Beneath the boiler, incorporated in the isolating valve set on some of our boilers, there will be a ‘filling loop' for introducing mains water into the boiler circuit for filling and topping up purposes. These generally have an isolating valve at each end. To introduce water into the system, undo one of these valves completely and gently open the other valve to control the rate of flow of water into the boiler circuit. Introduce water until the pressure gauge on the boiler reads between 1.5 to 1.8 bar. Turn both valves off fully. The filling loop should be only a temporary connection and should be disconnected after use. N.B. The filling loop may be sited a little further away, but usually very close to the boiler.