Abstract: A non-compressible coolant fluid system for cooling product merchandisers having heat transfer means constructed and arranged for maintaining preselected product zone temperatures, comprising an integrated closed circuit system having pumping means for circulating non-compressible coolant fluid, a first coolant fluid loop between the pumping means and the heat transfer means and including means for cooling coolant fluid in said first loop, a second coolant fluid loop between the pumping means and the heat transfer means in by-pass relation with the first loop and including means for heating coolant fluid in the second loop, and means for selectively controlling coolant fluid circulation by said pumping means through the first and second loops.