Upstairs radiators not getting warm

[hobbo47]

Conventional system with hot water cylinder, radiators and gas boiler new. While ago had new three port valve fitted, plumber did not drain system but went in loft to header tank to block what I guess was water feed. Since then upstairs radiators many times do not get warm. Have found solution is to turn pump from top speed to slow and then quickly back to fast again. works 9 times out of 10 but sometimes have to repeat again. Have had plumber back but says everything is o.k. feels as though there could be an air lock. Once I have turned pump up and down and back up again it runs fine. Any help greatly appreciated, Thanks.

 

[joni os]

Slacken large screw in centre of pump until small amount of water emerges then retighten.

 

[hobbo47]

Thanks will try that tomorrow, does it sound as though there is a slight air lock. Thanks p.s. do I do that with the pump not running

 

[hobbo47]

I have done that but problem still exists, might I have to vent a couple of times to get rid of at Thanks.