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re: help with passive solar hot water storage
29 may 1996
gene a. townsend wrote:
>76345.2033@compuserve.com (andym) wrote:
>>i have a 50 gal electric hot water heater in my attick . i was thinking of
>>running about 120 feet of 1.5 " pvc pipe back and forth in the attick to
>>bost the hot water supply...
i wonder if you can somehow put the pipe under the water heater... build a
shallow glazed box on the south wall of your house, with some pipe running
along the top edge, inside the box? or put the pipe in a sunspace, or put
it near the peak of the attic, and use a small circulating pump...
>>the attack maintains a temp of around 95-110 ( monitored useing a min-max
>>thermotere. at night it cools to around 75-90 depending on the outside
>>temp ( have an ovesized attack fan on a thermostat up there). my question is:
>>a) how long will it take for the water in the pipe to reach temp?
120' of 1.5" pipe has an area of about 47 ft^2, and the thin pipe wall has
a large thermal conductance (10?) compared to the still air film around it,
which has a thermal conductance of about 1.5 btu/hr-f-ft^2. if the pipe
contains 10 gallons of water, by gene's calculation, it will have an rc
time constant of 10gx8lb/g/1.5/47 = 1.13 hours. so if the water starts out
at say, 60 f, and it sits in a 100 f attic for t hours, its temp t(t) will
follow t(t) = 100 - (100-60)exp(-t/1.14)). after 1 hour, it should have a
temperature of t(t) = 100 - 40exp(-1/1.14) = 83.4 f. after 2, 93 f, after
3 hours, 97 f, etc.
>>b) has any one had any experience with this idea before is there any
>>thing i need to watch out for?
watch out for freezing... but if your water heater plumbing hasn't ever
frozen in your attic, chances are the pvc pipe won't either. if you don't
use any hot water at all in your house, how long will it take this pipe to
freeze solid and burst and flood your house, when your attic is -1 f, the
record min temp in columbia, sc? delta t would be 33 f, with u about
1.5 btu/hr-f, and the pipe has 47 ft^2 and there is 80 lb of water inside,
and it takes 144 btu to freeze a pound of water. so... 144x80 = tx33x1.5x47,
ie t = 5 hours. but the pipe might not freeze evenly. a little heat tape
with a thermostat might not be a bad idea.
another thing to watch out for is hydraulic short circuits. you don't want
the incoming cold water to somehow end up coming out of the shower instead
of the hot water that is inside the water heater.
>the pvc pipe must go on the cold water side of the heater...
seems like it might help a lot, if it's in a loop below the water heater,
so it can heat the water in the heater over and over, vs just heating the
incoming cold water in one pass.
>...now if only those pipes were painted black and put on the outside...
good idea. near the top of a shallow glazed box on the south wall? say the
box were 12'wide x 16' tall x 3 1/2" thick, over an edgewise 2x4 frame with
no insulation around the edges, and the glazing were 4 $50 4' x 12' sheets
of dynaglas corrugated polycarbonate plastic greenhouse roofing material,
with your 47 ft^2 of dark painted pipes in a 10' x 20" rectangle near the
top edge, and you had a $50 motorized damper behind them to allow some
warm air to flow into the house when you had enough hot water, but needed
some house heat (two thermostats), on an average january day in columbia,
with 1140 btu/ft^2/day of sun falling on the south wall, at an outdoor temp
of 55 f. if you kept the box at 130 f inside, you might collect 50k btu/day
of hot water that way on a 6 hour day, while 6(130-55)192ft^2/r1 = 86k of
heat passed back out through the glazing, and you could put the rest of the
sun's heat, ie 0.9x1140x192 - 86k = 111k btu/day into the house, saving
the heat equivalent of about a gallon of oil a day in the winter.
now why not make that shallow box on the wall a bit bigger, say a lean-to
sunspace, 8' wide at the base, with a pebble floor and a railroad tie
foundation, and a picnic table, and use a bit more pvc pipe and keep the
sunspace temp at 80 f in january, to add some interesting daytime living
space and collect more heat for the house?
nick
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