Air getting into system

It's been 2 years since he new boiler now, so I'm kind of assuming that's not it?

Hhmmm, spoke too soon - it's still happening. There was always a random element to when it would happen.

I'll try turning the pump to its lowest setting... plumber says pop into the attic and see if water is dripping into the header tank, which of course should tell me if there's a leak somewhere

 

You could try turning water off to header tank keep an eye on water level in tank to se if it goes down

 

How do you know it's air and not hydrogen from internal oxidation of the steel radiators? Did you try the test somebody suggested a few posts up? If it's hydrogen you just need to add Fernox to the system.

 

Sorry, yes I did - couldn't resist

Sadly no squeaky pop though - just air

 

Did the system used to be a gravity hot water system and changed to pumped?
Put a jar of water over the end of the expansion pipe and get someone to switch the pump on. If any water gets pulled in then you've found a pressure difference, which may be strong enough to pull in air. To make this problem more obvious the expansion pipe should be under positive pressure relative to the feed pipe, then you would see pumping over, but that test should show it up.

 

Sorry, don’t even understand the question

If you mean a change from an old unpressurised to modern pressurised system, then no. I imagine it’s original from when the house was built in 1994, except that the boiler, wiring and pump(s) were replaced a couple of years ago.

Originally it had two pumps, one for hot water and one for heating, and no room stat. It was converted to a single pump and some kind of switched valve, as far as I know, and a room stat. It has a header tank in the attic.

Does that help?

 

Any chance of photo of the 'some kind of switched valve..' ?

 

Heh - sorry. Make that TWO switched valves...

 

Two, 2 port valves, one for HW, one for CH..

I would guess that capped pipe bottom right, was where the second pump went?

 

I THINK so, but it was done by a plumber a couple of years ago, so not absolutely certain.

 

The new pump is probably stronger than the old one. Try the jar test and only switch the hot water heating on, assuming the feed and expansion pipes come off the hot water circuit. You could try various combinations also, for example it could even be when the heating and hot water are both on, then the heating turns off, and the sudden change is just enough to pull in some air.
The feed pipe is the one out the bottom of the small tank in the loft, the expansion pipe is the one that is suspended curving over the top above the water level.

The other option is that the pump is pumping "towards" the expansion pipe rather than away, and there's very little head meaning the suction side of the pump is under atmospheric pressure and pulling in air through a pinhole. But that would be less likely if the system is mostly in its original configuration

Maybe the experts (not me) have some further suggestion

 

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