A Guide to Troubleshooting Your Gas Furnace

gas furnace repair

No troubleshooting guide can cover every cause and solution for every gas furnace in existence. However, understanding the basic way a gas furnace works is a good start to diagnosing your furnace problems.

The first step in troubleshooting your gas furnace is to observe exactly what your furnace is doing – or not doing – and narrow down the possible causes. Often, you will find you don’t need a new furnace after all. Sometimes the simplest things can cause the biggest problems.

Inside a Gas Furnace

It starts with your thermostat. Your house is cold, the furnace needs to heat the house, and the thermostat begins the process. In general, your heating system follows a basic pattern:

  • The thermostat senses the house temperature and activates internal contacts that close the electrical circuit.
  • Closing the circuit signals the furnace to start.
  • Ignition begins. Older furnaces may use a standing pilot light while newer furnaces use an electronic ignition.
  • The gas flows to the main burners to heat up the furnace. Once it reaches the correct temperature, the furnace blower fan comes on, forcing air through the heating ducts and out the hot air vents, called registers.
  • The thermostat responds to the warmer temperature by opening the contacts, breaking the circuit. The fan will continue to run for a brief period of time – perhaps two or three minutes – to dissipate the heat remaining inside the furnace chamber.
  • The fan stops, the warm air ceases to blow and the thermostat waits for the temperature to drop again.

Of course, this is an oversimplification. To break down the complete process and explain every single part of the system would very complicated, and the repairs would be beyond the capability of the average DIYer. In addition, newer- and older-style furnaces operate slightly differently according to the type. At heart, each works in the same general way, however. Understanding the basics will help you identify the simple problems and solutions, possibly keeping more money in your pocket instead of going to furnace repair technicians.

Where to Start When Your Furnace Just Doesn’t Work

Sometimes your furnace doesn’t have any symptoms to clue you in to what is wrong. No smoke, no funny sounds or sparks. The furnace simply does nothing at all. It’s perhaps more frustrating than any other problem since other symptoms at least offer clues about what’s wrong. Quite often, however, the cause is something fairly simple. Troubleshoot the furnace by starting with the simplest causes and progress to more complex problems.

  • Check that your thermostat is set to “Heat.”
  • Is the thermostat’s temperature setting higher than the room temperature? Either double check the room temperature with a thermometer, or raise the temperature on the thermostat until you hear a click. If the room temperature is several degrees below the thermostat (allow a few degrees for thermostat inaccuracy) or the thermostat never clicks, continue troubleshooting the furnace.
  • Troubleshoot your thermostat. If you have a digital programmable thermostat, thermostat failure may be more obvious – the display is blank or flashing a code. Even older manual thermostats, however, can go bad or get too dirty to work. Troubleshooting your thermostat will likely take only a few minutes, so it’s a smart thing to check before admitting defeat and calling a furnace repair company.
  • Turn the fan switch on the thermostat to the “ON” position to check for power to the furnace. If it doesn’t operate, suspect either the furnace isn’t getting power or the fan motor, inside the furnace, is malfunctioning. If the fan isn’t working, heat will not come out of the heat registers, possibly making you think the furnace isn’t working even though it is.
  • Look for the SSU switch – formally called the Switch Supervisory Unit. Typically mounted on the wall near the furnace, the SSU looks like a bulky gray light switch box and features an “ON” and “OFF” switch. Check that the switch is turned on before proceeding.
  • Check your home’s breaker box to ensure the breaker that controls the furnace is on, or check the fuse box to make sure the fuse isn’t blown. Breakers, in particular, may react to a power surge by flipping the breaker to halfway between “on” and “off.” It’s obvious when this happens, as one breaker will be out of alignment with the others.
  • Flip the breaker off, wait two or three minutes and flip it back on again. Similar to a computer, sometimes simply resetting the power solves your problem.
  • Look at the pilot light. Is it lit? A standing pilot light should be visible when you open the combustion chamber door. If the pilot light is out, it must be re-lit, following the furnace lighting instructions precisely and observing appropriate safety procedures, such as turning the gas valve off and allowing the air to clear for several minutes. Have a professional light your furnace for you if you are not familiar with the procedure. If the furnace refuses to stay lit, consult a furnace repair technician.
  • Check the furnace gas valve to determine it is open. The valve will be located along the side of the unit, clearly marked, and forms part of the ignition system.

If, after checking every part and still not identifying why your furnace will not come on, consider calling a professional. HVAC service companies have special equipment and a deeper knowledge of your particular furnace’s inner workings.

