SYR Pressure Relief Valve 20mm 

This is only a pressure relief valve and doesn't regulate pressure.

Typically, a valve like this would be placed on the water line to release pressure from water hammer or a closed-loop heating circuit.

The valves are available in 3 Bar, 4 Bar, and 6 Bar.

This should not be used to try to regulate the system's pressure without a pressure regulator. 

There are many products that regulate pressure, but not all of them have a pressure relief valve. The relief valve allows it to release water to reduce the pressure after the valve. This will typically be used to release pressure from water hammer when a tap is closed quickly or when pressure builds up due to the geyser heating up. This valve is only the relief valve.

It is very important to limit the pressure before filter systems and vessel systems to under 6 Bar, and to have them protected by a pressure relief valve or a pressurised buffer tank.