Air in central heating system

[Jayluvit]

Hi all,
I have a gravity fed system in my house, hot water cylinder in a bedroom cupboard, boiler in another cupboard, main water tank and expansion tank in the loft.
I recently used Sentinal 400 to clean out the system which I have now flushed out and am now trying to bleed the system to get it up and running again.
My problem is that I am constently getting air in the system even though I have fully bled the system and it only seems to happen when the water is being heated up.
I can hear the air coming through my pump and with in about 10 seconds it then enters the tail rad in my bathroom which I then open up the bleed valve to let the air out.
It seems to happen every 10 minutes. I have had the boiler off with the pump running but do not get this problem.
I have also blocked off the vent pipe, which is also on the same pipe as the feed to the rads, for about 24 hours to see if the air was being drawn in through this but I still got the same results.
Anyone with some infromation would be greatly apprecieated.

 

[lightning]

Blocking off the vent pipe is crazy and dangerous, you could fill a jamjar up of water and hold it under vent pipe to see if it sucks

 

[Jayluvit]

Thanks for the reply.
Would the method I chose to detect if air was being drawn in through the vent pipe still prove that this is not the case? I did use a surgical glove to block it off thinking that if too much pressure and eventually the glove would burst but when I inspected it there was no change to it.
If I have proved that there is no air coming down the vent pipe where else could the air be entering the system?
If air is being drawn down the vent pipe what would the solution be?