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Your diagram shows two links marked W one for hot water the other for central heating which since from the hot water then you can't have central heating without hot water. However looking at the wiring diagram it seems the hot water connection is only a connection into the existing timer there seems to be no connection to the boiler other than through the existing timer. So here I am guessing that some where there is some motorised valve and the connection "HW" is not a input but an output to some exterior motorised valve. The picture
seems to confirm that, so in central heating systems I have worked on before the thermostat controls the valve and the valve controls the central heating. This
is the typical diagram.
Looking at your diagram for Hive it seems very similar to the programmer in my diagram, however the room thermostat is replaced by the Hive but the tank thermostat is not. So can't really help as to how Hive is wired, but likely it is wired to the valve not direct to central heating boiler.
Today we have a completely different system, instead of boiler switching on/off it switched up/down in flame height and only when flame height at minimum does it switch off, as a result the thermostats are fitted to each radiator these can link to things like Nest, but there is no thermostat as such on the wall they are built into the radiators and the device on the wall is just a hub gathering information from the TRV's and then telling the boiler to switch down or off as the room temperature is satisfied. It may at a glance look the same, but in real terms it is very different.
I know Nest is a IFTTT device and it talks to the eTRV's but not up with what Hive does, I expect it is the same? I also have an old boiler pre condensate and I intend to keep it since it has only broke down twice in 30 years, once a transformer failed, and second it got sooted up, all it needed was a clean and flame height adjusting to compensate for wear on the jets. I see modern boilers breaking down on a regular basis so not keen to update. However I accept some day I will be forced to update, and so when doing any alterations like adding TRV's I do it with update in mind. In my case separate boiler is used for hot water, but in your case I think you have to consider what next, and decide if a motorised valve or TRV's is the way to go.
Personally I think you have to consider carefully if you would want instant hot water in the future, the combi boiler was popular, but it does not integrate with solar panels or wood burning fires, so by many is considered as old hat, and they are putting tanks back in so they can use the solar power. So job one is sit back and work out what you want in 2025, and then select what you buy so it will integrate as you progress towards your goal. As a result in mothers house we have used eTRV's so we can at a latter date get Nest which will integrate. But since the eTRV allows you to alter each rooms temperature from your phone, not really sure why I need Nest?
I did look at Hive but it's a lot more expensive than Nest once you start adding eTRV's but cheaper than EvoHome.
Looking at your diagram for Hive it seems very similar to the programmer in my diagram, however the room thermostat is replaced by the Hive but the tank thermostat is not. So can't really help as to how Hive is wired, but likely it is wired to the valve not direct to central heating boiler.
Today we have a completely different system, instead of boiler switching on/off it switched up/down in flame height and only when flame height at minimum does it switch off, as a result the thermostats are fitted to each radiator these can link to things like Nest, but there is no thermostat as such on the wall they are built into the radiators and the device on the wall is just a hub gathering information from the TRV's and then telling the boiler to switch down or off as the room temperature is satisfied. It may at a glance look the same, but in real terms it is very different.
I know Nest is a IFTTT device and it talks to the eTRV's but not up with what Hive does, I expect it is the same? I also have an old boiler pre condensate and I intend to keep it since it has only broke down twice in 30 years, once a transformer failed, and second it got sooted up, all it needed was a clean and flame height adjusting to compensate for wear on the jets. I see modern boilers breaking down on a regular basis so not keen to update. However I accept some day I will be forced to update, and so when doing any alterations like adding TRV's I do it with update in mind. In my case separate boiler is used for hot water, but in your case I think you have to consider what next, and decide if a motorised valve or TRV's is the way to go.
Personally I think you have to consider carefully if you would want instant hot water in the future, the combi boiler was popular, but it does not integrate with solar panels or wood burning fires, so by many is considered as old hat, and they are putting tanks back in so they can use the solar power. So job one is sit back and work out what you want in 2025, and then select what you buy so it will integrate as you progress towards your goal. As a result in mothers house we have used eTRV's so we can at a latter date get Nest which will integrate. But since the eTRV allows you to alter each rooms temperature from your phone, not really sure why I need Nest?
I did look at Hive but it's a lot more expensive than Nest once you start adding eTRV's but cheaper than EvoHome.
