I don't have the boiler yet, but looking at the manuals it suggests you can seperate HW and CH temperatures. Frustratingly it makes no mention of needing their thermostat.
You don't have to use their thermostat, but to separate HW from CH output temperatures, you need the Vaillant system controls - without, it doesn't know what it is providing heat for.
It's no just a thermostat, it an actual temperature sensor, reporting actual temperature back to the controller, so it knows to ramp down it's heat output as it nears the desired cylinder temperature. Likewise, the CH - as it nears the desired, it can ramp down. So no wasted heat, or overshoot.
Disadvantage - because the two temperatures are different, it can only serve heat to one requirement at a time, so if there is a demand for cylinder heating, CH stops, until that HW demand is satisfied.
So you have the choice! Many people just swap the boiler for a new one, the Vaillant controls add a lot to the comfort and controllability of the system, well worth the extra I found, plus you get some extra economy of operation.
I’ve had this exact conversation with BG recently for my system change. Why am I paying for a modern boiler that can do so many things and you won’t use them and insist that you fit Hive which is a simple 240v single output switch control to the boiler for heat on or heat off regardless if it’s a DHW or CH demand.
Imho it’s no better than an old fashion and simple £50-80 dual channel Drayton with go faster stripes and glossy colour screen.
Infuriating doesn’t cover it.
I’ve asked for Vaillant controls to be able to use the system as designed, it’s with technical head office for a answer and decision.
I think the Vaillant when it gets DHW demand goes flat out, hence the greater output, usually for less than 30 minutes dependent on tank size. When it gets a CH demand it’s using its modulation.
With full vaillant controls it knows the tank and room temps, not just there’s a heat demand.
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