The ES2™ blends unmatched performance with long lasting durability and exclusive energy saving features. Designed as an optimal replacement boiler for millions of homes with existing chimneys, the ES2™ achieves an 85% AFUE. At the heart of the ES2™ is one of the most advanced cast iron heat exchangers in the industry and a well-insulated heavy-duty steel base for longevity. Even the ES2™'s control system is class leading. The IQ Boiler Control System allows the ES2™ to be easily equipped with an optional energy saving control package that can achieve up to a 15 to 20% reduction in fuel consumption according to most studies.