Whether in combination with solar heating or not, solar heating of swimming pools is an obvious choice, especially so since it is one of the few forms of pool heating system permitted by Spanish law. The season for swimming pool use coincides well with the availability of solar energy, allowing the pool temperature to be maintained at comfortable bathing temperature (27º) from May to September. Such systems are straightforward and low cost, as the collectors do not require such high thermal efficiencies as those for other applications.