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re: when should a/c kick on?
4 jun 2000
brettr wrote:
>>...the house starts warming up around noon... the 24000 btu a/c runs
>>from about 4p until 6a or so the next morning. it's usually warm in
>>the house the hole time the a/c is running. it starts cooling down
>>after 1a...
@>-->-->-- replied:
>...what you need to realize about a/c is... you aren't just cooling
>the air in the house, you are moving out all the heat that has been
>stored in all the surfaces in the house while the a/c was off...
well, maybe "volumes" vs "surfaces," eg 3,000 ft^2 of 1/2" drywall
with 0.5 btu/f-ft^2 of thermal capacity (1,500 btu/f altogether),
or the thick stone walls of an older house (100,000 btu/f?)
i wonder if that's significant here. an oldish 32' square 2-story house
might have r10 walls and ceiling and 200 ft^2 of r2 windows with 80%
solar transmission and 2 air volumes per hour of air leakage, ie a
thermal conductance of about 100 btu/h-f for the windows plus 180 for
the walls plus 100 for the ceiling plus 600 for air leaks, about 1000
altogether. direct sun shining in the windows would add about 40k btu/h
of heat on a continuous basis. a 20 f indoor-outdoor temp difference
would add 20k btu/h. the house might have only 3,000 btu/f of thermal
capacitance. if the house were perfectly insulated, the 24k btu/h ac
might cool that capacitance 10 f degrees in 30k/24k = 1.25 hours.
nick
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