Interstate Heating and Cooling
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Installs and Advice Articles

Installs and Advice

How Does a Forced Air Furnace Work?

Forced air furnaces are the most common way people heat their homes and come in many varieties.  Most run off of natural gas, propane, or fuel oil.  Although technology has advanced and improved forced air furnaces over the years, the basic concepts are still the same.



Air Flow

A furnace has a large blower, or fan, inside of it.  The furnace takes in cold air from your house through a cold air return vent, which can be identified by their size and lack of warm air blowing from them.  They then heat the air and blow it out of the top (or the bottom, if it is a counter flow furnace like a mobile home has), and into the supply trunk.  The supply trunk carries the air to the other smaller pipes which go to the different registers around your home.  Supply trunks tend to get smaller as you get farther from the furnace to maintain air pressure.


Heating Mechanism

To heat the air, the furnace must burn fuel.  However, the furnace can not simply shoot fire into the passing air, as carbon monoxide, a very dangerous gas that has no smell, is produced by the burning.  This is why furnaces have a heat exchange, which is a track made of metal where the heat and exhaust of the furnace travel through.  This makes the heat exchange extremely hot.  Your furnace then pushes the air from your home over the heat exchange, where it heats up without picking up any carbon monoxide.  The exhaust from the furnace is then let out a PVC pipe or a chimney.

Heat Exchange

Heat Exchange



When you think of controlling a furnace, you probably think of a thermostat, but there is a lot more going on behind the scenes.  A furnace is full of sensors that all hook up to a control board which monitors different factors to make sure it is safe for the furnace to run.  One of these is a flame sensor, which makes sure when fuel is going into the furnace it is actually burning, and not just letting gas into the house.  As long as the burners are lighting, the flame sensor lets the furnace know and it continues to run.  If no fire is detected when the furnace is "on", it promptly turns back off.  There is also the obvious controller, the thermostat.  This is a device with a mercury or digital temperature sensor.  When the temperature is below what you set the thermostat at, it completes a circuit with the 2 thermostat wires which tells the furnace it needs to turn on.  Assuming no sensors on the furnace are complaining, your home will begin to heat up!