | 10 | 75 |
| Birmingham | -3.40 | 9.8 | 96 |
| Edinburgh | -3.40 | 8.5 | 35 |
| Glasgow | -3.90 | 8.5 | 5 |
| Channel Islands | -0.20 | 11 | 0 |