Draining Central Heating System

[martin53]

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

 

[JohnD]

drain tap clogged

if you unscrew the end enough it will come out unexpectedly and dirty water will shoot out all over you.

if not, poke up it with a bit of wire, and dirty water will shoot out all over you.

if you can't take that risk, loosen one of the radiator taps (after closing the valves at both ends of the rad). you will probably find a garden hose connector to fit, once the radiator has gurgled out all its water into the tray you hold underneath.

 

[clf-gas]

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.

As John says to drain.

DO NOT put that pushfit cap back on, it should not be there.

 

[JohnD]

btw - a couple of extra things you can do -

bale out all the mud from your F&E tank and sponge it clean to avoid it washing down into the boiler

put these radiator valves with drain-off on at least one of your downstairs rads

give it a chemical clean with Sentinel X400 or similar when you are refilling, and run it for a few weeks before draining out all the old muck that it looosens, before rinsing out and refilling with inhibitor.