Your boiler is wired up using Vaillant's proprietary controls. They operate over ebus and the vaillant thermostat directly controls the boiler. This is why terminals 3 and 4 on the boiler are joined together. In a traditional system, your thermostat would control the boiler SL to operate it.
Does your CH have a motorised valve? If so, you probably have a Vaillant control panel (VR65) to operate it. If you remove this, you will need to ensure the Nest is wired through the motorised valve to operate it, and the valve would activate the SL on the boiler. (Edit - saw it's a combi, and therefore will probably not have an external valve - the boiler has an internal diverter valve).
Nest gen 3 supports opentherm and this would work best with your boiler. You need an additional vaillant opentherm module to use this, though. IMO, the nest working with on/off control is perhaps a retrograde step compared to the Vaillant thermostat you have currently.
Just to add - Dan is very knowledgeable in this area and genuinely helpful with his contributions. Fact is, you have showed a complete lack of understanding of even the basics of this installation, and so for your own benefit, I think his advice is spot on. You're best off getting a professional in if you really insist on having the Nest fitted.
Does your CH have a motorised valve? If so, you probably have a Vaillant control panel (VR65) to operate it. If you remove this, you will need to ensure the Nest is wired through the motorised valve to operate it, and the valve would activate the SL on the boiler. (Edit - saw it's a combi, and therefore will probably not have an external valve - the boiler has an internal diverter valve).
Nest gen 3 supports opentherm and this would work best with your boiler. You need an additional vaillant opentherm module to use this, though. IMO, the nest working with on/off control is perhaps a retrograde step compared to the Vaillant thermostat you have currently.
Just to add - Dan is very knowledgeable in this area and genuinely helpful with his contributions. Fact is, you have showed a complete lack of understanding of even the basics of this installation, and so for your own benefit, I think his advice is spot on. You're best off getting a professional in if you really insist on having the Nest fitted.
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