I've installed a 3-port valve in a bodacious hot-water-to-splasher-pool project. I put it in the heating circuit, and it all works nicely, except that sometimes when I put the 3-port in HW+CH position, it often "ticks" - about once per second something clicks on, then off again. I was fairly sure I wired it up correctly - I just wired it to a pair of on-off switches and labelled them accordingly. (I got the wiring diagram from the splendid article on the uk.diy heating and pluming page.)