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What overtemp control method for solar hot water heating?

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Posted by offgridman@cs.com on October 3, 2003, 1:48 am
I have just ordered two 3x7 solar hot water panels. I have a march 12
volt  DC pump on the way and plan on roof mounting the panels about 1.3
meters above the hot water storage tank. I will use a roof mounted PV
solar panel connected to the pump to circulate a glycol and water
mixture through the collectors and through a 60 inch long copper pipe
heat exchanger mounted on the side of the hot water tank. I plan on a
roof mounted automatic air vent. I also plan on a Pressure/Temperature
safety relief valve. I will install an expansion tank also. I have 70
gallons of hot water storage. I am located on Mon Louis Island Alabama.
The collectors will be south facing and elevated ~ 30 degrees.
Now my questions,
What if any over temperature control device should I use.
Anything I over looked?

Posted by Ecnerwal on October 3, 2003, 11:38 am

I guess the simplest overtemp control would be a combination of a
tempering valve on the hot water to house (to prevent scalding as the
hot water gets uncomfortably hot ) and the P/T relief valve on the hot
water tank side of the system (which should go at a lower temp than the
one on the glycol side, so you don't dump glycol unless there's a pump
failure, etc).

I don't think you can get a P/T relief with a low enough temperature
that the tempering valve is not needed, but I could be wrong.

The simplicity of this approach is that if the hot water tank gets too
hot, it dumps hot water, which brings in cold water, which cools it
down. If you have gravity feed water, it's no problem. If you're pumping
water with PV, you probably have plenty of sun to pump water when the
tank is overheating and venting water. If it vents water very often, you
might want to increase the storage size (add a second tank), or cover
part of the collector with shading in the hot season.

Anything else I can think of involves more complex electromechanical
bits that add to the cost - solenoid valves to divert hot water, an
automacitally deployed movable shade on the collector, etc - complexity,
things to fail, etc.

Cats, Coffee, Chocolate...vices to live by

Posted by offgridman@cs.com on October 3, 2003, 3:32 pm
 Thanks for the reply. I did forget to mention that I do have a tempering

valve for the system. I just have to install it. I do have PV powered DC
pumps that can compensate for a water release from the system with
cooler make up water.

I will see if I can locate a P/T relief valve with a slightly higher
setting for the glycol side of the system, that is a good idea. I will
have to check design limits of the components though.

I would prefer to not have to depend on a P/T valve for normal control
though. My experience with them has been they tend to start leaking if
opened on a regular basis.

YOu did give me a new idea though how about a uninsulated tank on the
inlet side that I could start or stop another (yuck, costly)DC
circulating pump on it that would have a exchanger between the hot water
tanks and solar panel on the return side of the glycol. Seems I could
preheat water, and temperature control the system this way with only
another pump, temperature switch, and piping costs ( I build my own heat

What do you think?

Ecnerwal wrote:

Posted by Ron Rosenfeld on October 3, 2003, 8:03 pm

You've probably got your design pretty set by now, but just some thoughts.
In a former home, I had a solar thermal system that provided about 1/2 our
heating and DHW needs.  The storage was a 700 gallon water tank in the
basement.  There were heat exchangers that preheated the water going to the
backup gas hot water heater, and also a heat exchanger that heated the hot
air plenum.

We used NO glycol.  If the solar thermal panel temperature was hotter than
the tank by some number of degrees, then a pump would turn on and push the
water through the roof panels.  If the roof cooled down (cloudy day,
night), then the pump would turn off and all the water would drain back
into the tank.  No danger of freezing in the tank in the basement.

There was another temperature controlled relay that would also turn off the
circulating pump if the temp in the reservoir got too high.

The concept seemed pretty simple, although between DHW and heat, there were
a number of pumps.

-- ron  (off the grid in Downeast Maine)

Posted by offgridman@cs.com on October 4, 2003, 11:36 am
 I considered this type of system, I thought it would need more energy to
operate  since the pump was pumping against head pressure and fritional
losses. Not having grid poser available I decided on the closed loop non
drain down system. I can use a small PV powered pump with it.

Ron Rosenfeld wrote:

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