What does a thermostat do!

[ericmark]

That's what thermostats do.
They are just an on/off switch which is operated by changes in temperature.

Most wall thermostats did exactly that, however today we seem to be getting a new breed, did not want to hijack the other post, but what should we expect from thermostatic control?

So if I set my bedroom TRV to 20°C what is a reasonable error? It will clearly depend on the valve which is different make to head, and likely how cold it is outside.

The post to which flameport replied was about a storage radiator, which to my mind is not central heating, every radiator is independent, and I am sure just because the electric was supplied from the "Central" electricity generating board does not make it central heating.

And the thermostat controls heat in not heat out, so nothing to do with heat of the room, but with a thermostat in a room it could switch on/off every 20 minutes to try and stop hysteresis, or 20 times a minute, and with electric heating the latter would be nearly analogue. The gas boiler can have an analogue thermostat connected to boilers ebus.

But does that really help?

When I read how Hive was controlling boilers, I thought well yes that will work with most boilers, so this heat on demand triggered by the TRV has a lot going for it. In fact likely better than Nest which although it has OpenTherm it tells the TRV what to do, not the TRV telling the wall thermostat what to do.

Be it allow more or less water to flow, or change fan speed on fan assisted radiators, it is not a gradual change but stepped, I hear my TRV heads adjust from time to time, and the motor runs for around 10 seconds but to fully open and close around 5 minutes, so it is adjusting the flow in steps.

Radiators take time to heat and cool so no need for a fine control, but back when I first fitted central heating, the room temperature likely had a sin wave of 4°C as thermostats have improved over the years this has reduced.

But what is today considered as reasonable tolerance from a thermostat control room? Is an off/on thermostat still considered good enough? Or is now modulating devices the norm?