Hot Water/Central Heating System possible air lock?

I drained my system to fit a towel rail. the water system did not appear to fill up after.
At first i thought it was a block in the pipe from the tank in the loft, but i have since discovered there is clear water and pressure in the system in 2 of the downstairs rads. There is still no pressure in one of the down stairs rads, or any of the upstairs one. I have tried the system air vent next to the 3 port valve and the tank and this does not release any air or water.
Only 2 of the downstairs rads can be bled.

Does this sound familiar to anyone? and what can be done to remove the air if this is the case?

Thanks

D

Sounds like a blocked cold feed....put a hose on one of your drainoff points that should be on one of your downstairs radiators. Open the drainoff.. then check the small tank in your loft & see if it empties..if it does not the cold supply to your heating system is blocked.Try that & post back.

i thought it was a blocked feed but 2 of the radiators have clean water in them and have plenty of pressure to bleed them. I guess that means it can't be the cold water feed?

Just because you have drained your system from one point downstairs does not mean that you have drained the downstairs rads. The only rads that you have drained will be the one at which you connected the hose & all of the upstairs rads. Try connecting to another & check the F&E tank.If this empties it will be an air lock.