Air in system - now not working

[Jo]

My upstairs radiators were cold at the top and rather noisy - water/ gurgling sounds. Thinking there was air in the system I turned the programmable whatsit to OFF then used the little key thingy - turning about 1/4 turn on each radiator until it started hissing, then tightening it again.
I turned the heating back on, boiler fired up - all good. After 20 mins I went to check each radiator but they were cold. The boiler had stopped running. I then discovered my cold water tank was nearly empty and the ballcock was stuck. After a bit of wiggling (the ballcock not me) it was ok but I had to hold it down to fill the tank. Boiler fired up - again all good.
Then it all stopped again. Despite the programmable thermostat telling me the heating was on, there was no flame in the boiler and the radiators weren't heating up. I checked the tank again and the water level was slightly over the ballcock, which is actually quite low in the tank.
Turned it all of now. Thought I was saving myself a call-out charge for a simple job but have I broken it now? Probably something simple but can anyone help me?

 

[Breesey]

You bleed the rad until water comes out of the bleed screw.

Sounds like you need a new ball valve, and the boiler has temped out.

 

[gas4you]

Leave your trv's on but turn the heating/pump off whilst bleeding the rads.

 

[Jo]

Thanks. DIdn't realise I had to wait for water to come out - thought I was just waiting for hissing to stop and probably did it a bit tentatively at first for fear of water gushing everywhere!
I've just done all the radiators as above and all seems to be ok now - bits of rads that were cold are now hot - hurrah! Although cold water tank was empty again and ballcock needed wiggling to unstick it and allow water in. I guess that needs replacing.

 

[Breesey]

Yep! Change the ball valve asap because you don't want the system running dry!

 

[TicklyT]

The central heating feed & expansion tank only needs a couple of inches of water covering the outlet connection, otherwise there's no room for the expansion part of it's role, which may then cause water to run out of the overflow until it finds it's own level again.

The ball valve should work correctly, and open before the water level does drop as far as the outlet. For the price of them, it is probably simpler to change it than repair it.