air in system

[hotncold]

Have a Gloworm spacesaver 38 vented system 2x2 port valves.

Noticed recently when in CH mode the sound of running water from the boiler like a trickling noise, could this be air in the system?
The noise is not there when boiler running on DHW only mode.

Also a couple of questions about Grundfos pumps.
Is it Ok to change the pump speed when the system is running?
What kind of noise do u get when there is air in the pump ? does it sound like a whirring noise (from the pump)?

Any help please

 

[ChrisRoberts]

Yes
Yes
& Yes.
turn off heating and try venting rads.

 

[hotncold]

Thanks for the reply.
There was some air in the system but pump still making noise (sometimes) mainly on setting 3.
Any other ideas?

 

[hotncold]

another thing

the boiler hums ( sometimes) although not sure if this noise is travelling down the pipes from the pump, could it?