Vaillant, 9 years personal experience, old boss 24years. In my nine years I've not had a Vaillant I've fitted breakdown on me. The only thing I've come across was niggles on the 937(under warranty), a small portion were produced where the aperture on the expansion vessel was too small, leading to farting noises when there was a hot water demand. Vaillant had a brief problem with the gasket seal on the burner chambres, they came across it on the in-house test models. They immediately notified every consumer who had had the relevent models fitted legally/notified and came out to resolve the fault. Most other manufacturer's dont do that, baxi-potterton spring to mind.