HELP!!!! Yet again, tried last night to sort out cold radiator problem. There are only four radiators in the flat - two doubles and two singles. When I click on the central heating, the hot water travels up from where the three way valve is situated and gets about 3metres along the 22mm pipe. There was some air coming out of the PRV in the loft but still no hot water getting to the radiators. Turned off heating........bled the radiators again - water coming out - cold. Turned off the valves to all but one radiator - still nothing happening when I turned the heating back on. Every time I turn the heating off I hear water running back down the pipes towards the hot water tank. Do I need to refill the system somehow. If so could someone please lead me through it step by step. Thank you so much.