On Saturday, 2 January 2021 at 22:18:47 UTC, Roger Mills wrote:
On 02/01/2021 19:33, bert wrote:

We have Y plan system (I think)
CH and HW are controlled independently by motorised valves which turn on
boiler/pump as required independently of each other.

More likely to be an S-Plan if there are separate motorised valves for
each zone and a by-pass valve. Y-Plan systems use a single 3-port valve,
and don't normally need a by-pass.
The boiler (Worcester Bosch) has a built in frost stat, but if that
activates the pump will only run on the bypass overrun circuit. It
cannot open the motorised valves to put the heating on. They are
controlled by the thermostats, one in hall for CH and done on tank for
HW always assuming the specific timer is ON.
But our system is over 10 years old. Maybe progress has struck again.
But if you *only* run the boiler and don't heat the rads, they and the
pipework could freeze up. You wouldn't want that, surely?!
--
Cheers,
Roger
Agreed Roger our frost stat was a separate unit not part of the boiler and controlled the CH valve just as the room stat hence the radiators getting lukewarm but unlike the room stat it was not wired through the programmer being solely triggered by the temperature falling into its set range.

Richard