Greenstar 24i System Boiler - Overrun Increasing Noise

[eltel]

Hi

System is:

24i Greenstar System boiler, 6 years old
14 radiators + HW (S plan) (2 replaced in August)
Magnaclean on return close to boiler
Honewell Auto Airvent on HW flow adjacent to Cylinder
Pump setting 2

Problem is:

Frequent bleeding of Magnaclean and highest radiator - air not gas (it doesn't burn or smell)

When the CH is satisfied the overrun in the boiler gradually gets noisier and sounds like bubbling, which I assume is the creation/release of air from the water by the pump and which is causing the problem. It hasn't always done this.

I have:

Flushed and re-filled with 1 1/2 L Sentimel X100 (system has always been protected with Sentinel.) water is always clean and there is minimal deposit in the Magnaclean.

Checked that the system pressure doesn't drop if it's not bled so I've assumed no leaks and none are visible - no damp walls, ceilings or carpets!

Changed the 3 O rings on the Magnaclean and checked all compression fittings (except those inside the boiler.)

Checked that the pump auto air bleed cap is unscrewed.


If I manually open the CH diverter valve when the pump is on overrun/bypass the noise goes, but am I right in thinking that in this situation there shouldn't be any circulation outside of the bypass? Or have I got that wrong?

If the system is drawing air in, is it reasonable to assume that water would escape when cool and the pressure would drop below 1 bar?

Also how long would you expect to be releasing air from a freshly filled system with inhibitor?

Many thanks to anyone who can suggest anything before I call someone in.

 

[oph]

S Plan?

Sounds like the bypass if its even fitted isn't man enough.

Has the heat exchanger been bled?

Ps, you cant do it yourself you need an RGI to do that I'm afraid but then again if the boilers due a service it should be done as a matter of course.

I would...

 

[eltel]

@oph thanks for your reply.

S plan? Yes I think that's right. Separate controls for HW and CH, 2 diverter valves - programmable room stat and programmer for H/W with tank stat.

Regarding "bypass if it's even fitted" - according to the 24i instruction manual I have it says there is an internal bypass fitted. (There is also an external one directly between flow and return above the boiler with a Honeywell valve - presumably in case the TRVs all close, although there are two rads without TRV's

I don't know if the heat exchanger has been bled though.