Size. The size of a gas boiler is given in terms of its heating capacity, in Btu/hour 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 have ratings in the range of 40,000 to 300,000 Btu/hour.
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 the annual fuel utilization efficiency (AFUE). The AFUE represents performance under a specific set of conditions, accounting for the effect of part-load efficiency and cyclic losses. The conditions, chosen to represent operation in an average climate with a particular usage pattern, include flue and infiltration losses during on and off cycles.
The AFUE is helpful when comparing two boilers under the same test conditions, but it’s less useful for predicting a boiler’s annual fuel use in the field, where local conditions may not match the AFUE test conditions and calculation assumptions. Keep in mind that the AFUE is based on a residential load profile, which may be different from the load profile of a commercial building.
Since 1992, the US Department of Energy (DOE), under the National Appliance Energy Conservation Act, has required that small gas boilers have an AFUE of at least 80%. In November 2007, the DOE established a revised minimum efficiency standard of 82% for residential boilers, which took effect in November 2015. The Energy Star program awards an Energy Star label to boilers with an AFUE of 85% or better. The most-efficient boilers on the market boast an AFUE of around 96% (figure 2).
Figure 2: AFUE ratings distinguish three boiler types
Condensing boilers typically have AFUEs greater than 85%. Contemporary, high-efficiency models use fully modulating burners to reach AFUEs over 92%. Fully modulating boilers, the most-efficient boilers on the market, boast AFUEs near 96%.
To find the ratings for a particular boiler, review the EnergyGuide label found on the unit, check the AHRI Directory of Certified Product Performance website, or consult manufacturer literature.
Sealed combustion. Boilers draw the air they need for combustion 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’s closed to occupied areas. This configuration virtually eliminates any risk that combustion gases could leak into occupied space. However, sealed combustion requires some complicated installation techniques, so check the manufacturer’s installation instructions carefully.
Load-matching/multistage/modulating burners. Most boilers are two-position devices: They either run at maximum output or are in the off position. 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%.
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 for new boilers (or retrofitted to existing boilers) come in varying levels of sophistication; the controls 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 postpurge, 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 user-determined priority order. These controls can increase the efficiency of noncondensing boilers by 10% and reduce idle losses to 0.3%.
The 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%. 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%.