Abstract: A heating and/or cooling system incorporates a number of modular refrigeration units each of which includes at least one compressor, an evaporator and a condenser. A variable speed circulating water pump circulates water through each evaporator/condenser, which are connected in parallel across the respective supply line and return line. A master controller activates and deactivates individual compressors in accordance with load demand. Shut-off valves close off the water flow through the respective evaporator and/or condenser of any units deactivated and the resulting changes in the pressure differential in the supply and return lines is sensed by sensor which sends a signal to the pump motor speed controller which changes the output of the pump to restore a predetermined pressure differential.