01 of 09
The Conventional Gas Furnace
This tutorial covers the conventional gas furnace (up to 89% AFUE). For a tutorial that supplements this tutorial and covers issues unique to the high-efficiency gas furnace (90% AFUE and above), see Troubleshooting a High Efficiency Condensing Furnace.
The Conventional Gas Furnace
A gas furnace is an appliance looking like a large 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.
Furnaces come in different efficiencies measured in AFUE.
Once in a while, things don't work quite right and you need to troubleshoot a gas furnace repair or relight a standing pilot if it has one. Conventional furnaces may have electronic ignitions which need special troubleshooting.
High-efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main difference between a conventional and condensing furnace is the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these regards, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than a conventional furnace. Both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.
In any case, let's take a look at the more common problems and furnace repairs you may have to make with a conventional furnace.Continue to 2 of 9 below.
02 of 09
Gas Furnace Produces No Heat or Not Enough 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
Gas Furnace Comes 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, which is why it's important to address. From a faulty thermostat to a dirty filter, there are several possible causes to look into.Continue to 4 of 9 below.
04 of 09
Gas Furnace Blower Does Not Turn Off
- Thermostat set to fan continuous
- Faulty fan limit control switch on furnace (if thermostat has no fan setting)
Continue to 5 of 9 below.
- Change thermostat fan setting
- Reset or replace furnace fan limit control switch
05 of 09
Gas Furnace Has Noisy Operation
Don't let a noisy furnace in your home drive you crazy. Troubleshoot this problem now before more long-term issues crop up. From a low-pitched humming to a high-pitched squealing to a loud banging, different sounds mean different problems.Continue to 6 of 9 below.
06 of 09
Furnace Pilot is Out—Relighting a Standing Pilot
A pilot light can go out due to a strong draft, dirty orifice or dirt in the gas tube. Luckily, relighting your pilot light is fairly simple. The thermocouple may also be faulty and is shutting off the gas supply. If this is the case, the problem is a bit more involved and may require you to replace your thermocouple.Continue to 7 of 9 below.
07 of 09
Electronic Ignition Furnace Problems
Newer furnaces do not rely on a standing pilot to ignite the gas burners. Electronic ignition occurs typically in one of two ways: intermittent pilot or hot surface ignition.
The hot surface ignition system uses an electronically controlled resistance heating element not unlike a light bulb filament, to... ignite the gas burner. Learn how to fix the electronic ignition.Continue to 8 of 9 below.
08 of 09
Mismatching the Furnace and Thermostat
Furnaces and thermostats are not mix-and-match appliances. Using the wrong type of thermostat with a furnace will cause operating problems and can be dangerous. Although thermostats look similar, they are designed very differently. 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.Continue to 9 of 9 below.
09 of 09
Furnace Problems Caused by Thermostats
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'll want to check the thermostat. Problems can show up as a furnace that produces no heat, wild temperature swings or cycling on and off too often.