Philmac Air Release Valves Size_4.5" Maxiplus Low and Neutral) Reefe EPS Digital Pressure Switch:Reefe EPS2 Digital Pressure Switch (no cables) Low and Neutral) They utilise a dependable ball and seat system – a simple automatic system that does not rely on mechanical parts or manual intervention When the pressure inside the pipe drops and water is no longer being pumped through the pipe the plunger inside the valve will drop and allow air into the pipe On start up the system needs to expel air from the pipe in order to operate effectively and a valve like this one can allow this to happen quickly and help prevent air locks in the system