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Solar Hot Water Heater - Storage Tank Cooling Problem

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Posted by trkess on October 6, 2007, 6:18 pm
Hello all,

I recently finished remodeling a large (7000 sq ft)  old house in
downtown Guadalajara, Mexico.  I installed a vacuum tube solar
collector with a differential controller and  recirc pump to a 130
gallon insulated steel tank on the roof.  The tank output feeds a
conventional hot water heater, so if there is solar water availabe ,
the boiler never  turns on.

Now that I've got all this operating, I've noticed one big problem:
When one starts consuming  hot water, cold water rushes into the roof
storage tank to replace it, and  quickly cools down the remaining
water in the tank.  The tank water will completely heat up to over
55C on a sunny day, but if a person takes a 25 gallon shower in the
afternoon, the remaining water in the tank cools considerably and
requires boiler heat to be useable for the remainder of the day.

Obviously some technique to isolate and use the stored hot water in
better synchrony with the sun input is desirable.  What techniques are
used to deal with this?

Tom Kessler

Posted by Sundug on October 7, 2007, 12:55 pm
On Oct 6, 1:18 pm, trk...@gmail.com wrote:

Is the hot water coming out of the tank at the top, and cold going in
at the bottom? Usually stratifcation will allow a fairly large
temperature difference. The taller the tank is, the better the
stratification. Also, the way the cold water enters should create as
little turbulance as possible. Doug

DIY Solar Hot Water Articles can be found here:



Gallery of Solar Homes- second one down-

SDHW and house-

Posted by trkess on October 7, 2007, 8:35 pm

Thanks for the nice response.  The cold water enters at the very
bottom front of the tank, and exits at the back of the top.
Unfortunately, the tank is not tall, about a 1 to 1 form factor;  I'm
afraid that is responsible for the less than stellar performance....

Now I know why hot water heaters have tall form factors!


Posted by Mike Scaife on October 8, 2007, 1:57 am

Try installing a copper tube diffuser, 18 - 24 inch long with 2 rows of 1/4
inch holes 2 inch spacing 120 degrees apart, pointed downwards, end of tube
blanked off and welded to the inlet fitting that screws into the tank cold
water entry.
Hopefully this will cause the cold inlet water flow over a large area
towards the bottom without causing large circulation currents that would
otherwise upset the natural water stratification..

Mike Scaife (NZ)

Posted by Jeff on October 8, 2007, 2:18 am
 trkess@gmail.com wrote:


   I'm confused here, Is there no heat exchanger in the tank?

  Is the water going through the solar collector the same water feeding
the house?


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