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re: how to calculate air-con btu
14 may 2004
"tkl"  wrote:

>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:

               minneapolis         phoenix

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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