re: how to calculate air-con btu
14 may 2004
>anyone know how to calculate total btu require for a room space?
>pls help with formula.
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.
dave harnish wrote:
>here's the 'ballpark' chart i've used for window units for many years:
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 :-)
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