Abstract: A control system is provided which automatically varies the degree of ventilation of an indoor swimming pool in response to variations in the outside temperature and comprises separate speed-regulating means for each of the fan-driving motors, each speed-regulating means being operated in response to signals received from a master controller connected to the output of an external temperature sensor. The control system also incorporates means for automatically lowering the level of the temperature to be maintained when the swimming pool is unoccupied, in inverse proportion to the difference between the outside and inside temperatures, this lowering being effected in small increments at relatively long intervals of time.