How Does a Gas Furnace Work?

A majority of homes today are heated with a gas furnace because furnaces are affordable, very efficient and easy to maintain and repair.  Here is a brief overview of how a gas furnace works.

When the temperature in the home drops below the temperature setting of the thermostat, the thermostat sends a signal to the furnace to generate heat.  A draft inducer motor starts – that’s the first thing you hear.  This motor gets air moving through the system and out the vent, to check that the vent is clear and that exhaust gases can leave the house.

After a few seconds, the gas valve will open, releasing gas to the burner where it is ignited by a glowing hot surface igniter.  All this happens in a sealed compartment known as the combustion chamber.  The heated air within the combustion chamber enters the heat exchanger, an essential part of the furnace that consists of a series of copper tubes. The heat exchanger typically gets hot enough to glow.  It functions something like a radiator because its job is to transfer heat from the combustion chamber to the air being circulated through the ductwork of your home.

The furnace blower comes on as the furnace is heating up.  Its job is to pull cold air into the furnace via the cold air returns and blow hot air back into the home through the ductwork in a continuous flow.  As the air is drawn into the furnace it passes over the heat exchanger and is heated before it circulates back into your home.  It exits the ductworks through heat grates in floors, walls or ceilings, providing warm air to the various rooms or zones of the house.  The combustion air leaves your home via the chimney, taking deadly carbon monoxide and carbon dioxide gas with it.

Standard efficiency gas furnaces range from about 80% to 90% efficient.  This mean that, in an 80% efficient furnace, 80% of the heat created passes into your home via the ductwork, while 20% of it is lost up the chimney.  Today’s most efficient furnaces can be as high as 97% efficient.  The way they achieve this very high efficiency is by adding a second heat exchanger.  Before passing out of your home the exhaust gases travel through this second heat exchanger where more heat is transferred into your home via the air that is passing over it.  The second heat exchanger is the primary reason that super-high efficiency furnaces cost significantly more than standard efficiency models.

The furnace continues to heat air and circulate it into your home until the air temperature sensed by the thermostat reaches the expected level.  Then the gas burner closes and the flame goes out.  The blower will continue to run for a short amount of time to blow the remaining heat in the furnace out and into your home.

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