Six Well-McLain boiler models in production as of 2015 are rated with an average fuel use efficiency of 95 or above, according to the Air Conditioning, Heating and Refrigeration Institute. Boilers with this high of an efficiency rating are considered among the most energy-efficient boilers available.
These six Well-McLain boilers with top efficiency ratings produce between 57,000 and 124,000 British thermal units of water heating per hour. All are models fueled by natural gas or propane that use an induced draft mechanism to increase combustion efficiency. These boilers are efficient enough to qualify for federal energy-efficiency tax credits for their purchasers as well.