I was hoping someone could point me in the right direction of the cause and possible fix for my boiler issue.

It’s a Baxi Platinum 33HE Combi and if you start running the hot water for a while (say a bath) a banging noise starts coming from the boiler. A friend said it maybe water hammer and to leave it running for a while and it’ll clear but it hasn’t. I’ve also noted the upstairs radiators also seem to come on at the same time. I don’t know to much about boilers but i’ve always assumed that the water and heating was two separate systems in the boiler. Do i have a leak somewhere in the boiler?

Sticking / faulty divertor valve.
£25ish for the parts if a repair kit is available, around £75 for a new valve.
Get it checked by someone who knows before ordering the bits.

Thanks Alan, i was worried it’d be a new boiler not a valve plus 1-2hrs labour i’m guessing.

I was thinking maybe heat exchanger?

If the H/E is partially blocked, it will cause boiling inside it when the gas is on, but the main clue here is that the radiators are getting warm when the water only is selected, so that points directly to a faulty divertor valve, though,of course, this cannot be confirmed without inspecting the boiler.

Alan plus 1

That’s not to say it ISN’T something else but the symptoms fit the diagnosis to a tee.

Good luck.