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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:

http://www.davesrepair.com/diyhelp/diyacsizingchart.htm

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 :-) 

nick

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 
http://www.ases.org/conferences/2004_call_for_papers/solar2004_prelim_program.pdf




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