Size. The size of a gas boiler is given in terms of its heating capacity, in British thermal units per hour (Btu/h) of gas input. A Btu is equal to the amount of energy it takes to raise 1 pound of water 1 degree Fahrenheit. In practical terms, 1 Btu is the heat given off by completely burning a single kitchen match. Most residential gas boilers are rated in the range of 40,000 to 300,000 Btu/h.
Efficiency. Boiler efficiency ratings are designed to help you readily compare boiler energy performance and make selections. The most common rating for small boilers is their annual fuel utilization efficiency (AFUE). AFUE accounts for the effect of part-load efficiency and cyclic losses that occur in actual installations; a single number represents performance under a specific set of conditions. 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. Additionally, AFUE is based on a residential load profile, which may be quite different than the load profile of a commercial building.
Since 1992, the U.S. Department of Energy (DOE), under the National Appliance Energy Conservation Act, has required that small gas boilers have an AFUE of at least 80 percent. In November 2007, the DOE established a revised minimum efficiency standard of 82 percent for residential boilers, which will take effect in November 2015. The Energy Star Program, which is run by the DOE and the U.S. Environmental Protection Agency, awards an Energy Star label to boilers with an AFUE of 85 percent or better. The most efficient boilers on the market boast an AFUE of around 96 percent (see Figure 2).
Figure 2: AFUE ratings distinguish three boiler types
Condensing boilers typically have annual fuel utilization efficiencies (AFUEs) greater than 85 percent. Contemporary, high-efficiency models use fully modulating burners to reach AFUEs over 92 percent. Currently, the most efficient boilers on the market boast AFUEs near 96 percent.
To find the ratings for a particular boiler, review the EnergyGuide label found on the unit, check the Consumers’ Directory of Certified Efficiency Ratings for Heating and Water Heating Equipment, available at the Air-Conditioning, Heating, and Refrigeration Institute’s Directory of Certified Product Performance, or consult manufacturer literature.
Sealed combustion. Boilers draw the air they need for combustion either from inside the heated space or directly from the outside. Drawing air directly from outside, typically through a plastic pipe that runs through an outside wall, is more efficient and safer. This method is usually referred to as sealed combustion because the gas is burned in a chamber that is closed to occupied areas. This configuration virtually eliminates any risk that combustion gases could leak into occupied space. It does, however, require some complicated installation techniques, so check the manufacturer’s installation instructions carefully.
Load matching/multistage/modulating burners. Most boilers are two-position devices: They are either running at maximum output or they are off. This on/off operation tends to send heat to the zones in pulses rather than as a steady flow. Contemporary, high-efficiency condensing boilers use modulating burners to reduce the number of on/off cycles (and cycling losses) and allow the boiler to operate for longer hours at lower firing rates, which improves efficiency. In 2005, researchers at the University of Dayton found that changing from on/off to modulation mode can improve average boiler efficiency by about 8 percent.
Controls. Electronic controllers for boilers have rapidly improved in capability and reliability since the early 1990s—allowing them to maximize comfort and minimize energy use. Controls of varying levels of sophistication can be purchased with new boilers or retrofitted to existing boilers and can increase equipment life, improve boiler efficiency, and enhance comfort.
Modern electronic controllers can reset boiler water temperature (especially important with condensing boilers), create time-delay relays, perform automatic post-purge, prevent warm-weather boiler operation, control the position of mixing valves, activate multiple boilers in stages, control pump speeds, and activate and deactivate boilers in a user-determined priority order. These controls can increase the efficiency of noncondensing boilers by 10 percent and reduce idle losses to 0.3 percent.
The purchase costs of controls vary dramatically depending on the size, age, type, plumbing configuration, and sophistication of the existing boiler. For example, add-on controls for conventional boilers typically cost anywhere from $150 to $1,000 and can reduce fuel use by up to 12 percent. Sophisticated controls come standard or as add-on options for condensing boilers and typically cost $500 to $1,000—but they can reduce fuel use by up to 20 percent.