How a Gas Furnace Works


forcedairheating_intro©Don Vandervort, HomeTips

Gas Furnace Diagram

This illustrated guide diagrams the various parts of a gas furnace and explains how they all work together.

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A gas forced-air heating system goes into action when the thermostat tells it that the room temperature has dropped below a preset comfort level. The thermostat sends a low-voltage electrical signal to a relay in the furnace, which signals a valve to open and deliver natural gas to the burners and for the blower to turn on.

The furnace’s pilot light or electronic ignition lights the burner inside the combustion chamber. This creates heat in the furnace’s heat exchanger, a metal chamber around which the moving air flows.

Once warmed, the air is pushed into the hot-air plenum and then out to the rooms through duct work. The combustion gases created by burning fuel are vented through a flue in the roof or, with high-efficiency furnaces, through a wall.

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One of the benefits of a forced-air system is that it can receive a whole-house air-conditioning unit, a humidifier, and an electronic air filter—all of which can take advantage of the furnace’s air handler and ducts for delivery of conditioned air to rooms. Duct work is generally metal wrapped with insulation or a flexible plastic-wrapped insulated material that helps retain heat.

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  • Jim

    I have a 90% efficient forced air gas furnace and when it was installed 4 years ago the guy didn’t hook up a fresh air intake pipe allowing the furnace to draw in air from the hallway(slab home). My question is there are two holes for intake and I need to know if I should use one for the intake and block off the other or does it require two pipes? There is already a 2″ exhaust leading outside thru wall.

    • Tom

      Your contractor installed the system as an indirect vent unit. This is permissible with some models, and not with others. Look at your specific manual. This is currently using inside air for combustion; the same as with an atmospheric system. While it may be correct (if the manufacture allows), this also depressurizes the house. A worthwhile improvement would be to have the combustion air drawn from outside, which can be done in this case. It will require an intake air pipe. You should not cover the existing hole if you choose to leave it as is. This will starve the furnace from combustion air and it will not operate.

      • Don Vandervort, HomeTips

        Tom, thanks very much for answering Jim’s question. We love having pros like you helping out our visitors!

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