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.

--
Do the impossible and your boss will add it to your duties.

Mail john rather than nospam...