I only asked for advice on the replacement boiler, was not looking to redesign & replace huge parts of the system. Though happy to listen on boiler options.
To be clear though to save repeat suggestions - I absolutely need to keep the thermal store ..... it is an integral part of the Underfloor Heating System (there are no radiators) the system designers & suppliers warranty their system & design, and it states explicitly that the UFH must never be run directly off a boiler ..... it is run off the stratified lower section of thermal store and its dedicated heat exchanger. Water temp is set to 60degree in the lower stratified zone.
I had assumed just swapping the Baxi out to another 'heat only Boiler', am happy to consider a system boiler if it can show any benefits, and importantly will work with existing control & input requirements, without modifying circuit boards on system.
The concern is the 'stir' function and how that could be achieved if pump control is moved into the boiler.
At the moment the boiler is nice an simple ...... heating zones work downstream of store, own pump etc. When they have exhausted heat from the store, store thermostat signals this to control board .... it operates relay and opens anti-syphon valve, when it is fully open it signals back to control board, it starts relay for primaries pump, then operates boiler relay and sends 220V to the boiler. (these relays are on-board on the system control board)
Boiler then runs/modulates as necessary until the store up to temp - at which point control board then drops 220V to boiler operates an overrun period then shuts down pump and anti-syphon valve.
In use on DHW once flow detected - after 30sec delay (in case of small water draw off) anti-syphon valve opens, signals to control board when fully open and it starts primaries pump but no 220V to boiler, this provides stir to Thermal store.
I accept none of this is rocket science – but it works, and works very well.