Upgrading large system - suggestions much appreciated

[XAPPO]

I have a large period listed home. 35 rads and 25 rooms. Currently it is served by a Potterton 65KW modulating system boiler with 300L tank but it has now died and I need to replace it. It was slightly over spec I think but has served very well for 20 years.

I have an existing evoHome system - dual controllers with 24 TRV's and HW control. It drives the Potterton in simple on/off fashion. It is integrated in Home Assistant. There are 3 zoned areas and 5 individual zoned rooms (mechanical valves and local wall sensor) .

Ideally I would like to install two cascaded boilers of about 35KW each and to drive them using openTherm although I am uncertain that this is advantageous (it appeals to my technical nature though). If a cascade is not appropriate I would consider a single boiler. I only chose cascade for better availability during a failure. Also I would prefer to use chimney fluing as I have currently but there is another to outside wall option.

I've looked at Viessman, InterGas, ideal (EvoMax2) and ATAG and probably favor either Ideal or Viessman and have discounted InterGas.

I do also like Worcester Bosh 8000 dual or single Condens 7000 but that uses an EMS bus incompatible with openTherm. I have been happy with evoHome but regrettably find myself considering abandoning it and using the complete Worcester-Bosch EasyControl setup instead. It seems to be evolving at a pace although I would have some issues still with zoning valves.

What might anyone suggest as their recommended system keeping evoHome and would you use openTherm? My budget from the quotes I have received is £10-15K .

If you are an installer (West Yorkshire) I'd be interested in you quoting for a system.

[Richard]

I am interested in why you have discounted the best OpenTherm boiler on the market (Intergas)? What are the grounds for your conclusion?

Firstly, I would be starting with a full heat loss calculation of your property using Heat-Engineer.com to actually see what you need. No point having massive/pointless redundancy, as it kills efficiency. You state you have 35 radiators, so its leading me to believe you don't have a 60kW heat loss (45kW at best and a guess?).

Personally, I would be using two boilers using an OpenTherm cascade controller. This would give you a better modulation range and better efficiency.

Depending where you are I could recommend someone, where in West Yorkshire are you?

[XAPPO]

I am in Huddersfield, if you have a recommendation of an installer I would be grateful.

I have moved off InterGas only based on the personal opinion of two installers who did not recommend them despite the fact they still sell them.
If your recommendation might involve a local company named M******* then that was one of them