FAO: any gas fitter

[Colin Wilson]

Not for me this one, but a colleague in work has a boiler than bangs whenever the DHW kicks in (although it's alright with the heating side)

Thoughts welcome (common causes ?)

[Ed Sirett]

1) Kettling due to reduced primary circuit flow and lack of inhibitor. 2) Internal thermostat has become insensitive and operating at higher than intended tempratures.

[John Stumbles]

What make & model is it? What sort of bang: is it as it ignites, or while the main burner is running?

If it's a combi I'd think about incorrect ignition and modulating burners: if it's not lighting cleanly but starting up at a low gas rate on CH but full bore on DHW you might get an ignition bang loudly on DHW but less/not noticeably on CH.

[Colin Wilson]

Thanks for the replies Ed / John - unfortunately I have no further info at present, as my colleague is off until (I think) next Wednesday...