On 3 Jan 2021 12:31:26 GMT, Roger Hayter wrote:

A properly wired frost stat should be wired in series with a
pipe
stat.

Don't you mean in parallel? A pipe-stat is a very 'local'
temperature
measurement and would turn off the boiler virtually as soon as it

fired up?

No. In series.

The pipe stat is supposed to be on the return pipe to the boiler
at say
20 deg.

As a matter of interest, what is that supposed to achieve?

Well you're only worried about the heat exchanger or pipe work
freezing. You're not overly worried about the loft/garage being
maintained at 7 C or WHY. Once the boiler has fired long enough to
raise the return to 20 C you don't need to run the boiler any longer.
I think I'd be happier with that return temp to be above the likely
maximum house temp though, so heat from the house doesn't turn off
the frost protection. Say 30 C.

Indeed one might get satisfactory frost protection by just running
the CH pump with the CH valve open. Would only work if the heating
was on, so the house was warm, and maintained warm, even if set back.

--
Cheers
Dave.