Water flow is detected within the boiler by a presure switch, remove front cover, about cntre behind control panel, a big valve unit, on end is a black covered unit, that is a microswitch, turn it slightly, so that you can see into it, get someone to turn tap on, you should see the disk in there move and operate the switch, if not, Either pressure to low, (water out should be same as water in, unless pipe work scaled up inside unit, probably difficult to descal, due to location. Cheap new one ebay) or the valve unit needs to be repaired, refurb kits on ebay about £11. if it moves, and presses on switch, poss switch faulty, standard microswitch available at electronic places. couple of quid. other than that, temp sensor, lft side, on the front pipe. Available new via ebay. You can get install manual with fault finding from Ravenheat website.
SD, January 2012