The GV90+ cast iron boiler for energy-efficient, comfort heating in residential applications features streamlined controls with simple setup, low maintenance and durable design for the long run. The unit features an annual fuel utilization efficiency (AFUE) rating of 91 percent. The unit features a high performance durable cast iron primary heat exchanger made from approximately 70 percent recycled materials. A corrosion-resistant, stainless steel condensing secondary heat exchanger recoups heat energy from exhaust normally wasted. Weil-McLain.