Investigating Your Furnace by Problem or Symptom

Sometimes your furnace works, but it isn’t working properly. Troubleshooting the thermostat, power or gas supply is a waste of time – you need to jump straight to the problem. Take a look at exactly what is happening and, with a general understanding of how a furnace works, you can often at least narrow down the possible causes.

  • The furnace cycles on and off too often: Figuring out the cause of an overworking furnace may require a little investigation. If your home is poorly insulated and drafty, for instance, your furnace will work more. Another cause is an over- or undersized unit, which will also affect your utility bill. If nothing has changed except the frequency with which your furnace comes on, however, check the thermostat to see if it is accurate and clean. A dirty air filter can also cause the furnace to overwork, so check and change or clean it as necessary. Finally, the fan motor may have a problem. Consult a furnace technician for further assistance.
  • The furnace is extremely noisy: It may be annoying, but is it abnormal? Some furnaces are simply louder than others, with pings, roars and squeaks a sign of normal operation. Sudden and unusual sounds may indicate a problem, however. Many sounds are due to slipping fan motor belts or fan parts that need lubrication, as indicated by a high-pitched shriek or squeal. A poorly adjusted standing pilot light may emit a low, rumbling sound when the burners are off. With the burners on, the same sound may indicate dirty gas burners. If you hear a grinding noise, it typically indicates the blower’s bearings are shot and need replacement. Have a technician inspect the furnace and oil, adjust, clean or replace these parts, as necessary, to correct the problem.
  • Pilot light won’t light or stay lit: If the furnace is turned on, a standing pilot light will stay burning unless there’s a problem. It may be something as simple as the flame being blown out by a strong draft. Other possible causes include dirty ignition parts, air in the gas supply line or malfunctioning thermocouplers or flame sensors, both parts of the ignitor responsible for extinguishing the gas if your pilot light is out or will not light. Gas and electrical components are hazardous and you should never attempt to service the parts unless you know exactly what you are doing. Instead, turn off the gas supply and call a professional immediately.
    Once you complete your furnace troubleshooting and have your furnace working properly again, stay and observe it through one complete cycle. If the fan doesn’t operate as it should – not coming on, not going off, cycling too often – you may have a problem with the fan limit switch.

A useful diagnostic test at this point – or any time you suspect a problem with your gas furnace – is measuring the temperature rise across the furnace. Using as accurate of a thermometer as you have available, first find the temperature of the hot air in the heat register closest to your furnace, whether in the ceiling or floor, when the furnace is blowing heat. Insert the thermometer as far as possible and avoid touching the sides of the duct. Next, check the room temperature. Subtract the room temperature from the vent temperature to find the temperature rise. Compare your results with the temperature rise listed on the furnace data plate. While your temperature measurement is unlikely to be as accurate as one performed by a professional with sensitive equipment, an extreme difference between your temperature rise and the specified rise may indicate a problem. Possible causes include a dirty air filter, improperly adjusted burners, a fan issue or a problem with the fuel delivery. While replacing your furnace air filter is simple, most other furnace internal and electrical repairs are not as easy. Consult a service professional for further information.

Furnace Preventative Maintenance

Cleaning your air filter regularly will help prevent future furnace problems. Over time, your filter

collects large amounts of dust, hair, pollen, dead skin cells, nicotine if a smoker lives in your house, and other contaminants. As the filter clogs, the furnace must work harder and harder, ultimately lowering the efficiency and costing you more money. Most manufacturers recommend replacing your filter monthly, although pleated filters, considered high-capacity, may require replacement every two to three months. “Washable” filters, meant to be reused, may need servicing once or twice a year. Check your furnace filter requirements and change as recommended.

Once a year, inspect the furnace to ensure everything is operating correctly. Before the inspection, vacuum out and wipe down the area around the fan and completely across the outside of the unit. More advanced preventative measures include cleaning each fan blade with a toothbrush, if the fan slides out or is accessible, and vacuuming up the residue. Older furnaces may also benefit from a yearly motor port oiling. Just two or three drops of appliance-grade motor oil, added to the ports near the motor shaft, help keep the bearings and motor running smoothly. Consult a service professional for more information and assistance.

Even though it may seem unrelated, proper home insulation levels and winterizing measures you can do yourself impact your gas furnace as well. No matter how new or efficient your furnace, it simply won’t keep up in freezing temperatures when the home isn’t weatherproof. Another problem is leaky duct work or poorly insulated ducts running through unheated areas such as the attic or crawlspace. Consider consulting a home energy audit professional or HVAC repair company to identify trouble areas in your heating system and home. Remember, an ounce of prevention is worth a pound of cure!