Modern condensing boilers are more energy efficient than traditional models, and you may be able to reduce your utility bills by replacing your existing boiler with a condensing boiler. Condensing boilers condense water vapor out of the combustion gases, resulting in an increase in the boiler’s AFUE rating.
Boilers are rated by their Annual Fuel Utilization Efficiency (AFUE), which must be posted on the boiler’s yellow Energy Guide label. The Energy Guide label is a federal requirement for many types of energy-using appliances. Condensing boilers have an AFUE in the 90+ percent range.
Condensing boilers burn either gas or propane to heat the boiler’s water. This hot water is circulated through radiators, convectors, fan coils, or a radiant slab in the home. Some boilers also heat the domestic hot water that comes from your taps. Condensing boilers work best with radiant slabs or fan coils because these heat emitters work well with cool water temperatures of 100 to 130 degrees, which encourage the condensing boiler to condense.
If your boiler has a old-fashioned pilot light that you can see flickering inside the combustion chamber, or if it is more than 15 years old, it probably has an AFUE rating of 70-80 percent. You should consider replacing an old boiler with a new condensing one if you have improved your homes insulation and air-tightness, which are a higher priority than replacing a heating system.
Another easy way to find the most efficient boiler possible is to choose one with an Energy Star® label. It should have an AFUE rating of greater than 80 percent, pilotless ignition, and a sealed combustion chamber. Visit the Energy Star® web site to learn about qualifying models.
Be sure your boiler is sized correctly to the house heating load. This sizing should account for any improvements you’ve recently made to the building shell, so your new boiler may have a smaller capacity than your old one. In fact, we recommend improving the insulation levels in your home before buying a new heating system. Ask your heating contractor to perform a full set of calculations to assure that your new heating system is both large enough to heat your home and small enough to be energy efficient.
See The Homeowners Handbook to Energy Efficiency or Residential Energy: Cost Savings and Comfort for Existing Buildings Chapter 6 for more information. For professional information about heating systems and energy efficiency see Saturn Hydronic Systems Field Guide.