In article , Roger Mills
writes
On 03/01/2021 12:16, Andrew wrote:
On 02/01/2021 22:18, 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?!
If that is likely then maybe you need to think carefully at how
good (or bad) your house is insulated.

Unless the boiler is in an unheated outhouse or attic, why would that
be any more likely to freeze than the pipes?
In my case its in the corner of the integral garage. With a belt and
braces approach I also have a greenhouse heater in there with a frost
setting.
--
bert