Here's the 'ballpark' chart I've used for window units for many
Hope that's of some help.
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Assuming the windows are shaded, divide their area in ft^2 by their
US R-value (or multiply by the U-value) to find the window conductance
then add the exposed non-window wall area divided by its R-value, then
add the air leakage in cfm to find an approximate thermal conductance
G = Awindow/Rwindow + Awall/Rwall + cfm in Btu/h-F. A room with volume
V and an air leakage rate of N Air Changes per Hour (ACH) has an air
leakage rate of NV/60 cfm.
Multiply G by the indoor-outdoor temp diff to find a basic AC load in Btu/h.
Add solar heat (about 200 Btu/h-ft^2) if some windows are unshaded, and
add heat gain (3.41W) if the room contains W watts of electrical stuff.
which says a 400 ft^2 room that's occupied upstairs needs 7500-8000 Btu/h.
Consider a 400 ft^2 room in Minneapolis (NREL record max temp 84 F)
and another in Phoenix (NREL record max temp 122 F), in a fixed font:
Awindow 32 64 ft^2
Rwindow R4 R2
Gwindow 8 32 Btu/h-F
dimensions 20'x20' 10'x40'
exposed walls 2 4
Awall 288 736 ft^2
Rwall R30 R20
Gwall 10 37 Btu/h-F
ACH 0.5 2
cfm 27 107
G 45 176 Btu/h-F
Tout 84 122 F
Tin 75 70 F
Basic AC load 405 9152 Btu/h
unshaded window area 0 32 ft^2
sun 0 6400 Btu/h
electrical power 10 1000 watts
heat gain 34 3410 Btu/h
Total AC load 439 18962 Btu/h
It looks like the chart is accurate to within
a factor of 43, if we ignore dehumidification :-)
Need an answer or a method for figuring which window, insulation, heating
system, etc to use? Ask us.
Join solar guru Steve Baer and PE Drew Gillett and PhD Rich Komp and
me for an all-day workshop on solar house heating and natural cooling
strategies ("HVAC Nonsense") on July 9 in Portland, OR--see page 25 of
The best you can do is a ballpark number. There are far too many
factors to take into consideration to do it by room size.
If you are buying a one room A/C it might be OK to use ballpark.
However if you are doing the whole house, get it done right. Have a
professional come out and do the measurements and plug then into the
formulas to give you an accurate answer. It will be cheaper in the long
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