Why is 2.0 bar the minimum pressure for an unpumped accumulator installation?

An accumulator vessel is a pressurised vessel. To allow the incoming mains water to be able to force water into the vessel, the pressure within the vessel must be lower than the pressure of the incoming mains.

For AccuBoost accumulators, the pressure within the vessel should be 1.5 bar below the pressure of the incoming mains. This size of pressure difference ensures that the incoming mains can fill the vessel and that there is enough pressure to force the water out of the vessel and to the outlet.

In pumped accumulator installations, the pump will ensure that the pressure within the vessel is at 3.0 bar and the pre-charge pressure of the vessel is set to 1.5 bar, creating a difference of 1.5 bar.

In unpumped accumulator installations, the minimum pressure of the mains must be 2.0 bar so that the vessel can be set to 1.5 bar below (0.5 bar). If the vessel is set to 0 bar then the positive impact of the accumulator is negated.