On Fri, 10 Mar 2006 15:35:59 -0000 someone who may be "Mary Fisher"
wrote this:-

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.

?

Provided it is under enough pressure water can be heated to above
100C. It will not turn into steam, unless the pressure is reduced.
Such systems are used in big heating systems.

However, this is very much something for those who know what they
are doing. It is best to think of such high temperature hot water
systems as liquid steam, because if it gets out the results are very
nasty. My staff once had to recover the bodies of two men who had
been killed by such a system and it haunted them.

The application to solar heaters is more complicated than I wanted
to explain, but I will now. In any heating system one wants an
adequate margin between what happens normally and what might happen
in a strange situation. If an open vented solar water heating system
was running with the water coming out of the collector at 95C then
there is little margin between hot water coming out and the hot
water turning into steam. There is little pressure because the
header tank is likely to only just be above a roof mounted collector
and thus the water will turn into steam at 100C. This isn't so much
of a problem with flat plate collectors, which are unlikely to reach
this temperature, but it will be with evacuated tube collectors.

With a pressurised system there is more margin, so that in odd
conditions the collector could get to say 120C without too much risk
of a problem. This could happen if the pump is not working for some
reason, such as a power cut. With the Solartwin and Imagination
systems the collector will never get to this sort of temperature
anyway and the pump is solar powered and thus fewer things can go
wrong with its electricity supply.

You'll also note at the bottom of the page, "Solartwin offers minimum
disruption when installing, and then is as equally easy to un-install and
load in a van when moving house.

That is one of the advantages of the system. I think that if they
could reduce the price of the unit they could corner a large slice
of the market. When they started selling it some years ago ISTR a
price of £1500, rather then the current £2000. However, reducing the
price means increasing the volume sold and that depends on reducing
the price...


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54