Advanced gas furnace designs and features will increase the options and benefits available to consumers. This capacity is used regardless of changes in weather conditions. Because the capacity is fixed, the furnace run time changes as the weather changes. In warmer weather, the furnace tends to run for short periods of time, but in very cold weather, the furnace runs for longer periods of time. The ON-OFF cycling of a furnace is controlled to limit the temperature swings that affect occupant comfort.
Several manufacturers offer furnaces capable of operating at two different capacities.
In addition to the full capacity necessary to handle the coldest weather conditions, these two-stage furnaces offer a reduced capacity stage that can be used in the milder conditions that predominate during most of the heating season. The reduced heating capacity may be between 50 and 75 percent of the maximum capacity, depending on the furnace design. Because most furnaces have some reserve capacity, a two-stage furnace will operate at its lower capacity most of the time. Low-speed operation has several advantages, including reduced temperature swings. Comfort can be improved by maintaining a more consistent temperature in the conditioned space.