On Sat, 01 Mar 2008 23:57:50 +0000 someone who may be Andy Champ
wrote this:-

If I have hot water in my cylinder, and there's not much sun, I may lose
heat by pumping the hot water out onto the roof.


The answer to this question for the Solartwin has already been given
several times http://www.solartwin.com/technical_faq.php


"Q: During the winter months when most of the hot water will be
generated by the boiler, what happens if there is enough sun to set
the pump in action. At which time there is not enough thermal energy
to heat up the water sufficiently so that when it is returned to the
hot water tank it reduces the temperature there?

"A: The pump runs only on solar energy in the form of electricity.
It has no temperature sensor, only a high pressure bypass in case
the panel or its pipes are frozen. In response to your question:
First - best not to have the hot water system on all day since this
is wasteful anyway and does not allow for optimum solar performance.
Most boilers have separate timers for this, but not all. Ideally
time the boiler to add heat to the domestic hot water after 4pm.
Second, even in winter some hot water is made by Solartwin, not all
by the boiler as you say. Third - the panel is well insulated and so
will still raise the temperature of water going into it since it
collects heat from the sun and not the air. Fourth - at 100% sun and
a water input temperature of 50C and air temperature of freezing our
mathematical model (based on extensive tests at Napier University)
suggests that the water will still leave the panel at least 10C
hotter than when it went in. Fifth - if they really want to put cold
water in under these circumstances they can connect a second
cylinder behind the first and draw water off it! This will also
allow for more summer hot storage and is a neat solution for people
with AGAs and Rayburns."


--
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