"Matt" wrote in message
...

No one except you was ever advocating a 15 deg C thermostat setting, a
more practical 18 or 20 deg C - just below the main occupancy rooms
is required.


From your post

"
Lots of thermostats are located in hallways with
no TRV's on the rads and controlling to a much *lower* temp than the
main living areas.
"

So a much lower temp is well above 15C then.

The main rooms will still stick at 18 deg C forever and a
day with no boiler firing because of the lack of thermostat call, BUT
a thermostat sited in a main living area set to 23 deg C with
additional heat sources present could conversely in extreme conditions
leave a bedroom at 18 deg C below zero.


That is why you may need more than one stat wired in parallel.
Just like a frost stat is wired in parallel with the room stat.

Single room stat system are a bodge.
They offer almost no control.