On the outside of the Aspen™ condensing boiler is the sleek, smart, ergonomically optimized styling that US 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.