If you can it would make sense to split upstairs and down And have a thermostats per floor
Why? I know with my home rooms are reassigned in their use, so a dinning room becomes a store room, or a bedroom and office, or a craft room etc. So with TRVs we can swap to programmable heads, and each room can be independently controlled, yes a wall thermostat can be handy where the radiator is on the outside wall, so cooled by the wall, so want a wall thermostat linked to the TRV head, but in the main to heat multi-rooms with single control is looking for control problems.
I have an old 20 kW oil boiler, in a three story 5 bedroom, total of 14 heated areas, and DHW, and it is ample, the radiators are a bit on the small size, and the micro bore also slows down recovery time, but that's not the boilers fault.
I would say a 2 kW oil filled radiator would heat the large living room, and a 750 watt oil filled radiator would heat every other room in the house, so ignore the hall, shower room etc, and down to 10 heated areas, so 13.5 kW would heat the whole house, call it 14 kW by time the three bathrooms are included, they are small, only a shower in one, so why did anyone fit a 20 kW boiler?
I will admit to start with I was cold, but this was a control error, not the boiler, I could hear the boiler cycling off/on, so it was very plain that was not the problem. My problem was the hall cooled down too slowly, so the wall thermostat was late turning the central heating back on.
I made a mistake, and we learn from mistakes, I fitted a Nest Gen 3 wall thermostat which I was told would work with my Energenie TRV heads, it does not. Since it also works the DHW in an emergency should the immersion heater fail, it has been left in place, but a Drayton Wiser hub added, with a wall thermostat in living room, and TRV in wife's bedroom, so we now have three devices which can fire the boiler. (Four rooms are rarely used, so they have a forth thermostat, pump and motorised valve, which controls what we call the flat.)
So step one is work out what type of boiler you have, we have simple on/off (like mine) but only normally found with oil fired systems. Then we have the modulating boiler, designed to gain the latent heat from the flue, and in the main it turns up and down the flame hight depending on the return water temperature, so is controlled by the TRV heads, but these can't turn the boiler fully off, and there must always be a route back to the boiler, so we need a by-pass valve for when all TRVs close, and some form of hub to take signals from selected locations to control it, be it linked TRV heads or wall thermostats, but not all thermostats need to be linked, so for example if using two bedrooms, only the coldest bedroom needs a linked TRV head/wall thermostat.
Because a TRV head can be programmed
showing my cheap eQ-3 head which cost me £15 each in 2019, we can set a sequence of heating, so living room not heated 11 pm to 5 pm the next day for example, and bedrooms heated 10 pm to 8 am the next day, every room has its own schedule but need to ensure it opens when the boiler is running, so kitchen at 4:30 pm, dinning room 4:40 pm, and living room at 4:50 pm and wall thermostat in living room turns boiler on at 4:30 pm this will get radiators warm in sequence, so kitchen warms up first. But if I used 4:30, 5:00 and 5:30 the boiler would turn off as return water too warm, so one does need to consider the time delay carefully.
