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Gas Heat Furnaces: An Overview

A home with natural gas hookups likely uses gas to power the furnace as well as the stove. In your HVAC career, you’ll encounter a fair number of gas heat furnaces that need repairs or maintenance. Since these heating systems work quite differently from electrical furnaces, it’s important to know how both should typically work and the crucial differences in their makeup and functions.

A gas heat furnace has its pros and cons compared to an electric heating system, as well as significant differences in energy efficiency and how the system works. A gas furnace also has the potential to be dangerous to the occupants of the home if not installed or repaired correctly. Before you start work on a gas furnace, keep yourself well-informed of what to expect and how to handle the machine.

Gas-powered furnaces need different types of work and repairs than electrical heating systems. Do you know the differences? Learn how gas furnaces work and how to repair them here. #TheTrainingCenterofAirConditioningandHeating Click To Tweet

How Gas Furnaces Work

Gas furnaces usually run on natural gas piped into the home from a utility company. If the home doesn’t have natural gas hookups, the fuel will likely be propane from specialized tanks. A propane provider will periodically visit the home to refill the tank and keep the heater running.

Propane typically enters a furnace gas valve at a maximum of 11 inches of water gauge pressure as measured by the proper manometer. Once the propane furnace is running, the actual manifold pressure is usually about 10 inches. Natural gas pressure tends to be about the same amount, averaging about 10.5 inches at the entry point. However, when the furnace is running, the manifold pressure of natural gas will drop to about 3.5 inches of water. Check the nameplate on the furnace to determine which pressure level you should be seeing.

If you take a look at a gas furnace, you’ll see the box is maybe 4-5 ft. long and contains a fan and the furnace itself. One end connects to the duct coming from the filter grill. The other end of the furnace connects to the AC evaporator coil. The fan pulls air from the air filter and, if the furnace is running, it warms the air below and sends it along the ductwork throughout the house. Since the heated air passes through the evaporator coil, it’s just as important to keep the AC coil clean in the winter as it is during the summer. A dirty evaporator coil can cause a furnace to overheat and pose a fire hazard.

A gas furnace and its accompanying flue pipe also requires open space around it (typically 30 inches) to keep them both away from potentially flammable objects. This is especially important for the flue pipe since, even with its double-walled material, it can still get very hot. As the exhaust pipe of the gas heater, the flue handles a lot of hot air. The pipe’s sealant must be sturdy to keep hazardous gases such as carbon monoxide from escaping, and the pipe must never be closer than 1 inch to wood or flammable materials or it poses a significant fire hazard.

A properly installed gas heater connects to the thermostat and waits for the signal to turn on. The thermostat receives 24 volts from the red T stat wire and sends the power to the furnace through a white wire, which activates the furnace. The furnace sparks the gas inside to create heat and begin warming the whole house.

Pro Tip: Propane and natural gas furnaces are designed for very specific and very different fuel sources. NEVER try to run a propane furnace with natural gas or vice versa! The differences in pressure will lead to severe problems.

Running a Gas Furnace

When installed correctly, a gas furnace runs like this:

  • The combustion fan motor, a special fan that pushes or pulls air through the fire box, activates for about 30 seconds. Any raw gas in the sparking area is flushed out through the roof. A small pressure switch connected to this fan tells the thermostat the fan is running. Once the thermostat gets the signal from the switch that the purge fan is on and has run for 30 seconds without a problem, the next step begins.
  • The thermostat checks that all the safety switches are in the proper open or closed positions. Located in several places on the furnaces, these switches detect overheating or other potential dangers. If these switches are in the proper position, the thermostat keeps moving forward.
  • The thermostat sends power to the hot surface igniter until it glows red hot. If you have a spark igniter furnace, this is when it starts sparking. The thermostat can even tell if the hot surface igniter is hot or not by measuring how much resistance it gives the electricity.
  • Now that the furnace has a spark or red hot igniter, it opens the gas valve and lets in a little gas. As the gas travels past either the sparker or the hot surface igniter, it lights.
  • Using a flame sensor, the thermostat verifies that the gas ignited safely.
  • The thermostat then activates the main gas and lights the main burners. Now the furnace is fully in heating mode with up to 100,000 BTUs of heat being created.
  • Finally, the thermostat turns on the main house fan and start blowing the heated air all around the house. Eventually, the temperature will reach the maximum specified by the homeowner and the thermostat will deactivate the heater. If the thermostat sense any problems or potential dangers, it will immediately deactivate both the heater and gas and potentially place both into lockout mode to prevent an accident.

Troubleshooting a Gas Furnace

One important step in troubleshooting a gas furnace is to pay attention to the circuit card, a computer card inside the thermostat. A diagnostic light on the card will flash at a certain rate to indicate the necessary repairs. Always take note of the flash rate in case you have to reset the light–this is valuable information that helps you determine the problem.

To diagnose the issue, try to ignite the furnace and watch each step as it happens. As soon as you notice the furnace faltering on a particular step, you’ve discovered the potential source of the problem. Work from there to find the answer.

Pay attention to the flue pipe as well. An older, less efficient furnace will likely have a flue pipe that tends to stay very hot and very dry, since about 20% of the generated heat was wasted through exhaust fumes. However, newer high-efficiency furnaces will waste far less heat (to the point that plastic piping is often safe to use for their flues) and thus may allow moisture to collect in the flue pipe. The furnace then collects and disposes of the moisture. These kinds of furnaces are called condensing furnaces. Check your manual and manufacturer’s recommendations to determine the proper type of flue pipe for each heater.

The length of the flue pipe is also important. Calculate the length of pipe extending outside the home based on the roof’s pitch and the manufacturer’s recommendation. If you overcompensate and make the extension far too long, you run the risk of the pipe breaking off in high winds and allowing deadly exhaust gases back into the home. Support the flue pipe very firmly and never make the exhaust portion too long. When the life of your client is in your hands, it’s better to be overly cautious than to just meet minimum standards.

A client’s air filter might also be a cause for concern. To see if the filter is causing the problem, watch the flame in the furnace. A normal furnace flame is bright blue. Any yellow flames indicate incomplete combustion caused by poor air flow through a dirty filter. This is a serious condition that you must address immediately. Until the flame is blue again, the problem is not resolved.

Finally, do you have all the tools necessary to work on gas furnaces? Check your HVAC supplies and make sure you own all of the following equipment:

  • Gas pressure tester
  • Combustion analyzer
  • 3 amp fuses

Above all, remember that while trial and error is typically a legitimate way to learn, it is drastically unsafe to experiment with gas heaters. Even the slightest mistake could lead to potentially deadly consequences for the homeowners. Never take a guess or try to figure out something unfamiliar on your own. Consult an expert or the manufacturer when necessary. An abundance of caution is far better than rushing a job and making a dangerous mistake.

Providing Excellent Work to your Clients

Working on a gas heat furnace presents its own set of challenges that an electrical system won’t have. Fortunately, with the right amount of study and hands-on practice with both kinds of heating systems, you’ll be well-prepared to help your clients keep their homes warm this winter.

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