Gas Furnace

Overview

A gas furnace heats a building by burning fuel gas and transferring the released heat to a circulating air stream that is then ducted through the structure. This high-efficiency condensing model reaches about 96% AFUE by extracting latent heat from the flue gases, and modulates between two firing rates with a variable-speed blower for quieter, steadier comfort.

Its major subsystems are the Cabinet, the Gas Train that meters and burns fuel, the Heat Exchanger that recovers the heat, the Combustion System that moves combustion gases, the Circulating Blower Assembly that moves house air, and the Control System that sequences everything safely.

How it works

On a call for heat, the Control System starts the draft inducer in the Combustion System, which pulls combustion products through the furnace and proves airflow on a pressure switch. The integrated furnace control then energizes the silicon-nitride hot-surface igniter and opens the two-stage gas valve in the Gas Train. Gas metered through manifold orifices feeds a bank of in-shot burners, and a flame-rectification sensor confirms ignition.

Hot combustion gas flows inside the aluminized-steel primary clamshell of the Heat Exchanger, then through a stainless secondary coil where water vapor condenses, releasing its latent heat; the resulting condensate drains through a trap. House air is drawn across the outside of these surfaces by the Circulating Blower Assembly, an ECM-driven squirrel-cage blower that delivers conditioned air to the supply plenum after passing a Return Air Filter.

Spent gases vent through the sealed PVC Flue & Vent Kit, while high-limit and flame-rollout switches in the control system shut the furnace down on any over-temperature fault.