The Rheos boiler/water heater combines 87-percent combustion efficiency with boiler modulation circuits that sense the heating demand and automatically adjust the boiler with the range of 1.2 to 2.4 MBtu. The high-density heat exchanger uses a double-row copper tube design to maximize the heating surface and create high-density heat transfer per square inch of floor space, the company says. The unit has a footprint of less than 34.5 inches. The manufacturer states that the small footprint and the sidewall venting allows for easy, flexible installation.