On Wed, 17 Dec 2003 12:36:21 +0000, John Laird wrote:

On Wed, 17 Dec 2003 10:54:24 -0000, "Christian McArdle"
wrote:

Does your last sentence make sense ? If the heating zone valves are
already open, then this can only be because one or both of their timers
are on and one or both of their stats are demanding heat. In what way
is heat in the radiators then "unwanted" ?

It is unwanted because it happens despite you switching the central heating
off. This switch should override the programmable thermostat.

So, the idea is to provide a master switch to turn off the heating. If that
master switch is after the zone valve microswitches, then the programmers
may call for heat and turn on the zone valves. The switch only prevents the
zone valves firing up the boiler. However, if the boiler fires up for DHW,
then the valves are already open and will pass water.

To work properly, you need to put the switch before the zone valves, so they
aren't open unnecessarily. This is easiest done by switching the feed to the
programmers, as doing so after the programmers requires relays, or a multi
pole switch.

I think I missed the point about the master switch ;-) Sounds a bit like
overkill as the two zone programmers can be co-located anyway.

I over-hauled a large heating system last year.
4 Heating Zones and DHW. All this used a Keston C40 which has seperate
temperature control for heating and HW. It is true that for a short while
the boiler output is at a higher temperature whilst the tank is being
reheated.

However the extra heat in the radiators is not a particularly big problem
and is further minimized by the fact that the water circuit for the DHW
coil is very much lower than for the radiator zones. (Short run of 22mm
pipe right off the 28mm main manifold). This means that very little water
is inclined to go through the radiator for the 10 minutes (typically)
whilst the tank is reheating.

Another way would have been to use a large 28mm 3 port diverter valve with
DHW priority which would kill the heating during DHW reheat.
However experience shows that some people like to turn the heating & HW
off if the go on away in winter and resent waiting the _entire_ cylinder
reheat time say 25mins (170 litre, nominal 14kW coil) before the radiators
_start_ to heat up.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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