The Design of the Gas Furnace
The original gas furnace consisted of a heat exchanger, burner, gas control valve, and an external thermostat, and there was no blower. Natural convection or forced air flow was used to circulate the air through large heating ducts and cold air returns to and from each room. This system was very inefficient—allowing over half of the heated air to escape up the chimney.
Today's gas furnace consists of a heat exchanger, secondary heat exchanger (depending on efficiency rating), air circulation blower, flue draft blower, gas control valve, burners, pilot light or spark ignition, electronic control circuitry, and an external thermostat. The modern furnace is highly efficient—80-90%, allowing only 10-20% of the heated air to escape up the chimney.
When heat is requested from the thermostat, the burners light and throw heat into the primary heat exchanger. The heated air then flows through the secondary heat exchanger (90% efficient furnace only) to the exhaust flue and chimney. The average furnace has three heat exchangers each producing 25,000 BTUs for a total of 75,000 BTUs. A flue draft blower is placed in the exhaust flue to supercharge the burners and increase efficiency. The heat exchangers perform two functions: transfer heated air from the burners to the home and allow dangerous exhaust gases to escape up the chimney. The transferred heated air is circulated throughout the home by a large blower and heating ducts.