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You are here: Additional Info » Warm-Air Gas Furnaces

Warm-Air Gas Furnaces

In a forced warm-air system, a blower fan pulls air from various rooms throughout the house into the cold-air return duct. The air passes through a filter and into the furnace where it is heated. The warm air is then forced into the rooms though ducts and registers that are located adjacent to exterior walls near windows or doors.

Maintenance of Forced-Air Components:

  • Clean or replace the air filter monthly during the heating season
  • Clean the fan blower blades at the start of each season
  • Check the belt for wear, alignment and tension (1/2” to 3/4” deflection is normal)
  • Examine the ducts for leaks and seal with duct tape
  • Keep cold-air returns clear of blockage by furniture
  • Check auxiliary components (humidifier or central air conditioner) for leaks into the unit

Gas burners are usually associated with warm-air furnaces but could also fuel hot water or steam heating systems. When the thermostat calls for heat, the gas valve opens, allowing gas to flow into a manifold and then into venturi tubes where it mixes with air. The mixture is ignited by an electric spark or constant pilot light. The heat exchanger separates the circulated warm air and exhaust gases. (Check these for cracks every few years.) The thermocouple closes the gas valve if the pilot light is not working.
Conventional furnaces have a 60-70% efficiency and draft naturally to exhaust gases up the chimney. Mid-efficiency units achieve a 75-85% efficiency by incorporating an induced draft fan. High-efficiency furnaces achieve a 90-97% efficiency due to a secondary heat exchanger that extracts heat from the exhaust gases, which would normally flow up the chimney.

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