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re: materials & sizing q
28 feb 2001
timla  wrote:

>so i finally have a home in the woods.  after i drop some of the towering
>firs near the house (the ones that are either dangerous or block most of the
>winter sun), i am seriously looking at various solar adjuncts.  yes, nick
>pine, this is a blatant solicitation for your (and others) expertise...
hmmm. free advice :-) where did i put my super solar cape (tm)?

>house is 40x40, two story, 6" external walls, listed as 2100 sq ft (loft,
>etc) you can see pix at to give you an idea.
>located east of seattle, in the cascade foothills... exactly 1000 feet up.

near the mt. baker hot spring? :-)

>currently has an 18 year old heat pump, which is not only old and near
>retirement, but the previous owner (a self-appointed expert in all things,
>from what i can tell) installed the indoor heat exchanger upside down,
>so in a/c mode, the condensate falls into the blower, rather than the
>drain tube. a rusty mess...

sounds like you are somewhere between seattle (average outdoor temp of
40.5 f in december, with 420 btu/ft^2-day on a south wall) and spokane
(averaging 27.8, with 580 btu/ft^2 on a a south wall.) let's say the
average temp is 34 f, and 500 btu/ft^2-day falls on a south wall in
december, and you might collect 55% of that as beam sun.
>i'm thinking of replacing the heat pump with a ground-water and solar-
>assisted heat pump.  i have a backhoe and 30 acres, so finding space
>to put the horizontal ground heat piping should not be a problem.

with all that space, you might keep horses, with a 16' tall x 32' long
x 16' deep parabolic run-in shed that solar heats water. it might be near
the house, with 512 ft^2 of solar aperture, and a 4'x8'x16' tank containing
32768 pounds of water with 448 ft^2 of r26 sides (6" of fiberglass plus
1.5" of foam) in the basement, so rc = 32768btu/fx26ft^2-f-h/btu/448ft^2
= 1902 hours (79 days), semi-seasonal storage...   

on an average december day, the shed might collect 0.9^3x275x256ft^2
= 103k btu of beam sun in 200 f water. it might store 32768(200f-100f)
= 3.3 million btu of useful heat (and 4096 gallons of useful rainwater)
enough to keep a 100k btu/day house warm for 33 cloudy days in a row.

>would it be better to put in a large, water based (with large, well
>insulated water tank) solar collector, using that to add some btu
>to the returning groundwater line (as well as preheat dhw), or go 
>the air-based method as an independent supplement to hvac?

well, heat pumps aren't very efficient, considering that a utility might
burn 3 kwh of gas or coal to deliver 1 kwh of heat to you, which you then
convert back to 3 kwh of heat using expensive equipment with limited life.
why not simply solar-heat the house?
>i have a woodstove rated at 68k btu max that managed to keep
>the house at 68... when the outdoor temp never got above 20...

if the conductance g = 68k/(68f-20f) = 1417 btu/h-f, with 1600 ft^2 of
ceiling and 1280 ft^2 of walls (totaling 2880 ft^2 of external surface),
the overall r-value is only 2880/1417 = 2. you might improve that a lot
with more caulking and insulation. 

keeping the house 65 f in december with 103k btu/day of solar heat
and 300 kwh/mo (34k btu/day) of electrical usage means lowering g
to 137kbtu/(24h(65f-34f)) = 184 btu/h-f, ie raising the average wall
and ceiling r-value to 2880/184 = 15.6.


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