My 15 year old WB CDI 35 combi has been adequately reliable with just a disintegrated flow turbine adaptor 4 years ago (rubbish plastic not suitable for chlorinated water) and replaced one set of electrodes at the same time (sight glass risk issue). I have put in Evohome in 2016 which operates very well on TPI control. I am now planning the replacement for the boiler.

For the replacement I want to use stainless steel heat exchanger, the WB has been corroding over the years into the condensate trap, and I dislike the use of the aluminium with acidic condensate . The system is clean , I replaced all the radiators myself 2 years ago and flushed system very well and added a good mag filter and spirotech deaeration + sentinel X100 inhibitor. I live in manchester we have very low tds and very low hardness water typical for north west.

I would like to use the Viessmann 200-W, because of the 316 SS HX and modulation down to 1.9 kW , this low modulation would be perfect for me because often only 1-2 radiators in use with Evohome in my house . However I don't want the weather compensation game , I would much prefer to use Opentherm , but the 200-W is not ready for this out of the box in the UK.

I read else where there was some opentherm extension module plugin (not supported by Viessmann UK) that could be got that would allow the 200-W to accept open term signals from a Honeywell R8810a opentherm bridge , but I am not sure what is the exact module is called (perhaps it is called Viessmann EM-EA1 Extension) . If this extension module does exist and allow opentherm my next problem will be does the boiler accept the max flow temperature to be preset when in opentherm control (lots of probems with the for the other 100-W model) .

if Opentherm is not possible it will back to TPI control with Evohome and set the boiler flow temperature to 65 deg C which where I have the WB at present.