Where oh where is this air coming from?

[sammathias]

sorry don't know if I phrased that correctly. some 3 port valves are mid position. i.e always an open circuit. but some are a closed circuit and only open with a demand...

 

[Ric2013]

Yes, always (at least) one circuit open. Merry Christmas!

 

[Ric2013]

Right, I said I'd get back to you.

I followed up the suggestion above which was a dodgy washer on the pump and applied Hylomar to my existing rubber washer (which wasn't all that tightly done up, as I think I stated above. Also, to be pedantic, I took the gland nut off the pump gate valve and shoved some PTFE behind it in the usual manner (even though the graphite packing seemed to be in good order).

After a couple of weeks there was no more air. Boiler still kettles horribly, but I can move on to powerflush or whatever for that. At least we seem to have solved the air problem, which will, of course, reduce further corrosion of the system.

Thanks for your suggestions and for finally forcing me to check the bloody obvious, because, logically, a system like this can only normally draw in air between the pump and the neutral point, but I couldn't see anything wrong with that section - must have been though!!!