Hi, we have a Vaillant combi boiler, either the turbo- or eco- (it has a black plate with green knobs). Today the hot water pressure out of the taps is very low. I think I have mangaed to repressurise before by opening the valves either end of the braided pipe under the boiler - but only one end has a valve: the other end does not! How do I repressurise the boiler? It is showing 1.7 bar at the moment. Thanks