On the outside of the Aspen condensing boiler is the sleek, smart, ergonomically optimized styling that U.S. Boiler pioneered last year – an elegant design so striking that it has actually been adopted by others in the industry. But that’s where the comparison ends. On the inside you’ll find a proven stainless steel firetube heat exchanger and an advanced combustion system, designed to set new standards for reliability, efficiency, ease of installation, and operation.