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re: maintain or setup setpoint
11 aug 1997
jonny  wrote:

>is there any savings (comfort not an issue) in cooling mode between
>maintaining a constant set point when unoocuipied vs comming home and
>starting the unit in a hot house?

yes, unless you live in a house with masonry walls on the inside, in which
case it may be better to sell the air conditioner and ventilate the house
with cool night air and button it up during the day. and solar heat it in
winter, of course.

>for example:  
>1.  it's 85f outside and while i am at work a set my thermostat at 72f
>all day long to keep temperature consistent.

if the house has little internal thermal mass and a thermal conductance of say,
200 btu/h-f, this takes 24h(85-72)200 = 62.4k btu of cooling, eg a 5000 btu
window ac running about 12 hours per day at a cost of something like $1/day.

>2. come home from work and try to cool a 85f house to 72f.

if you are away from home for 10 hours, this takes about 14h(85-72)200
= 36.4k btu, close to a 40% savings. drywall has a thermal capacitance of
about 0.5 btu/f-ft^2, and a typical house might have 2,000 ft^2 of drywall, 
with an overall heat capacity of 1000 btu/f. cooling this from 85 to 72 f
requires another 13k btu/day, which still makes for a 20% savings. 

nick



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