Had some feedback from my uncle who I recommended a pair of these too, he installed the hot side to his shower first, turned it on and nothing, checked a couple of things then phoned flowflex up, couldn't get much help from them as the guy who deals with these pumps was off. anyway he was put through to inventor and he finally told my uncle to swap the one on the pipe to the one he hadn't put onto the cold yet(he bought two seperate ones not the two linked together) worked fine, he got back on the phone to arrange a return/replacement and all fine he was even been given £30 back, but still waiting on replacement 9 days later, haven't spoke with him since but when he told me the story I told him to get straight on the phone to them as they deliver next day when he first ordered. since this is the second problem out of two orders I know about there maybe some teething problems.