Lennox and Carrier furnaces are design to deliver reliable and economical warmth. They have efficiency ratings starting from 80% all the way up to 95+% AFUE. And many of them are energy star qualified, and rated, which means they can significantly lower your utility bills when compared to conventional models. For optimal comfort, reliability, safety and efficiency, a gas furnace can be combined with an electric heat pump in one dual-fuel system.
A home furnace is a one of the biggest appliances that is permanently installed in the basement, attic, crawl space, garage or closet of the house. It provides heat to an interior living space through intermediary fluid movement of air. The fuel source of today's furnace is natural gas, liquefied petroleum and heating oil.
There are electric furnaces and heat pumps that use electric source. Because, furnaces burn fuel they need and must be vented to the outside of the house to exhaust poison gases like carbon monoxide and other chemical gases. The venting is done through double wall or single wall piping generally routed through the roof for non-condensing furnaces. Other method for venting is through PVC piping for condensing furnaces which are most efficient. They extract additional heat from combustion byproducts through heat transfer in the secondary heat-exchanger. The condensed moisture drains out of the equipment to an approved drain location such as floor drain or condensate pump.
You can typically save 20% to 35% in utility costs, assuming the existing furnace is in the 60% AFUE range. Components of modern furnace are: the cabinet, heat-exchanger, burners, draft inducer motor, gas valve, circuit board, igniter, venting, safety devices like pressure, limit and flame rollout switches, and blower motor.
Order of furnace operation
Thermostat calls for heat. During a call for heat the circuit board first checks to make sure all safety devices are closed and operational. Next the control board energizes the induced draft motor which establishes a negative draft through the heat exchanger and out through the venting system insuring that there are no obstructions in the venting system that would cause an unsafe condition. Once this occurs the negative pressure air proving switch will close allowing the ignition sequence to start.
First the ignitor will be energized, and once it reaches a high enough temperature to ignite the fuel gas the gas control valve will be energized. When the gas control valve is energized it will open allowing the fuel gas to flow through the burners and ignite. The flame sensor will send a signal back to the control board that a flame is present allowing the gas control valve to remain energized and open until the thermostat ends the call for heat. After a pre-determined amount of time the circulating fan motor will be energized and allow the warm air to be circulated throughout the home until the thermostat ends the call for heat.