On Tue, 16 Oct 2007 09:12:59 +0100, David Hansen
wrote:

On Mon, 15 Oct 2007 14:41:58 +0100 someone who may be Peter Parry
wrote this:-

The other possibility is that the original poster thought that a
second hot water cylinder was necessary, to pre-heat water going
into the main cylinder. Such an approach was advised in the past as
a way of avoiding replacing the main cylinder.

It is still advised as a method of getting the most from an active
solar system.

"neither [flat panel nor evacuated tube] system can be guaranteed to
continue to give the maximum possible output once more intrusive
auxiliary heating schedules are introduced.

If the "intrusive auxiliary heating system" is controlled separately
from the solar system. However, if they are controlled together, by
the same controller or controllers which speak to each other, then
any such problem does not arise.

Apparently they do as it has little to do with control and more to do
with patterns of usage. Most people want hot water in the morning so
auxiliary heating (for most of the year) kicks in before they rise as
the storage tank will have been depleted by evening use (the other
peak period). Temperature stratification in the relatively small
storage tanks typical of domestic installations is fairly poor so the
solar system for the rest of the day is trying to preheat already
warm water. Giving the solar system its own pre-heat tank means that
at the start of the day the pre-heat tank is invariably cold (as most
domestic hot water usage is evening and morning) and during the day
the solar panel can work at its best. For those with room for a
pre-heat tank it allows them to get the most from a solar hot water
system and makes the system overall relatively immune to the usage
pattern.
--
Peter Parry.
http://www.wpp.ltd.uk/