a heating , ventilation and air conditioning system includes a condenser flowing a flow of refrigerant therethrough and to an output pipe and a falling film evaporator in flow communication with the condenser and having an evaporator input pipe located vertically higher than the output pipe . a plurality of riser pipes connect the output pipe to the evaporator input pipe . the flow of refrigerant flows through selected riser pipes of the plurality of riser pipes as required by a load on the hvac system .