Two Stage Gas Furnaces

Two-Stage Operation

• Two-stage furnaces change the rate at which natural gas is consumed by the furnace. This rate is known as BTU, or British thermal unit, which is the amount of energy needed to heat 1 lb. of water from 39 to 40 degrees Fahrenheit. These furnaces use a microprocessor in conjunction with a gas valve to control the amount of gas supplied to the burners. When exterior temperatures are mild, approximately 60 percent of burners ignite to supply the building with heat. The percentage climbs to 100 percent on frigid winter days when maximum heating is required.

Cycling

• The cycles at which single- and two-stage units operate are different. Single-stage units, forcefully delivering hot air at a high-rate of speed, cycle on and off frequently. Two-stage furnaces, especially when operating at lower capacity, run for longer periods of time because heat is delivered at a slower rate. This difference in operation results in lower electricity bills, but the savings in natural gas cost is negligible because of the longer burner time. The real advantage of two-stage gas heating is overall comfort through fewer temperature swings.

Two-Stage Motors

• Further augmenting benefits provided by two-stage furnaces are two-stage variable motors, which are optional on newer units. Two-stage blower technology works in a manner similar to two-stage heating by sensing when more air is needed to achieve comfortable indoor air temperature. These blowers are particularly efficient when used in conjunction with zoning, taking into account opened and closed dampers, duct work, dirty air filters and other variables that can affect the efficient delivery of heat. They also maintain a constant airflow despite a range of external static pressures in the ducts.

Other Advantages

• In addition to comfort, two-stage furnaces provide several other advantages. Quieter operation occurs with insulated blower compartments plus inducer systems that slowly ramp the unit up to speed. Two-stage units run primarily at lower capacity, which also means quieter operation. More consistent airflow occurs with two-stage furnaces, resulting in better filtration and ultimately better overall air quality as more particles are trapped and removed from the environment.