In an earlier contribution to this discussion,
Christian McArdle wrote:

With your existing setup, all you can really do with a room stat is
switch the pump on and off - so, whenever heating is selected by the
main programmer, the boiler will cycle on its own stat - and the DHW
will continue to get hot(ter) by gravity circulation.

That isn't quite true. On an old boiler setup like this, you would
set the programmable stat to cut power to the boiler, so it won't
short cycle. Even without any additional zone valves at all, you can
install a cylinder stat.

But surely, if you do this without additional relays, you have to power the
boiler and pump from the same feed - effectively connecting them together.
When (in the summer, say) you turn the boiler on to get DWH, the pump will
come on too.
--
Cheers,
Set Square
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