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One thing to consider maybe is that Viessmann doesn't have installers in London. When I contacted them they could only give me one company in Cardiff.
Good call... did you go for it anyway?
I asked a heating engineer what he thought, he concurred that installation is hard (extra training he hasn't done) but that maintenance is a dream.
He recommended that if I'm looking at that level of quality that I investigate Worcester Bosch as there were more installers, efficiency is nearly equivalent. He suggested something like this https://www.worcester-bosch.co.uk/products/boilers/lifestyle
Though most of what he told me was not to change the power of it without doing a full site survey as the gas run might not support a higher rating anyway and then it would be less efficient. So his main advice was: Do nothing and make no choices until you've had a full site survey.
So it's not like I'm making any decision for a long time.
One thing to consider maybe is that Viessmann doesn't have installers in London. When I contacted them they could only give me one company in Cardiff.