Consider ideal central heating, as any room want heat, that room is heated, and when it does not want heat either as warm enough or room not being used, then that room is not heated. Sounds great but there is a flaw, programmable TRV heads can only heat the room if hot water is flowing, so we need a method of turning the flow of hot water on when required, but allow it to stop flowing when not required.
There is a second consideration with some boilers, where the boiler extracts the latent heat, it also needs to control the return water temperature, this can be done in two ways, with a device connected to boiler ebus, or the temperature of the return controlling the boiler output.
So we are looking at a method to stop and start the boiler and pump as and when required that will not interfere with the extraction of latent heat. So a thermostat which switches boiler on/off using a mark/space ratio to ensure the temperature do not over shoot may give a good hysteresis control, but would also mess up the extraction of latent heat. So good control does not mean economical control.
The problem is OpenTherm is only fitted to some boilers, and using zone valves only EPH seem to have designed a system using master/slave that will work with both OpenTherm and zone valves, and it does not seem to have any link to the TRV heads. Basic interpretation of government rules has made builders think the best option to comply is two zone valves, and this has caused problems when trying to improve on the basic installation, however it is simple enough to work both zone valves together, so easy fix.
But now to general idea of Hive, the Hive wall thermostat is a simple on/off switch, no connection to any ebus, but to get the modulating control it relies on the TRV heads sending a heat demand signal to the wall thermostat, so any room with a Hive TRV can switch on the wall thermostat, it is considered this complies with the government requirements without using zone valves, but clearly needs some Hive TRV heads to work. I personally think it is a great idea, it allows any boiler to modulate as required without the need for OpenTherm and the like.
However the point of all this, is if Hive TRV heads are used in some selected rooms then only one Hive wall thermostat is required, but without programmable TRV's then you need two Hive wall thermostats, so how it is wired depends on if using Hive TRV heads. That is assuming a gas modulating boiler, there are oil modulating boilers but the amount they modulate is so small not worth worrying about.
I have talked to guys about the whole idea of IFTTT (if this then that) and in theory you could use simple sonoff wifi switches linked to TRV heads to control the central heating, however not found anyone who has done it. But mothers house I did use two thermostats one at front and one at back of house to switch on boiler, both wired in parallel and it worked. And I could use IFTTT direct to TRV heads for geofencing, however the problem was the anti hysteresis software in the TRV heads.
What I found was overnight at 16°C and set to rise to 20°C in morning at 7 am and by 10 am still not at 20°C, so set at 22°C at 7 am and 20°C at 8 am and then it was at 20°C at 8 am, however can't really do this with geofencing, only with set times, however using the Nest wall thermostat Gen 3 and geofencing it has actually worked, it seems some cleaver algorithms in the software works out at what point to stop heating, reading it seems Drayton TRV heads also have these algorithms built in, so can re-heat the home fast.
To an extent the lock shield valve can help, I found with the energenie TRV heads which show on the PC current and target one could adjust the lock shield valve so it heats to spot on what is set, or over shoots, but if set to over shoot it also over shoots when you don't want it to.
I was critical of the so called heating engineers calling themselves engineers, I felt to be an engineer you should be trained to degree standard, however when I realised how hard it was to set up my mothers central heating, I realised to get it right first time really takes some skill, my first two house simply worked, so I thought it was easy, but sun through the bay window of mothers house took some sorting to get it working A1. So I take of my hat to the heating and ventilating engineer who can set it all up spot on first time in any home.
The wiring is easy, what takes the skill is where to fit wall thermostats, and how best to wire for that home. With mothers house intention was to fit Nest, however once the programmable TRV's had been fitted and set up, never really needed Nest.
