As far as I recall vaillant and grunfoss had economical issues with eachother (tail wanting to wag the dog), vaillant started using another manufacturer(wilo), some problems appeared, all the ones I knew of were rectified under warranty. In 08 I was told all is fine again, dog and tail are back in unison. Vaillant is my manufacturer of choice, the 831 the most common I fit, have never considered leaving nor have I had reason to.