In an earlier contribution to this discussion,
Clive M wrote:

Does anyone have a diagram of how a system with 2 pumps and no valves
is wired together to give interlock? Secondly how does pump overrun
work if 2 pumps are used (i assume you have to dump into the CH
cylinder)

I don't have an official diagram, but I've just drawn one on the back of an
envelope which I think will work. If I describe it, you should be able to
replicate it. I will deal with only the live connections, leaving you to put
in all the neutrals and earths.

On the left is the programmer, with feeds for HW and CH
To the right, one above the other, are the cylinder stat and room stat. Both
are normally closed switches, opening when the set temperature is reached
To the right of these is a mains changeover relay
To the far right is the boiler

Take a wire from the programmer's HW feed to one side of the cyl stat
Connect the other side of the cyl stat to the HW pump and to the NC contact
of the relay
Take a wire from the programmer's CH feed to one side of the room stat
Connect the other side of the room stat to the CH pump, the relay coil, and
the NO contact of the relay
Connect the relay's Common terminal to the Boiler switched live
Connect a permanently live feed to the Boiler permanent live
Connect the Boiler Pump terminal to the NC relay contact (and hence to the
HW pump)

If I am right, this will fire the boiler - plus the appropriate pump
whenever either the CH or HW or both are demanding heat, and will shut down
when neither is.
If the boiler's pump over-run stat operates, the HW pump will run until
things cool down.

I can see one potential problem with this, which may require a mod to the
boiler's internal wiring. On my boiler, the pump outlet is connected to
switched live below the boiler stat temp, and to permanent live above the
stat temp. My diagram, as it stands, would result in the HW pump running
whenever the boiler was on - even if the HW demand was satisfied. Severing t
he internal link between switched live and pump would do the trick - only
allowing the *boiler* to switch the pump on *above* the stat temp.

HTH.

--
Cheers,
Set Square
______
Please reply to newsgroup. Reply address is invalid.