In an earlier contribution to this discussion,
The Medway Handyman wrote:

What's the +/- tolerance on a bog standard wall thermostat?

I mean, if you set it to 20c will it trigger on at 19c & off at 21c?

Are you sure that you mean tolerance - 'cos what you've described is
hysteresis. If by 'bog standard' you mean a mechanical by-metallic stat,
that will inevitably have a difference of several degrees between the switch
on and switch off points. As someone else has said, these often have a
built-in accelerator heater. This is a small resistor which is powered when
the stat is 'on' and warms up the innards so that the stat switches off
slightly before the target room temperature is reached. Of course, the rads
don't cool immediately, so the room goes on getting hot for a bit - so the
accelerator heater just helps to reduce overshoot.

In terms of absolute accuracy (which would be closer to my definition of
tolerance), I suspect that the relationship between the number shown on the
knob and the actual temperature at which it switches is probably not a very
close one! It's probably best to ignore the numbers, and set it to give a
'comfortable' temperature - or hang a mercury thermometer alongside it if
you want to calibrate it.

Now, digital stats are different altogether - and not *that* expensive.
--
Cheers,
Roger
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