What am I missing? (besides the obvious). I am trying to drain my central heating system (Valliant gas convection boiler) to replace a few thermostatic radiator valves. I turned off the power to the boiler, turned the room thermostat to minimum and turned off the central heating and hot water programmes at the timer. I tied up the ball cock on the header/vent tank, attached a hose to the drain tap downstairs and opened it. Nothing came out. So I opened up several radiator bleed valves upstairs, although water then started to leak from these. Finally I thought there must be a further vent valve and found a rising pipe on the outlet next to the header tank with a push on cap. So i took this off. Still nothing going through the hose or even from the drain valve when the hose was loosened. What obvious thing am I missing? Thanks