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

No, hang on, you'd also need a relay. Otherwise, turning the boiler
on for HW would also bring on the CH!

There's all sorts of variations. Some need relays, some don't.
However, you don't need to install zone valves to get 80% of the
benefit of the cylinder stat.

Christian.

OK, I think I can see a way of doing it without either zone valve or relay.

How does this grab you?
* CH feed from programmer connected via room stat to pump
* Link in programmer to force HW on when CH on
* Cyl stat of changeover type, with Common connected to boiler
* NC connected to HW feed from programmer
* NO cross-connected to room stat output/pump

That should only fire the boiler when either or both stats require heat, and
only run the pump when CH (room) stat requires heat. Like other systems
without zone valves (or dual pumps) it won't stop the HW getting hotter than
required when the CH is on.

As you say, you probably get 80% of a zoned system benefit.
--
Cheers,
Set Square
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