I have an internal and external bypass on my Vaillant Ecotec plus boiler system - I'm trying to figure out if having two by-passes is a good thing or a bad thing. I was instructed by a Vaillant engineer to add an external bypass years ago because he said the internal one was too 'weak'. I'm trying to reduce some system noise by adjusting the external by-pass but not sure whether I need to do something with the internal by-pass. And of course, my 637 boiler is way oversized (13 radiators, about 2KW of UFH and a 290L cylinder).
