8 Common Problems With Conventional Gas Furnaces

Repair Technician Removing Furnace Service Panel
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Conventional gas furnaces are those rated up to 89% AFUE (annual fuel utilization efficiency). These are often older furnaces that may have standing pilot lights and are usually noncondensing furnaces that vent exhaust gases through chimney flues rather than through PVC piping. High-efficiency condensing furnaces with AFUE ratings of at least 90% have become increasingly popular, and they require different troubleshooting techniques.

How Forced-Air Furnaces Work

A forced-air gas furnace, whether conventional or high-efficiency, is an appliance that does the following:

  • Takes in cold air
  • Cleans it with an air filter
  • Heats it up with a gas burner using a steel heat exchanger
  • Distributes the warm air with a blower motor through your home's ductwork

The heated air then cools down throughout your home and returns to the furnace through return air grills and ductwork. The cold returning air enters back through the air filter into the furnace to complete another heating loop. Sometimes there is a humidifier mounted on the furnace or the return air ductwork.

Conventional furnaces are generally simple in design, but occasionally things don't work quite right. Here are eight common problems you may encounter with a conventional gas furnace.

No Heat or Insufficient Heat

When cold weather hits, it is imperative that your furnace is working properly and producing enough heat to warm your home. If your furnace stops producing heat or is blowing cool air only, there are several possible culprits. You might be able to address some repairs on your own while others will require a professional. Easy potential fixes include:

  • Making sure the heat registers are open
  • Checking the thermostat settings
  • Verifying that the furnace has power

Furnace Frequently Cycles On and Off

Cut down on energy costs and prevent damage to your furnace by addressing a gas furnace that's frequently cycling on and off, or "short cycling." In very cold weather, this activity might be normal and necessary to keep your home at a comfortable temperature. But otherwise, it could signal a problem with your furnace. There are several possible causes to investigate, including a dirty filter, a blocked exhaust vent, or a faulty thermostat.

Blower Runs Constantly

If your furnace blower runs constantly, two likely reasons should be considered:

  • The thermostat is set to the FAN setting. This will cause the blower to run constantly. In a few instances, this might be desirable, such as when you want to filter the air or dry out a house that is too humid. But in most cases, it simply puts wear on the blower motor. The solution is to change the blower setting on the thermostat back to AUTO, which ensures the fan will run only when the system is heating or cooling.
  • The fan limit control switch might be faulty. The fan limit switch is a safety device that monitors the temperature inside the furnace and shuts off the components when set temperatures are reached. If the blower runs constantly, it may mean the limit switch is malfunctioning. To address this problem, reset the fan limit control switch. Or replace it if it's damaged.

Furnace Is Noisy

Excessive noise in your furnace almost always indicates a budding problem that can turn into an expensive repair if you ignore it. You might hear loud banging, high-pitched squealing, or just a low-pitched humming. Some problems you can fix yourself, but it might save you money in the long run if you get a professional in right away. Potential simple solutions include replacing the filter, as a clog could be making the noise, and oiling the blower motor.

Pilot Light Is Out

On old furnaces, the standing pilot light can go out due to a strong draft, a dirty orifice in the pilot burner, or dirt in the gas tube. Or the thermocouple might be faulty, causing the gas supply to shut off. The pilot light going out is one of the most common reasons that a gas furnace fails to produce heat. Luckily, relighting your pilot light is fairly simple. And it's also easy to replace the thermocouple if it is faulty.

Technician Cleaning A Natural Gas Furnance
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Electronic Ignition Is Failing

Newer conventional furnaces don't rely on a standing pilot to ignite the gas burners. Instead, they make use of electronic components. Electronic ignition occurs typically in one of two ways: intermittent pilot or hot-surface ignition. ​​The intermittent pilot system uses an electronically controlled high-voltage electrical spark to ignite the gas pilot, which subsequently lights the main burners when the thermostat calls for heat. A hot-surface ignition system uses an electronically controlled resistance heating element, not unlike a light bulb filament, to ignite the gas burner.

If the electronic ignition has a problem, your furnace might produce little or no heat, it might cycle frequently, or you might have an overactive blower. Fortunately, problems with electronic ignitions sometimes can be fixed yourself. You can try replacing the filter and checking the power and gas line to your furnace. If simple fixes don't do it, it's probably time to call in a pro.

Thermostat and Furnace Are Mismatched

Furnaces and thermostats must be correctly matched to one another in order to function correctly. Pairing the wrong type of thermostat with a furnace will cause operating problems. For instance, your furnace might overheat or underheat your home in spite of how you've set your thermostat. There are three types of thermostat systems used today: millivolt, low voltage, and line voltage.​ Make sure you have installed the correct type for your furnace.

Thermostat Is Faulty

Some common symptoms exhibited by your furnace might actually be due to a faulty thermostat. Once you've ruled out the common culprits with the furnace itself, such as a dirty filter, check the thermostat. Problems with a thermostat can manifest as a furnace that produces no heat, wild temperature swings, or a furnace cycling on and off too frequently. Make sure there is power to your thermostat and that there is no dust or debris in its components. If it's still not working, consult with a professional.

Article Sources
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  1. Furnaces and Boilers. U.S. Department of Energy, Office of Energy Saver.