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The best way is to fit a 3-port "diverter" valve to give a priority
system.
A diverter is similar to a 2-port valve with just another port. The boiler
will need to be on full temperature. A simple clamp-on pipe stat on the
flow
can be fitted to keep the CH flow temperature down if that is a problem.
Better still, if the cylinder is old replace it with a quick recovery
cylinder to reduce time heating the cylinder. Travis Perkins do the
Telford
Typhoon very reasonably. Doing this will transform the system for little
outlay.

IMM,
I prefer to have valves that allow heating water and rads simultaneously if
needed,

On demand for hot water, it's sometimes important that all the heat
output of the boiler is directed in that direction. It would have to be
a quite powerful boiler which could handle both simultaneously

Don't forget that the CH will probably already have a lot of heat in it
which takes some time to cool

so I would not be happy to use the "diverter" W-Plan option.
Looks like it is going out of favour now also,as I did not see it as an
option from some manufacturers and found an article recommending them to be
replaced where found see -
http://www.gasman.fsbusiness.co.uk/f...ed_systems.htm

Thanks for your suggestion on quick recovery cylinder - I want to get one,
but so far only found the "Superduty" by Albion.
And thanks for your recommendation of good valve makes.

Dave



--
geoff