Variable primary flow chilled water pumping systems allow flow through chiller evaporators to vary in response to changes in load. Variable primary flow proponents claim that this approach uses less energy and is more economical than other pumping schemes, in particular, the well-established primary/secondary system. This paper investigates these claims via a parametric modeling study. Parameters varied included plant cooling load profile and climate, number of chillers in the plant, chilled water temperature difference vs. load, and others. Construction cost and operating cost differences were calculated and systems were compared on a lifecycle cost basis. Variable flow, primary-only systems reduced total annual plant energy use by 2%–5%, first cost by 4%–8%, and life-cycle cost by 3%–5% relative to equivalent primary/ secondary systems for the assumptions and range of parameters considered..