I've got a large house with gas central heating which I use during the
day. It gets cold overnight in the bedroom though, so I use a 2KW
electric convection heater with the thermostat set to about 18 C on a
timeclock which switches on between midnight and six am (boiler comes
on again at six).

This was a great improvement last winter, and it only uses one or two
KWh overnight which must be cheaper than heating the whole house with
gas.

This year I want to improve the solution to keep the temperature more
constant (there's a big hysteresis on the convection heater
thermostat) and remove the bimetallic thermostat clicking noise.

I think I want electronic switching to remove the clicking noise and
allow a lower hysteresis plus an external temperature sensor. Maybe
something that's closer to a temperature controlled dimmer switch
instead of a standard bimetallic thermostat.

My plan is to put this temperature controller between the timeclock
and the heater, and set the heater thermostat to something like 25C as
a fail-safe.

Has anyone found or built something like this? I'm sure I can't be
the only person who want a constant overnight bedroom temperature
without running the main house heating.