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Re: Solar water heating system value - Page 10

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Posted by daestrom on March 12, 2006, 2:43 pm

Not entirely.  To prevent forming steam in the top most portion, the
pressure there must be kept above the saturation pressure for the
temperature.  So if the collector outlet temperature is 180F, the pressure
must be kept above 7.57 psia.  With the return line open to atmosphere (14.7
psia), then the vertical fall from the collector to tank is limited to
somewhere in the range of 16 ft.

If you arrange so the major friction loss is on the return line, say a
throttle valve right before the tank return, then the pressure drop across
it will add to the pressure 'seen' at the panel (i.e. the backpressure
raises the pressure in the panel).  This would support more vertical height

The *worst* thing would be if the throttle valve/restriction were on the
riser side.


Posted by Derek Broughton on March 8, 2006, 7:41 pm
meow2222@care2.com wrote:

There is no such thing as zero risk.  It's OK for you to not worry about
freezing.  We just had two weeks at -10C and lower.  I'm not risking
exposing water pipes to that.  The idea of a draindown system is to _not_
permit that - but how do you guarantee that it's always sufficiently

I could install a drain-down system, but (a) I'm not confident I won't have
a flood, (b) nobody in this area will sell or service them (because _they_
aren't confident, either), and (c) I'm _definitely_ not confident that _I_
could build, or even install one, that would be low-risk.  otoh, I can get
a glycol-loop system, built locally, installed professionally, with proven
reliability - and still save money over heating water with propane.

Posted by David Hansen on March 9, 2006, 11:15 am
 On Wed, 08 Mar 2006 15:41:59 -0400 someone who may be Derek

Agreed, but that seems at odds with what you then typed.

  David Hansen, Edinburgh
 I will *always* explain revoked encryption keys, unless RIP prevents me

Posted by Derek Broughton on March 9, 2006, 1:26 pm
 David Hansen wrote:

I don't follow?  My entire point is if you can't guarantee that - or even
that it's a vanishingly small likelihood - then it's too risky for most
Canadian homeowners.  It'll work for most folks in the UK.  When I was a
child outside London, we would only rarely have frozen pipes in completely
uninsulated outside walls.  A little insulation eliminates the problem
completely.  When you have to spend weeks with temperatures well below
freezing, it's a whole different matter.

Posted by David Hansen on March 9, 2006, 2:30 pm
 On Thu, 09 Mar 2006 09:26:23 -0400 someone who may be Derek

If there is no such thing as zero risk then wanting a guarantee is a
trifle strange.

Zero risk and as low as reasonably practical are two different

Indeed. There are a number of countries that have lots of snow to
contend with, for at least part of the year, including Canada.

  David Hansen, Edinburgh
 I will *always* explain revoked encryption keys, unless RIP prevents me

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