A forced-air furnace has four main sections:
1. the blower chamber;
2. the combustion chamber;
3. the return duct; and
4. the supply duct.
When your thermostat calls for heat, the burners will kick on and begin to heat
up the heat exchanger. The heat exchanger contains all the dangerous gases
produced by combustion and vents them through the exhaust stack.
When the heat exchanger gets hot enough, the blower starts. The blower pulls
cooled air through the return duct, passes it over the warm heat exchanger and
returns the warmed air to the rooms.
Furnaces vary quite a bit in design, so yours may be somewhat different from
this illustration but the process is the same.
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