air in central heating system

[pict]

I've got an oil-fired open-vented system which is making a rushing noise around the pump from time to time - sounds like air being drawn in to me. It happens mainly on the hot water setting, though that also includes four small radiators and a couple of towel rails ( it's a big house!). The sound happens after the system has warmed up ( half an hour or so), and a couple of minutes after the boiler has finished firing. It's intermittent not continuous. I can't find any visible leaks, and there's no air to bleed in any of the radiators ( just a very small amount in one of the towel rails). The system was drained down to instal a new boiler at the beginning of March, but that should be sorted out by now?
Should I be worried? If so, what can I do about it?
"Pict"

 

[kevplumb]

which side of the system is the pump on flow or return?

 

[pict]

The pump is right by the boiler, pushing water into it. I guess that makes it on the return?

 

[kevplumb]

if the pump is on the return the system is under negative pressure
IE any small leak tends to draw air in instead of pushing water out
rad valves are a favorite slight weep on the gland air in system
i would start there

 

[pict]

Many thanks - I'll have a check around. Does "gland" mean some specific part of the rad, or do I just look at all the joints?

 

[MANDATE]

'Gland' refers to valve spindles and the housing into which they fit.
Within the housing is a form of seal that prevents water escaping past the spindle.The seal is compressed by the gland nut. OK!