The external filling loop is a silver-coloured braided hose which is connected between the central heating return pipework and a cold water mains pipe. It should be located close to the boiler and is often found under the sink if the boiler is in the kitchen. The braided hose will have a valve fitted on either one or both ends. This is one example of what external filling loops look like: How to use the external filling loop Make sure both ends are securely attached to the pipes Open one or both of the valves on the braided hose to allow water to flow into the heating system Watch the pressure gauge rise slowly until it reaches about 1.5 bar (in the middle of the green section) Return valves to the original position to stop the flow of water into the boiler