For me no problem using an oil boiler, which does not modulate, but with gas very different. The big question is when fitting any on/off wall thermostat is will it mess up the boilers built in control system.
Mothers house this
was fitted, it would have been very good with this oil boiler, but the anti hysteresis software made it unsuitable for gas modulating boilers.
As the target was approached the thermostat started to switch off/on slowly changing the ratio between off and on so that the boiler average output slowly dropped. Rather clever really.
However the gas boiler monitors the return water temperature and as it increases it modulates the boiler (turns down output) and in some cases also output temperature also reducing hysteresis and more important gaining the latent heat.
Once the boiler is turned off, it in the main resets, so restarts as max output, and if turned off before it has modulated then far more energy held in boiler which is then loss up the flue.
So what we want is for the thermostat to use very clever algorithms and work out what point to turn off so it will not over shoot, and keep the number of on/off to a minimum. However this is hard for it to do, as the TRV is also doing the same thing, and also leaving doors open or closed will also affect heating and cooling times.
So to work it needs the same air flow all the time and no TRV in the system, i.e. hot air central heating as used in USA. I actually had a hot air system in the UK, and there was a vent in every door so air could return to main room, and since air circulated all around the house one thermostat would control whole house, it also resulted in warm air being circulated past single glazed windows so heat loss was silly.
But last house used Myson fan assisted radiator again ensuring a flow of air so one thermostat controlled whole house.
But mothers house and this house has internal doors on all rooms not just bedrooms, 15 areas in all, some very small like shower room and utility room, but to counter for wind and sun and other cooling and heating each room at varying rates using lock shield valves only to proportion the heating will not work.
So most homes need TRV heads to vary the heating for each room, so big question is if the TRV sets room temperature what is the wall thermostat for? Well the TRV can tell the boiler with return water temperature to turn up or down, and even when to turn off, but it can't tell it when to turn on again, so we fit a on/off wall thermostat often in the hall which is kept cool so as summer arrives it will turn off boiler.
OK now for the bit I don't understand, why fit zone valves? OK with my house the flat under the main house not often used, so turning off whole flat makes sense. But within the main house we have a dinning room hardly ever used so turned down heating, kitchen and living rooms used in the evening, craft room and office used in the day, and two bedrooms used over night, splitting heating to upper and middle floors would be pointless. And if done would want it timed not set be temperature. The rooms all come off one central area, hall, stairs and landing, which is naturally an average of all rooms. No other room could be used as a single point to monitor temperature, and to turn off heating in upper floors would cool hall as would lower floors, so it is an all or nothing, either one thermostat for whole house, or thermostat in every room (TRV).
So step one consider why you have zone valves, and are they appropriate for how the home is used? Do children use bedrooms for homework, is the bathroom heated which ever zone is on. Think about it first and ask yourself do I want zone valves the way they are now, or would TRV heads do a better job.
If you go for TRV heads then better if some at least can link to the wall thermostat, which means Nest is not suitable as least until the temperature sensor units are released in UK. Nest would have been great in last two home with hot air circulating but as found out, not this house.
