Not been to this yet but had call from my friend who has asked me to have a look,far as i can gather is following: Had new ballvalve, one mixer tap in bathroom,non return valve on hot side. New cylinder , Both header and f&e tank on same level. Vent pipe not too short. Hot water cylinder stat seems to be working and set at 60 degrees. All seems to have happened after new fit. It`s unvented,header tank in loft cylinder below, standard 36 x 18 indirect. Anyone offer any advice? Thanks.