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re: enertia building systems passive solar houses
2 dec 2000
steve spence wrote:
>have to disagree on this one nick. i have lived in log homes most of my
>life. warm and toasty all winter, even when -40 outside.
but probably not solar-heated :-)
>less airflow than stick built. 12" logs do have thermal mass...
sure. say 1000 btu/ft^2 of sun falls on a south wall on an average 30 f day,
and your 8' r12 cube with an r2 glazed south wall with 80% transmission and
5x64ft^2/r12+64ft^2/r2 = 59 btu/f of conductance is 30+0.8x64x1000/(24hx59)
= 67 f inside...
say the wood stores c = 5x64ft^2x1ftx28btu/f-ft^3 = 8960 btu/f, like 20
55 gallon water drums. suppose that mass is in the center of the wall,
and (30+67)/2 = 48.7 f on an average day. on a cloudy day, it loses heat
from the inside surface to the outdoors through the r2 south window via
half the wall's r-value and from the outside surface to the outdoors via
half the wall's r-value, so we have an equivalent circuit like this
(in courier font):
r2/64ft^2 r6/(5x64ft^2) r6/(5x64ft^2)
30 f -----www----------www-----*-----www----- 30 f
window <----- |
i --- 8960 btu/f
---
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_
rc = 0.01364f-h/btux8960btu/f = 122 hours, so after 1 cloudy day, the wood
cap drops to 30+(48.7-30)exp(-24/122) = 45.4 f, losing i = (45.4-30)/0.05
= 308 btu/h from the inside to the outdoors via the window, making the room
30+308r2/64 = 39.6 f. after n cloudy days with no woodstove, the room temp
becomes 30+11.7exp(-24n/122), 37.9 after 2 days, 36.5 after 3, 35.3 after 4,
and a warm and toasty 34.4 after 5...
nick
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