What’s this? Its got a Schrader valve on top and had air in, so because our boiler is doing intermittent foghorn impressions I tried anything that thought it might be a place for air to collect. Now there’s no air but the boilers still grumbling.

It looks like a expansion vessel.

That’s what I thought but its not really giving anything much room to expand into -the bricks behind show how big it is.

What pipe is it connected to?
If it is on the mains cold feed, it could be an ‘anti hammer’ device.

Yep its on the main, so should I put some air back in it?

Yes.
They are ‘shock absorbers’ to stop your pipes banging when taps are turned on/off suddenly.

Thanks! So if there’s no air in the system whats causing the foghorn noises?

It’s an anti water hammer device.

Should be set to around 2.5 bar of pressure to work properly.

boiler noise could be caused by bad combustion, probably worth getting a boiler service and have the gas flow rate checked

Limescale in the heat exchanger?

Noise seems worse when its starring from cold which is why I thought it was air accumulating, but is only happening on heating rather than hot water. Seems to be mainly when heating demand switches on and off, but not exclusively. Service is probably a good start!