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re: tankless or solar water heater
20 feb 2003
loren amelang   wrote:

>obviously you have freezing temperatures to deal with. i struggled with 
>each of the solar freeze protection systems in turn: the dribble valve 
>wasted too much heat and too much water and eventually buried itself in ice 
>anyway. to get the water up to the roof, the drainback system needed a big 
>pump that was too noisy and too power hungry to run on pv. the draindown 
>valve couldn't handle my hard water and needed continuous attention... 

sounds like you put in your struggling time. did you consider letting
the downgoing water help lift the upgoing water in a draindown system
with a downpipe restriction that would put the solar collectors under
a slight vacuum? acorn solar houses worked this way.

>i've ended up with a system i believe solves all these problems. there is a 
>"primary loop" with antifreeze which runs through the input coils (five 
>parallel 50' coils of copper tubing) of my exchanger tank...

...250' of 1/2" tubing would have about 33 ft^2 of surface and store about
3 gallons of water. i'd guess ntu = 30x33/(3x8x60) = 0.68, after the first
3 gallons, with e = 1-e^-ntu = 0.49... 50 f water in a 140 f tank might
emerge from the coil at 50+0.49(140-50) = 94.5 f.

>i was concerned about the efficiency of the double heat exchange required
>in this system, and i probably went overkill on the amount of tubing i used

seems like the right ballpark. more might help.

>but i've been extremely pleased with the result. with a 3 gpm flow and 90f
>rise, the dhw comes out within a degree or two of the water in the top of
>the exchanger tank.

sounds lovely. have you measured this after the first 3 gallons?

nick




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