Integrating a solar pool heating system with an indoor pool is more complex due to the interaction between the pool and the conditioned space around the pool. While the mechanical system for an outdoor pool is concerned only with water quality and temperature, the systems for an indoor pool also provide space heating, manage moisture, and maintain indoor air quality.
All of these systems can function independently, but they affect one another. Suppose the water temperature feels cold and the pool owner increases the temperature setting on the pool heater. Without making other changes, the dehumidification system will have to operate more as a result of increased evaporation from the pool. Since this process is handled automatically, the owner may not be aware of the repercussions—until the increased utility bill arrives. What the owner will immediately notice is that swimmers feel colder when exiting the pool though the air temperature in the building has not changed. This is due to the relative temperatures between the pool water and the air. The air temperature in most indoor pools should be 2°F higher than the water temperature. If the pool owner increases the thermostat setting, the higher indoor air temperature would increase comfort and reduce evaporation, but the building would then lose more heat to the outdoors and the energy bill will climb even higher.
For solar pool heating systems, the differential controller defines the high limit for pool temperature. With an outdoor pool, an owner might set the differential controller to deactivate the system when the pool temperature reaches 85°F, even if 82°F is comfortable. Since the additional heat is essentially free, the only trade-off would be more evaporation. With an indoor pool, this approach would have different consequences. While the heat provided to the building from the solar pool would be free, there could be significant costs for the additional energy required to heat, dehumidify, or ventilate the building.