Powerful, efficient, and reliable, MegaTherm® pool heaters are designed for commercial, institutional and large residential pools where economical performance and rapid heat-up are required. Available in indoor and outdoor models in both natural and propane. MegaTherm is design certified to the ANSI® Z21.56 Standard for gas-fired pool heaters. (Outdoor 500-1825 units are certified for use without the addition of a draft diverter or external vent. Indoor units incorporate a built-in draft diverter and require only vent piping.) Salt water models (500-1825) are available with cupronickel alloy heat exchanger tubes and bronze headers and header covers.