Gas furnaces that use a two-stage gas valve can improve energy efficiency and comfort when heating a home by efficiently regulating the flow of gas to the furnace to match demand. This improves efficiency by only delivering as much gas as the furnace needs instead of starting and stopping combustion.
Single-stage gas valves for furnaces have two positions: closed and open. Under mild conditions, a furnace using a single-stage valve must open and close the valve repeatedly to maintain temperature, starting and stopping the furnace each time. This leads to more wasted gas, less even heat and more electricity use for the blower and starter in furnace models that use electrical versions of those components.
In contrast, a two-stage valve has a low-heat setting and a high-heat setting. If less heat is needed to keep a particular temperature as set by the home's thermostat, the two-stage gas furnace can release only enough gas to keep the temperature with the low-heat setting. The furnace starts and stops less and maintains a more even temperature. A furnace with a two-stage valve can usually run in low-heat mode 90 percent of the day, while still maintaining a comfortable temperature and using much less gas than a furnace with a single-stage valve. Most furnaces with two-stage gas valves pair them with two-stage blower fans to reduce energy costs further when the furnace needs to produce less heat.