nick at early.com
Thu Feb 19 06:22:42 CST 2009
Benjamin Pratt writes:
>... As a non-engineer, I appreciate and admire nick's Math... I was
>actually thinking of getting him to help my argument that we need
>windows on the south side of the building I work in, and how much they
>would save us in the long run.
NREL says the average daily max temp is less than 60 F from October
through April in Eau Claire, WI, with 7759 F heating degree days
(37/day) and 6990 Btu/ft^2 of sun (999/day) on a south wall, so 1 ft^2
of U0.25 window with 50% solar transmission would save about
0.5x999-37DDx24hxU0.25 = 278 Btu/day, or about 7x30x278 = 58.4K
Btu/year, roughly $1, with natural gas at $2/therm.
December is the worst-case month for solar house heating, when 790
Btu/ft^2 falls on a south wall on an average 16.8 F day with a 25.3 high
and an average daytime temp of about (16.8+25.3)/2 = 21.1, so 1ft^2 of
U0.5 twinwall polycarbonate air heater with 80% solar transmission would
save about 0.8x790-6h(65-21.1)xU0.5 = 500 Btu/day.
Gennaro Brooks-Church writes:
>... Why the hell are we using windows if energy efficiency is 90% of
Good question. People say my new kitchen layout needs a window over the
sink, through a 2' north stone wall with outdoor insulation. I'd like to
hang a flat-screen TV with an outdoor camera there, but why should it
run when nobody's in the kitchen? I could turn it off along with the
lights using a half-hour motion detector, but how can I turn it on
without pushing a button to see the picture?
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