Viessmann are supposed to be very good boilers. A problem I find with some plumbers, is they each have their favourite manufacturer and tend to hate some of the others. Best bet is to get a good installer to fit and then service/maintain the boiler if you can. It’s then their boiler recommendation, install and service work. Nobody else left to blame.
I’ll install any boiler a customer wants however I’ve been installing Vokèra boilers for 32 years and I like Vokera as a manufacturer, thier technical support is excellent and it’s run by the engineers who repair the boilers, they don’t take no shit either pretty old fashioned in that sense.
Worcester Bosch have excellent customer service but lately in Glasgow it hasn’t been great relative to their normally excellent service however their technical support is also excellent and friendly, boilers are very expensive relative to most other boiler manufacturers though but customers love Worcester Bosch.
I’ve very little experience of Valiant as there’s very few of their boilers installed here and Baxi/Potterton & Ideal are all decent.
I wouldn’t recommend Viessmen, their boilers are overly complicated & expensive and parts prices are eye watering but that doesn’t really matter with long warranties.
I’ve installed three Vokera combis in my own houses over a period of 30 years and couldn’t have been happier with the boilers there’s no way I’d spend double on an equivalent Worcester Bosch combi, Vokera’s new Unica Max is easily as good as a WB 8000 life at about £500 cheaper with more or less the same warranty and Vokera, like WB, don’t try to wriggle out of the warranty unlike a few other boiler manufacturers do.
I service all of the manufacturers boilers some are really easy to service and some are really complex but I don’t knock the work back cause it’s a decent earner however boilers are now becoming like cars insofar as you can’t really adjust anything in the more recent models if anything is out of range you have to get the manufacturers engineers out for a warranty call out.