At £100 for thermostat and no wifi if it does need replacing would be looking at other units. However it has always been a good quality unit, and it has fail safe.
The Hive thermostat starts at around the same price, and this is clearly the time to look at upgrades.
So we have Hive, EPH, Nest, EvoHome, Tado and a whole load more which can use the internet in some way or can control the boiler analogue rather than digital, and for temperature control analogue is far better.
Broadly speaking there are two types of boiler, those which modulate (turn up/down) and those which only switch on/off.
With the modulating I would split into three. 1) Can only use return water temperature to control modulation. 2) Can only use their own special thermostat to control modulation plus return water. 3) Those with OpenTherm so a whole host of thermostats that will work.
There is no perfect system one unit fits all, but in the main it is the thermostatic radiator valve (TRV) which does all the work, the wall thermostat is just a hub collecting information or in some cases little more than a summer/winter switch to tell boiler when to run.
Each thermostat has plus and minus features, and often a plus feature when used with one boiler is a minus feature when used with another, and example is the anti-hysteresis software used with the thermostat you have, it is really good when used with a non modulating boiler, and oil boilers rarely modulate even today, by using a mark/space ration it stops the temperature over shooting.
Mark/space means on/off, and with a modulating, condensing boiler every time the boiler switches off any heat in the boiler is lost out of the flue, and each time it switches on again it returns to full output so an uneconomic way to control, it will do a good job of controlling but wastes fuel.
We can debate until the cows come home which is best, but a cheap eQ-3 TRV head starting at £10 each is how I control up stairs, I use a more expensive one down stairs and a very expensive Nest Gen 3 wall thermostat but not convinced worth the money.
Some people due to design of their house have little option, as far as I am aware only EPH does an OpenTherm thermostat with a slave and master configuration so it can work in homes where they have zone valves.
Nest does not link to TRV heads any more, Hive does not have OpenTherm and basic problem is every system has some plus and some minus features.
There is a reason why we call the installers heating and ventilating engineers, it is rather complex, above the level 3 most tradesman have, but there are many who call themselves engineers who do not have a degree standard qualification. And mine is not in heating and ventilating.
But one would hope with that thermostat you don't have a modulating boiler, and the old on/off boiler is more expensive to run, but easier to understand.
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