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Re: Solar water heating system value - Page 6

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Posted by meow2222 on March 10, 2006, 2:39 am
nicksanspam@ece.villanova.edu wrote:

yes, thats what I had in mind. Perhaps I should have said drainback
rather than draindown, though I'm not sure about that one.

Derek Broughton wrote:

If a system design depends for its survival on a component that will
fail, then yes it is incompetent enginering.

If the collector gets cold enough to freeze in a draindown system, that
collector will have been empty for hours.

why? (thats not exactly what I said anyway).


Posted by AJH on March 9, 2006, 10:22 pm
On 9 Mar 2006 14:05:11 -0500, nicksanspam@ece.villanova.edu wrote:

Yes this was my assumption, the water header being sufficient cross
section, and within the insulated area of the house, that once the
pump stopped it just filled up a bit. The only drawback I could see
was that the pump would always work against the head from the header
tank up to the panels. In a sealed system the pump only circulates
against the friction in the system.

Most of UK is a temperate marine climate with few excursions below
freezing in recent years.


Posted by David Hansen on March 10, 2006, 8:45 am
 On Thu, 09 Mar 2006 22:22:35 +0000 someone who may be AJH

shows the unit promoted by one supplier. In addition to this box
there is just the panel, cylinder and some piping/wiring.

The drainback system does depend on the valve working, just like
human life depends on valves in the heart working. I suspect that
the valve in the unit is chosen to have a very large number of
working cycles, as it will be operating a minimum of once a day and
so the possibility of it failing and the panel to freezing is remote
enough to be placed with the other remote possibilities, like the
panel being hit by a meteorite.

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

Posted by AJH on March 10, 2006, 9:54 am
 On Fri, 10 Mar 2006 08:45:34 +0000, David Hansen

Thanks I was considering a more simple open vented system. In general
I have a preference for sealed systems (only just becoming common in
UK and then almost never for wood burning). Once the option for sealed
system is taken I can see little advantage in drainback over a glycol
or food grade antifreeze system.


Posted by David Hansen on March 10, 2006, 1:51 pm
 On Fri, 10 Mar 2006 09:54:57 +0000 someone who may be AJH

In general I have a preference for open vented systems, in small
installations. In such installations the advantages of a "sealed"
system largely fall to the installer and maintainer. The occupier
gains little with such systems, in many/most cases. Large
installations (I have been responsible for some very large heating
systems) are a different matter.

However, for solar water heating the disadvantages of an open vented
system include the trouble of placing the header tank high enough,
then maintaining it and the inability to heat water above 100C. In
this case I think the advantage lies with a "sealed" system.

The system at http://www.imaginationsolar.com/system.htm  has a
number of advantages. One of these is that it does not use mains
electricity, which can exterminate 20% of the savings gained by
using solar hot water heating [1]. The control system is very
simple, if the sun is shining it pumps water into the panel and
starts warming up the cylinder. If the sun is not shining, or there
is snow on the PV panel, then the pump is not running and the panel
is drained of water.

The system at http://www.navitron.org.uk/solar_collector_panel.htm
is more sophisticated and so will extract more heat from the sun. On
the other hand it needs mains electricity 24 hours a day, because
the controller might need to turn on the pump occasionally to warm
up the panel header a bit on cold nights (assuming it is run without
anti-freeze). Mains failure means possible freezing in winter and
overheating in summer, though these are rare in most of the UK.

The system at http://www.solartwin.com/easy_to_plumb_in.htm  involves
much less plumbing, so is easy to fit and has simple PV powered
controls. Provided that the existing hot water cylinder is well
insulated and of an adequate size this has great advantages.

These are just three possibilities, but they do show a range of
possible solutions in one small part of the world.

[1] - "Side by Side Testing of Eight Solar Water Heating Systems"
DTI/Pub URN 01/1292

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

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