Damage new boiler by slightly increasing mains water pressure?

Another_Level

Just to add, the new boiler may need an expansion vessel on the cold water supply, its to do what is already installed.

showers 

Bendy_House

SubSeb, almost certainly there is nothing more amiss here than the S/C not being opened enough. So try that and report back, please.

The boiler installer clearly had to shut off your mains supply in the street then? And they didn't ask if you wanted the internal S/C replaced whilst they were there?! Why on earth not? That's just weird.

Is your internal S/C is an inaccessible place, or what? You really should have it replaced in case of an emergency, when you need to shut off the flow very quickly. What's you gonna do - grab a torch and a spanner and head outside at 3 in the morning during a storm? Sheeeesh.

SubaruSeb

Reporting back on this, I increased the water pressure using the tap in the street, upping it to almost fully open, but the shower still gets turned into a dribble if someone uses another tap in the house at the same time. This didn't used to happen, but might coincide with a plumber replacing the bath combination taps a while ago, which was before we had the new Worcester Bosch boiler fitted.

I've not tried opening the internal stopcock more, though, so will try that.