Point of No Pressure Change

Point of No Pressure Change – Expansion Tank

Mike Belanger Service Tip of the Month


The Point of No Pressure Change Demonstration video by Mike Belanger, a R.L. Deppmann Service Technician, is made up of two identical pumps, both of which have the following features: pressure gauges to measure discharge and suction pressure, an air vent, as well as an expansion tank.

Each of these pumps are piped in the same piping loop, but the main differentiator is that one pumps away from the expansion tank while the other pumps towards the expansion tank. The expansion tank is piped into the loop at the point that is referred to as the “Point of No Pressure Change”.

When the pump that pumps away from the Point of No Pressure Change is turned on, it creates enough pressure to pump the water around the system, vent any air at the top of the system, and return to the suction side of the pump at a positive pressure.

When the pump that pumps into the Point of No Pressure Change is turned on, it creates the same differential pressure as the other pump. However, the majority of the pressure is absorbed by the expansion tank, the water level will decrease in the air vent, and there’s not enough positive pressure to go all the way around the loop. At the inlet of the pump, the pressure will be negative and any automatic air vents will be drawing air into the system.

As each of the pumps is turned on, one pumping toward the expansion tank and the other pumping away from the expansion tank, the positive and negative pressures can be observed on the discharge and suction pressure gauges.