A furnace produces heat by burning natural gas, propane, or fuel oil (gas furnaces dominate residential SC), or by passing electricity through resistance coils (electric furnaces, less common). A blower fan moves room air across the heat exchanger and through the duct system.
Modern gas furnaces are rated by AFUE (Annual Fuel Utilization Efficiency). High-efficiency condensing furnaces reach 95% AFUE — meaning 95% of fuel energy becomes heat, 5% goes up the flue.
In Charleston, gas furnaces are usually paired with an AC for summer (split system). They make sense where there's already a gas line and the homeowner prefers warmer supply-air temperature; otherwise a heat pump is typically the better choice for total cost of ownership.
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