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Posted by andre.julien on September 7, 2005, 11:41 pm
I have bought a new house and it currently has a 60 Gal Cascade Hot
water tank. The house also has oil fired hot water heating so I plan to
convert to a combination heater that will heat the house and provide on
demand hot water (tank less).

I would like to preheat the water going into the tankless water heater
with solar. Is there a conversion kit to change an electric tank into a
solar tank? I figure if I preheat the water in the tank using a solar
based heat exchanger it would reduce the amount of oil needed to heat
the water to the desired temp.

The alternative would be to build my own conversion kit by putting
copper or pex tubing in the tank and running glycol through the solar
panel and then through the tank.

I figure that copper would transfer the heat better then PEX but PEX
seems to be easier to work with. What is the difference in heat
transfer capability. After all they put PEX in the floors, not copper?

Posted by Gary on September 8, 2005, 3:21 am
andre.julien@sympatico.ca wrote:

One simple and cheap way you could use your existing hot water heater
is to turn it into a batch type solar hot water preheater.  In this
scheme the tank and collector are integrated in one unit.  Take a look
here for how this works:
Look under "batch  water heaters".  If you live in a climate that gets
winter freezes, you would have to bypass and drain the preheat tank
for the mid winter months -- this is common.

Another scheme would be to use a heat exchanger that is external to
the hot water tank.  There are commercial units that can be used this
way.  These usually require two circulation pumps -- one on the
collector side that circulates antifreeze, and the other on the hot
water tank side that circulates potable water.  This has to be a
double wall heat exchanger if you use antifreeze on the collector
side.  For example, the "QuadRod" heat exchangers listed here:

Another scheme that allows you to place a double wall heat exchanger
inside of a standard hot water tank is shown here:
These are not as efficient due to the limited surface area of the heat
exchanger, and the small flow passages, but you only need one pump for
this one.

I don't know of any scheme that would allow you to "put copper or pex
tubing in the hot water tank", and it would be hard to make this
double wall.  If you work something out, please let us know.


"Build It Yourself" Solar Projects

Posted by Robert Scott on September 8, 2005, 12:13 pm

Is it considered a double-walled heat exchanger if you wrap PEX with
antifreeze around the outside of the tank?  I can't see how such a
system could contaminate potable water.  If either the PEX or the tank
were to leak, it would just spill onto the floor.  The antifreeze
could not get into the water system.

-Robert Scott
 Ypsilanti, Michigan

Posted by Steve Spence on September 8, 2005, 1:22 pm
 yes, that would be a double wall heat exchanger. the coolant has a wall,
and the water tank has a wall. It would require a double failure for the
two to mix.

Steve Spence
Dir., Green Trust, http://www.green-trust.org
Contributing Editor, http://www.off-grid.net

Robert Scott wrote:

Posted by Gary on September 8, 2005, 1:34 pm
 Steve Spence wrote:

It also provides for leak detection when there is a leak, which is
something a good double wall heat exchanger should do.  That is, if
there is a leak you will see it leaking fluid and fix it.

There is a commercial tank that uses copper tubing wrapped around and
somehow bonded to the tank wall, so the concept must work OK.

If you use Pex, you would have to work very hard on getting a good
thermal connection between the tank and the Pex coil.  Even with
copper, getting a good bond will be a challenge.


"Build It Yourself" Solar Projects

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