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re: no power? --- sleep warm anyway
5 jan 2000
george ghio writes with incredible density:
>>>>the present ashrae standard is 15 cfm (7 liters per second) of continuous
>>>>fresh air flow per person...
>>>but how the hell do you figure people into it?
>>15 cfm (7 l/s) per person.
and this is still unclear? (doh.)
you might be able to live with less--19th c. coal mining persons tended to
pass out with less than 5 cfm. warmer people or sleeping people or little
people should need less air, proportional to their surfaces. ashrae-standard
105 pound persons have about 20 ft^2 of heat-losing surface, and they must
breathe enough air to make enough heat to stay warm. larger persons have
surfaces proportional to the square of the cube root of their weights...
>yes nick but what is the heat loss for the model and how is it affected
>by the actions of people entering and leaving the room. how much heat
>must be provided to maintain a given temp.
surely you've heard of "ohm's law for heatflow." the room's thermal
conductance is the sum of each exterior surface divided by its r-value.
if it leaks air at c cfm, we add about c btu/h-f to that conductance.
for instance, your proverbial 8' r50 strawbale cube has 6x8'x8'/r50 =
7.7 btu/h-f of bulk wall conductance, so if it's airtight, it needs
(70-30)7.7 = 307 btu/h to stay 70 f in 30 f air, about the same heat
output as an ashrae-standard person at rest.
make one wall a dramatic r4 picture window with 50% solar transmission,
and the conductance increases to 5x8x8/50+8x8/4 = 22.4 btu/h-f, so when
the sun shines right into the window the room temperature rises to about
30f+125btu/h-ft^2x64ft^2/22.4btu/h-f = 387 f (with this linear model),
and it drops to 30+307/22.4 = 43.7 f at night.
add 15 cfm for breathing (with no air-air heat exchanger) and the
room temp drops to about 30f+307btu/h/(15+22.4btu/h-f) = 38.2 f
at night (see how nicely the units work out?)
if we open the door to the room, say, 5 times a day, for say 10 seconds
each time, and each time all 512 ft^3 of warm room air is replaced with
cool outside air (unlikely, i'd say) that only adds 5x512ft^3/(24hx60m/h)
= 1.8 cfm or btu/h-f to the room's air leakiness and conductance, which is
why "airlock entries" aren't considered great energy savers for houses
(vs department stores.)
nick
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