SDHW overheat loop/window insulation [RE-wrenches]

[Conrad Geyser]

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Carl,
    I already explained that you don't.  The system will self regulate by
losing more and more heat to parasitic losses as temperature increases until
the gain is practically zero.  The pipe runs, tank, circulator etc. all lose
heat.  If the occupants are away on summer vacation and not using any hot
water, I usually tell them to leave the circualtor on 24 hours as this will
help dump heat at night (at least out of the bottom of the tank) and keep
the max temp. a little less crazy.
    One thing to note is that conventional hot water tank
Pressure/Temperature relief valves will weep once the tank temp. goes over
~160F.  Code doesn't recognize this and doesn't provide for an alternative
so you have to make your own....
    Glycol systems should not have any overheat shut downs becuase the fluid
will boil in the panels.  This breaks down the glycol with time.  They
should just go right on circulating.
     PV driven ones will go on circulating past when they should if they're
not controlled, and the 30 VDC controller is expensive (but very nice).

    On the window question, does anybody have a favorite night time
insulation?  I just installed some symphony shades and was a little
dissapointed with how much space there is around the edges.  I actually have
made my own automatic night cover using tekfoil (bubble pack) along the
lines of what Jay was looking to do on his panels.  It's a big cover for the
solar gain front on my shop using awning track and sail bolt rope.  Works
great.  Gearmotor/limit switch/timer controlled.

    Thanks for all the great discussion!

Conrad Geyser
Cotuit Solar
----- Original Message ----- 
From: "Carl Emerson, Free Power Ltd." <freepower at freepower.co.nz>
To: <RE-wrenches at topica.com>
Sent: Wednesday, May 18, 2005 5:29 AM
Subject: RE: el sid question [RE-wrenches]


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> Can someone explain why there is a need to have overheat protection at all
> if the loop is all copper, the fluid is 50% glycol, there is an expansion
> tank in the circuit, the panels are built to stand the temperatures and
> pressures, and the loop is pressurised to prevent a boil ???
>
> This situation will happen every time there is sufficient sun and a power
> failure.
>
> In the case of a PV powered pumping system the solar supply should be
> controlled by an over temp. thermostat.
>
> What are we trying to protect ???
>
> Carl Emerson
> Free Power Ltd.
> Auckland N.Z.
>
>
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