Method of testing a backflow preventer assembly, the backflow preventer assembly comprising at least: - a backflow preventer comprising: • a housing comprising an inlet and an outlet, • an upstream check valve assembly positioned in the housing downstream of the inlet and being biased to a first closed position, • a downstream check valve assembly positioned in the housing downstream of the upstream check valve assembly and being biased to a second closed position, • an intermediate chamber positioned between the upstream check valve assembly and the downstream check valve assembly, the intermediate chamber comprising a discharge opening at a bottom side thereof for emptying the intermediate chamber, • a relief valve located in the housing, the relief valve being positioned upstream of the downstream check valve assembly and being configured to move between a third closed position in which it closes off the discharge opening and an open position in which the discharge opening is open, - an upstream test cock positioned upstream of the upstream check valve assembly, wherein the upstream test cock is configured to be attached to a liquid pressure measurement device, - a downstream test cock positioned downstream of the downstream check valve assembly, wherein the downstream test cock is configured to be attached to the liquid pressure measurement device, - an upstream shut off valve positioned upstream of the upstream test cock, wherein an upstream zone is defined between the upstream check valve assembly and the upstream shut off valve, - a downstream shut off valve positioned downstream of the downstream test cock, wherein a downstream zone is defined between the downstream check valve assembly and the downstream shut off valve, wherein the method of testing the backflow preventer assembly comprises: - testing the downstream shut off valve and the downstream check valve assembly by measuring a development over time of a first pressure in the downstream zone at the downstream test cock, - testing an opening pressure of the relief valve by closing the upstream shut off valve, gradually lowering the pressure in the upstream zone by tapping off liquid from the upstream zone via the upstream test cock and by measuring a second, absolute pressure at the upstream test cock at which fluid starts to discharge via the discharge opening and comparing said second, absolute pressure to a required threshold pressure for the relief valve. - a backflow preventer comprising: • a housing comprising an inlet and an outlet, • an upstream check valve assembly positioned in the housing downstream of the inlet and being biased to a first closed position, • a downstream check valve assembly positioned in the housing downstream of the upstream check valve assembly and being biased to a second closed position, • an intermediate chamber positioned between the upstream check valve assembly and the downstream check valve assembly, the intermediate chamber comprising a discharge opening at a bottom side thereof for emptying the intermediate chamber, • a relief valve located in the housing, the relief valve being positioned upstream of the downstream check valve assembly and being configured to move between a third closed position in which it closes off the discharge opening and an open position in which the discharge opening is open, • a housing comprising an inlet and an outlet, • an upstream check valve assembly positioned in the housing downstream of the inlet and being biased to a first closed position, • a downstream check valve assembly positioned in the housing downstream of the upstream check valve assembly and being biased to a second closed position, • an intermediate chamber positioned between the upstream check valve assembly and the downstream check valve assembly, the intermediate chamber comprising a discharge opening at a bottom side thereof for emptying the intermediate chamber, • a relief valve located in the housing, the relief valve being positioned upstream of the downstream check valve assembly and being configured to move between a third closed position in which it closes off the discharge opening and an open position in which the discharge opening is open, - an upstream test cock positioned upstream of the upstream check valve assembly, wherein the upstream test cock is configured to be attached to a liquid pressure measurement device, - a downstream test cock positioned downstream of the downstream check valve assembly, wherein the downstream test cock is configured to be attached to the liquid pressure measurement device, - an upstream shut off valve positioned upstream of the upstream test cock, wherein an upstream zone is defined between the upstream check valve assembly and the upstream shut off valve, - a downstream shut off valve positioned downstream of the downstream test cock, wherein a downstream zone is defined between the downstream check valve assembly and the downstream shut off valve, - testing the downstream shut off valve and the downstream check valve assembly by measuring a development over time of a first pressure in the downstream zone at the downstream test cock, - testing an opening pressure of the relief valve by closing the upstream shut off valve, gradually lowering the pressure in the upstream zone by tapping off liquid from the upstream zone via the upstream test cock and by measuring a second, absolute pressure at the upstream test cock at which fluid starts to discharge via the discharge opening and comparing said second, absolute pressure to a required threshold pressure for the relief valve.