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re: water heater opinions
17 oct 2000
rlstager  wrote:

>this may fall more under an efficiency group...

dunno of any, but conservation or improved efficiency is renewable energy,
in the sense that it never runs out... :-)

>does anyone have any experience pro or con with running hot water from
>gas hot water heater through coil in solar storage tank as an added heat
>source when shower, washer or dishes are being done?

after it's used for showers, dishwashing, etc.? the opposite might work
better: preheating the cold water going into the gas water heater with
a solar heat store, but that requires lots of heat transfer surface, if
the pressurized cold water only flows through the heat store once.

>i figure since most of the hot water the family uses goes down the drain
>anyway why not try to be more effective with its use? i use hot air from
>the gas exhaust now to help warm floor space, why not let it help heat
>more water?

you might use something like the gfx heat exchanger, $300 for a large
piece of plumbing with 60% (?) efficiency. the water from showers, etc.,
falls down a 5'x3" diameter copper pipe hard-crimped inside a smaller
spiral copper pipe carrying fresh cold water in the other direction. 

but heating air may be simpler and less expensive than heating water, and
equally valuable in the winter if using gas for both, vs electricity for
water heating. you might use 50k btu/day of gas or oil water heating at
a cost of about 50 cents per day, vs $1.50 for electric hot water at 10
cents/kwh. 

you might cut the tops off a few free 3' tall 55 gallon plastic drums under
a basement workbench to collect warm graywater, and connect them together
near the bottoms with a few (-1) $5 3/4" "bulkhead tank fittings." these
3-piece plastic things have a flange connected to a smaller threaded pipe,
a rubber washer, and a nut that threads over the pipe. drill a hole in each
drum and put the flange inside one with the nut inside the other and the
washer between the drums. put a submersible pump with a level switch (eg
northern's $47 item# 108981-c151) in one of the drums, adjusted to keep it
nearly full, and pump it back up into the sewer drain or out onto the garden.

with 120 gallons of 110 f water entering every 24 hours, warming 70 f air,
this might be an ashrae hof heat exchanger with minimum fluid capacity rate
40 btu/h-f and 0 capacity rate ratio, so ntu = 25ft^2n(1.5btu/h-f-ft^2)/40 
= 0.94n, and effectiveness e = 1-exp(-0.94n), ie 61% for 1 drum, 85% for 2, 
94% for 3, and 98% for 4... 

nick




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