Even though they are designed for a different market, they can play an effective role in commercial buildings as well. Residential boilers are manufactured in larger quantities than commercial units, which makes them relatively inexpensive and readily available with a wide variety of options. Their small size enables them to be easily moved into and out of buildings, and the most efficient residential units are more efficient than the best of their commercial counterparts. In addition, multiple small boilers can be staged to provide high efficiency over a wide range of heating loads. 1 degree Fahrenheit.
Boilers typically operate most efficiently at full load.
As they are turned down, they can fall off by as much 20 percentage points in efficiency. The conditions, chosen to represent operation in an average climate with a certain usage pattern, include flue and infiltration losses during on and off cycles. AFUE serves well for comparing two boilers under the same test conditions, but it is less useful for predicting annual fuel use in the field, where local conditions may not match the AFUE test conditions and calculation assumptions. AFUE is based on a residential load profile, which may be quite different than the load profile of a commercial building. Since 1992, the National Appliance Energy Conservation Act has required that small gas boilers have an AFUE of at least 80 percent.