combi pressure loss when running hot water

[Sadgran]

my combi is running fine on heating side and maintains pressure fine
Just recently turning on the hot tap causes a pressure drop on the meter and the hot flow to stop. By opening the filling loop the pressure builds and then the hot feed starts to run again. The hot flow continues whilst the filling loop is open and the rate is controlled to keep the pressure within limit. If the hot tap is turned off the boiler pressure builds untill the fill loop is closed Boiler is a baxi duotech.

 

[cross thread]

Have you had plumbing work done

 

[Sadgran]

One rad replaced but system pressurised and heating fine with pressure maintained
Only drops when hot tap opened