I've noticed that there is a short length of 15mm pipework complete with gate valve connecting the 28mm flow & return pipes. I would be grateful if someone could explain what this is for and should the valve be open or closed ?
valve is used to maintain a constant differential pressure in a heating system. It reduces flow noise in a system, particularly as thermostatic radiator valves are closed.
The boiler return temperature is raised and this reduces corrosion caused by flue gas condensation. In addition, when radiator valves are closed it also maintains flow over the temperature sensor on the boiler to ensure proper operation of external temperature compensation controls.
On gas-fired water heaters it guarantees a minimum flow circulation when thermostats or radiator valves are closed.