On 04/05/2021 22:13, Tim+ wrote:

I thought my problems would be solved by going digital with one of these.

https://www.amazon.co.uk/dp/B07YPQRC...ZZEB6SY8BFE7VE

Today however, it was getting uncomfortably warm in our hall. The
thermostat was set to twenty, it was displaying 20.5 and was still calling
for heat ( flame symbol in display).

Shouldnt it have turned itself off at 20 or are digital ones designed to
keep calling for heat for a period after reaching the target temperature?

From the manufacturer's blurb it looks like it uses electronic
proportional control logic instead of relying on a physical hysteresis.
As the temperature approaches the set point it modulates the heat supply
by progressively reducing the duty cycle in order to avoid overshooting
the target. The call for heat indicator doesn't necessarily mean that
the heater or boiler is actually running because the indicator remains
on during both the 'on' and 'off' parts of the duty cycle.

Even at the set point, or even slightly above this, its indicator still
shows a call for heat because a low level of heat input is still needed
to balance heat losses. Our Honeywell CM927 works this way maintaining
the temperature within +/- 0.5 deg C of the set point, and it does still
show a call for heat when 0.5 deg above.

Incidentally with this type of thermostat the theory of temporarily
turning it up to a higher value can in some cases actually result in
heating the house slightly quicker because you don't get the reduced
duty cycle during the last few degrees as you approach the temperature
you want.