How a Gas Furnace Works to Keep You Warm

How a Gas Furnace Keep You Warm

When the temperature drops, you may look to your gas furnace to keep your home warm. Instead of wearing layers and layers of clothing inside your home, your central gas heating system was designed to increase the indoor temperature by warming the cooler indoor air.

The Mighty BTU


The heating capacity of a gas furnace is measured in British Thermal Units (BTUs). A BTU equals the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. The higher the BTU output, the more powerful the heating system.  

In real-world terms, the energy released by a single burning match is approximately equal to one BTU.1 A typical home may require thousands of BTUs during to keep you cozy and warm during the cold months. Ironically, the BTU is rarely used in Great Britain because it is a non-metric measurement. 


The Goldilocks Principle


No one like the uncomfortable feeling of being ‘too hot’ or ‘too cold!’ A central gas heating system should be able to provide a consistent amount of warmth to keep you comfortable inside your home. However, that means your gas furnace must be sized correctly with the appropriate amount of BTUs for your home. A “just right” size furnace can provide the precise balance of comfort and cost-efficiency. To establish this precision, it’s important to have a licensed professional HVAC technician calculate the right size gas furnace for your home.

If your furnace is sized too small, it might not be able to keep up with the demand on cold days, leaving you with a consistent chill in the house. Depending on the indoor vs. outdoor temperature difference, an undersized may have to run continuously to try to maintain your thermostat setting. Over time, this strain can diminish its performance, potentially increasing utility bills and resulting in unnecessary wear and tear on critical components.

An oversized gas furnace can create bursts of warm air. If your furnace is too large for your home, it will heat rooms very quickly and then shut off. This rush of heated air can trick thermostats into shutting off the system before the whole house is at temperature.

This can leave you reaching for a sweater in between cycles! Repeatedly turning on and off can be hard on your furnace, potentially reducing its lifespan.

To make sure your gas furnace is sized properly, contact your licensed professional HVAC technician. 


How a Central Gas Furnace Works


Simply put, a central gas heating system creates a cycle of warming cooler air. Here is the simple version:
  1. Propane or natural gas generates controlled heat in the furnace's burner.
  2. The heat produced passes through a heat exchanger, making it hot.
  3. Air from the home's ductwork is blown over the heat exchanger, warming the air.
  4. The furnace's blower then forces the heated air into the supply ductwork, distributing it throughout the home.
Of course, there are many system components must work together to keep you cozy and comfortable inside your home.
Temperature Control: The temperature control, which is regulated by the furnace control board, turns on the ignition switch. This starts the heating process when the thermostat or control system calls for heat.
Ignition switch: Gas flows over the igniter to establish a flame. This flame is drawn through the burners and used to heat the heat exchanger.
Draft Induced Fan: The draft induced fan draws air into the burner assembly. The air also allows the burners to warm the heat exchanger.
Gas Burners:  When the thermostat or control system calls for heat, the gas burner valves are open to deliver gas and burn fuel.
Heat exchanger: The part of a gas furnace that adds heat to the indoor air. The gas combusts inside the heat exchanger, creating heat that is used to heat the passing air. The design of the heat exchanger can add energy efficient operation of a gas furnace.
Blower Fan: Uses the return venting to blow air over the hot heat exchanger.  The conditioned air is then sent throughout your home via ductwork. Some furnace models offer a blower fan that can run at multiple speeds to improve efficiency.
Flue: A flue or chimney acts as an exhaust for gaseous by-products of combustion used to create heat. 

Gas Furnace Options


Gas furnaces come in a variety of shapes to fit your space. However, they can also be categorized by one of the following:
  • Non-condensing furnaces - vent exhaust gases out of the home, typically through the roof.
  • Condensing furnaces - uses a second heat exchanger to heat the air from condensed exhaust gases to reach higher efficiencies.
  • A modulating gas furnace - continuously regulates the amount of fuel burned to maintain the set temperature of your thermostat. This modulating component can minimize indoor temperature fluctuations.

Amana® ENERGY STAR™ Gas Furnaces
1 Energy Explained. (n.d.). Retrieved from U.S. Energy Information Administration: