01 of 09
The Conventional Gas Furnace
Note: This article discusses common problems with conventional gas furnaces, those rated up to 89% AFUE (annual fuel utilization efficiency). These are often older furnaces that have standing pilot lights, and they are usually noncondensing furnaces that vent exhaust gases through chimneys rather than through PVC piping. Since 2012, high efficiency condensing furnaces with AFUE ratings of at least 90% have been increasingly popular, and they require different troubleshooting techniques.
How a Forced-Air Gas Furnace Works
A forced-air gas furnace, whether conventional or high-efficiency, is an appliance looking like a large metal box 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 in your home's various rooms 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 vs. High-Efficiency
While the basic functions of all forced-air furnaces are similar, it is important to know the difference between conventional furnaces and condensing, high-efficiency furnaces. Furnaces come in different efficiencies, measured in AFUE. Those below 90% AFUE are generally known as conventional furnaces, while those at 90% AFUE or above are considered high-efficiency condensing furnaces.
High-efficiency condensing furnaces are a bit more complex than conventional furnaces. The main differences are the heat exchanger technology used to extract heat from the combustion process, and the method used to exhaust the combustion gases. In these functions, the two furnace types are very different. The combustion process is actually quite similar in condensing furnaces and newer conventional furnaces— both use gas burners with electronic ignition. The real difference is that the condensing furnace has a more efficient heat extraction process after combustion. By wringing out virtually all the heat from the combustion gases, the remaining exhaust is quite cool, making it possible to vent it from the house through plastic PVC piping rather than a metal flue and chimney, as is needed with a conventional furnace.
Conventional furnaces are generally simpler in design, but occasionally things don't work quite right and you'll need to troubleshoot or relight a standing pilot if it has one. Some conventional furnaces may have electronic ignitions, which need special troubleshooting techniques.
Here are 8 common issues you may need to address with conventional furnaces.Continue to 2 of 9 below.
02 of 09
Conventional Furnace Produces No Heat or Insufficient Heat
Come fall and winter, 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, there are a number of possible culprits, starting with your thermostat. Find out how to fix the problem before you freeze.Continue to 3 of 9 below.
03 of 09
Furnace Cycles On and Off Too Frequently
Cut down on energy costs and prevent damage to your furnace by addressing a gas furnace that's overactive or "short cycling". Besides the need for consistently comfortable temperatures, this issue could cause long-term safety issues to your home. From a faulty thermostat to a dirty filter, there are several possible causes to investigate.Continue to 4 of 9 below.
04 of 09
Blower Does Not Turn Off
If your furnace blower runs constantly, two likely reasons should be considered:
Continue to 5 of 9 below.
- Thermostat is set to FAN setting, which 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 here 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 may be faulty. To address this problem, reset the fan limit control switch, or replace it if it is faulty.
05 of 09
Excessive noise in your furnace almost always indicates a budding problem that can turn into a long-term issue requiring expensive repairs. Troubleshoot this problem immediately, whether it involves loud banging, high-pitched squealing, or just a low-pitched humming. Some problems you can fix yourself, but even a professional service call will be cheaper if you address the problem quickly.Continue to 6 of 9 below.
06 of 09
Furnace Pilot Is Out
On older 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 may be faulty, causing the gas supply to shut off. Luckily, relighting your pilot light is fairly simple. And it's also easy to replace your thermocouple if it is faulty.Continue to 7 of 9 below.
07 of 09
Problems With Electronic Ignition
Newer conventional furnaces do not rely on a standing pilot to ignite the gas burners, but rather 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.
Many problems with electronic ignitions can be fixed yourself.Continue to 8 of 9 below.
08 of 09
Thermostat Mismatched to the Furnace
Furnaces and thermostats must be correctly matched to one another. Mismatching the wrong type of thermostat with a furnace will cause operating problems and can be dangerous. There are numerous types of heating systems and thermostat systems, and they need to be coordinated for safe and proper operation. There are three types of thermostat systems used today: millivoltage, low voltage, and line voltage. Make sure you have installed the correct type for your furnace.Continue to 9 of 9 below.
09 of 09
Some common symptoms exhibited by your furnace may actually be due to a faulty thermostat. After you have confirmed that the furnace is not the problem, you should check the thermostat. Problem with a thermostat can manifest as a furnace that produces no heat, wild temperature swings, or a furnace cycling on and off too frequently.