In hot climates, pool owners put canopies over their pools to avoid the overheating that can occur from passive solar gain. In these locales, SPH systems may be installed primarily for the shoulder seasons. Since the solar differential controller has a high limit to keep the SPH system from heating the pool above a certain temperature, there may be portions of the summer during which the system remains dormant.
Some differential controllers have a “nocturnal cooling” function that activates the SPH system when the temperature at the collector sensor is sufficiently below the pool temperature and the pool temperature is above its high limit. By pumping the pool water through the collectors, excess pool heat can be radiated to the cooler night sky. As a result, in some climates the SPH system may also serve as a solar cooling system.