Clive Arthur wrote:

Our pressurised central heating boiler is making rattling/banging
noises, but only when the hot water is on but not the radiators. It
sounds like air in the system; all the radiators are bled.

So I wondered if there was a way to bleed any air from the hot water
tank coil. I can't see any way, but there are two air vent valves on
the inlet and outlet. They look a bit like this...

https://www.jtmplumbing.co.uk/heatin...ltecnic-valves
-c421/altecnic-caleffi-robocal-bottle-air-vent-p9592/s9594?utm_source=goog
le&utm_medium=cpc&utm_term=altecnic-caleffi-robocal-bottle-air-vent-15mm-1
5mm-15mm-502649&utm_campaign=product%2Blisting%2Bads&gclid= EAIaIQobChMIova
q7Zec2QIVzrXtCh1MZgwIEAQYBCABEgKf4_D_BwE

...or even...

https://goo.gl/2ctbUC

On ours, the plastic cap/button is screwed down tight. I think it
should be loose. Should it?

Cheers

Yes, the button should be turn and a half or so open to let out air.
But don't leave them open after all the air's gone. In theory that
should be a good idea, but in practice they get gummed up with
limescale, leak water and don't let air out any more if you leave them
open too long.

--

Roger Hayter