There is no key its just a loop of flexible pipe that has an isolation valve on each end that is connected to the cold water and central heating pipes.
Jon
Hi
I have a Worcester boiler for my central heating and hot water. I am trying to add water to the central heating. The system has a loop from the cold water into the central heating pipe. There are isolating valves on each end. I added water in the past by just turning the valve near the...