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 ?
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Whats this for ?
- Thread starter surfrider
- Start date
bypass valve it should be open enough to suit the system 
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.
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.
Its meant to give a constant flow through the system should all other ports be closed.
leave it open app 1/4 
if the boiler starts banging open it a bit more
if the boiler starts banging open it a bit more Unless of course its on a y plan system, in which case it should be closed.
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