On Mon, 28 Jan 2008 21:41:40 +0000 someone who may be Andy Champ
wrote this:-

I can't be sure exactly how much hot water we use each day.

Do you think anyone can say exactly how much hot water they will use
in a day? Solar water heating works by slowly warming the water and
storing enough of it to overcome variable draw offs.

Your system pulls in cold water from the bottom of the tank, warms it a
bit, then dumps it back in the top. Where it will mix with the *hotter*
water left over from the last boiler run. And ruin the stratification.

"Ruin" is a rather strong term. It would be the correct term if the
Solartwin system involved high flow rates, but it doesn't.

What *would* work would be a second cylinder to preheat the water before
delivering it to the hot one, which would be gas (or something) heated.

That used to be the fashion. However, as well as cost it has the
disadvantage of extra heat losses, due to the extra volume of water
stored at above ambient temperature). This reduces the effectiveness
of the system. For a number of reasons few offer such systems these
days.

OK, in summer it'll be more effective - but in winter it'll be doing
nothing much, or freezing wouldn't be an issue.

Any freezing generally happens overnight, when there is no sun to
warm the collector. When/if the sun rises the frozen collector is
rapidly thawed out and will then start warming the water in the
cylinder. On a very cloudy day the collector might stay frozen all
day, which is no problem.

With a drainback system the collector will be empty overnight and
thus not frozen. When/if the sun is out enough to make collecting
its heat worthwhile the panel is filled up and it is then obviously
not frozen.

With a non-drainback system the system either has antifreeze added
or the controller uses a little heat from the store to keep the
water temperature above freezing. When/if the sun comes out hot
water production starts.




--
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