Modulating gas furnaces are very high efficiency furnaces, usually 95% and higher. These models also use computer technology along with special gas valves to control the burner output. The burner levels on these models operate at varying levels hence the term “modulating”. When the room thermostat calls for heat, the furnace fires and starts operating at a “medium” level. The furnace operates at this level for a short period and as the temperature gets closer to the set point, the furnace “modulates” to the lowest level until the thermostat is satisfied. In cases where the thermostat is set significantly higher than the room temperature or the heat demand is high, the furnace will operate on the highest stage in order to warm the home up as fast as possible.
Two-stage gas furnaces are available in both 80% and 90% efficiencies. However, two-stage furnaces use a microprocessor along with a two-stage gas valve to control the burners at a high or low rate, depending on the heat demand. When the outside temperatures are moderate, the burners operate at a lower, more efficient rate. If the outside temperatures are very cold, then the furnace will operate at the higher rate in order to keep up with the heating demand.
Single-stage gas furnaces are available in both 80% and 90% efficiencies. The term “single-stage” means simply that the furnace’s burners fire at one fixed heating level. When the thermostat tells the furnace to heat, the gas valve opens and the burner flame ignites. When the thermostat is satisfied, the burners shut-off.