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Two-stage furnaces, although little or no more efficient than single-stage furnaces, reduce indoor temperature fluctuations and improve comfort in many homes. Two-stage furnaces accomplish this by utilizing a low-fire and high-fire position on their gas valves. Two-stage furnaces also have multi-speed or variable-speed furnace blowers to move more or less air through the ducts and two-stage draft fans to move more or less combustion gases up the chimney. During mild weather these furnaces operate on their low-fire stage, and with lower airflow in the ducts and less chimney gas flow. During cold weather they use their second high-fire stage and higher airflow and chimney gas flow.
A properly-sized furnace provides just enough heat for your home during the coldest day of the winter. During mild weather, a conventional furnace must cycle on and off frequently to maintain a comfortable temperature. This on-and-off operation causes the indoor temperature to fluctuate slightly, and reduces the efficiency of the furnace.
Two-stage furnaces work best in small to medium sized homes with simple floor plans. They also work well in larger, more complicated homes with zoned duct systems. Zoned duct systems have separate thermostats that control different parts of the house by adjusting airflow dampers in the ducts. Two stage furnaces may not work so well in large complicated homes with large duct systems, since the first stage of heat may not have an adequate blower speed to push air to all parts of the home.
If you plan to replace your heating system, be sure to ask your heating contractor about two-stage furnaces. We suggest upgrading insulation and improving air-tightness before replacing your heating or cooling system. The keys to efficient two-stage furnace operation are a good initial design followed up by a high-quality installation. The two-stage furnace’s high stage should be sized for the severest weather, and the ducts should be designed and sealed to give good airflow for a range of weather conditions.
See The Homeowners Handbook to Energy Efficiency or Residential Energy: Cost Savings and Comfort for Existing BuildingsChapter 6 for more information. For professional information about heating systems and energy efficiency see Saturn Hydronic Systems Field Guide.