I appear to have a noise coming from the pump area in the airing cupboard.It sounds like tiny air bubbles, a sort of 'rushing' noise, similar to rising main pressure noise.
It only happens when the system is hot, and the mid position valve has moved to CH only, from CH and HW.
I can't attribute it to any particular event, although I noticed it probably 12 months ago after fitting a new pump,as the other died after many months of knocking.The system did initially have FERNOX in it, but since draining I have not replenished it.I have also since fitted thermostatic valves to all rads, bar the bathroom one.When I recently drained the system, I noticed the water was black.Perhaps adding fernox again may help, as it appears as if bubbles are being created when the circulation is at its shortest. IE when only flowing through the rads.When the noise is present,I can cure it by turning up the cylinder stat,which moves the mid position valve across to HW and CH again, and the 'bubbles' disperse.
Direcly above the pump is a device I believe to be an air separator(MYSON AERJEC II).Could this be the problem ?
Someone else suggested filling the system with distilled water as this couldn't oxygenate and cause bubbles.
Could anyone cast any light on this problem, further info if required.
many thanks