Furnace troubleshooting advice.

22 Jan, 2021

There’s nothing more frustrating than waking up to an icy house and realizing that instead of blowing in warmth, your heating system is pumping your home full of cold air or—worse—no air at all!

If you discover your furnace is not working, don’t panic. You can resolve many furnace malfunctions with some straightforward furnace troubleshooting

Before contacting a repair professional, read on for our step-by-step guide to gas furnace troubleshooting and DIY solutions.

Furnace Not Working: Now What?

Modern furnaces are designed for reliability, but sometimes, they break down. To keep your furnace functioning properly, you should perform regular maintenance. However, even a properly cared for heating system can leave you in the cold. 

If you find your furnace not working correctly, you can take a few steps to resolve the problem yourself before calling in a professional. You’ll need to know what kind of heating system you have and have a general idea of how it works. Then, you can determine what you’ll need to fix based on its symptoms. Once you’ve figured out the problem, you can try a simple DIY repair to get your furnace up and running again.

Before you start, however, take a minute to inspect your thermostat to ensure that it is set on “Heat” (or “Auto”) and the temperature is set at least five degrees higher than the room temperature. If your thermostat runs on batteries, check that they are charged; if it is wired into your home’s electrical system, check the breaker and fuse. Finally, on a programmable thermostat, make sure the date and time are correct, as that may affect the furnace timer. 

If your thermostat is working correctly, it’s time to move on to the furnace. If you have a forced-air heating system run by a conventional or high-efficiency furnace, check for these common furnace problems:

Furnace Troubleshooting

1. Furnace Not Turning On 

If you’ve checked your thermostat and your furnace still isn’t working, you may have to power it back up. Here are a few common power-related problems:

  • The furnace power switch is off. The power switch, located on or near the furnace, looks like a common light switch. If it's been accidentally turned off, you simply need to flip it back on. The furnace might have a short delay before starting, so wait a few minutes before moving on.
  • The circuit breaker is off. Switch it back on. If the circuit continues to trip after you turn it back on, there might be a short in the furnace wiring that you’ll need to get serviced by a professional.
  • Dirty filters often cause the furnace to shut off. Check the filter, then clean or replace it if need be.  

2. Furnace Not Heating

When your furnace turns on but blows cold air, check for these issues:

  • The gas is turned off. Check the gas valve on the gas line of your furnace. If it is open, the valve should be parallel to the gas line. If the valve is perpendicular (off), your gas isn’t getting to the furnace. 
  • The fresh-air intake is blocked by snow or debris. Clear the area in and near the intake vent.
  • Other causes include a blocked gas control valve, a failing ignitor, or a malfunctioning draft motor. Any of these three issues require part replacement or professional diagnosis.

3. Furnace Switches Off

Does your furnace switch off before reaching the temperature setting on the thermostat? You may have an issue with the limit switch. Here are some furnace limit switch troubleshooting steps:

  • Clean any dirt or grime from the fan limit switch. Build-up can inhibit the sensor from operating correctly. 
  • Check the furnace filter to confirm it isn’t clogged. A clogged air filter may overheat, causing the furnace limit switch to turn off.
  • If your furnace is continuously activating and deactivating throughout the day, your blower motor may be to blame. If the blower assembly overheats, it can trip the furnace limit switch. 
  • Other common issues that may trip the furnace fan limit switch include malfunctioning gas valves, thermostats, and circuits. 

4. Draft Motor Runs, But Furnace Shuts Down

Maybe you can hear the sound of the draft motor (also called the inducer motor), but the furnace doesn’t turn on. Possible causes include:

  • A blocked intake or exhaust vent. Clear snow, debris, or bird and rodent nests from the vent or chimney.
  • You may be dealing with a failed motor control board, gas valve, or ignitor. These issues require part repair or replacement.

5. Furnace Clicks On But Doesn’t Fire Up

If your furnace doesn’t fire up after clicking on or initiating the start cycle, you may have a faulty or dirty ignitor or sensor. 

  • Shut off the power and gas to your furnace, remove the ignitor or sensor (refer to your user’s manual to see where it is located), and carefully clean the probe with medium-grain sandpaper. Replace the part, restart the power and gas, and turn up the thermostat temperature to see if the furnace fires up.
  • If the furnace still doesn’t fire up, you may need to replace the ignitor or sensor. 

6. Furnace Runs But Can’t Heat Home Sufficiently

Everything seems to be in working order, but you find your furnace not heating your home enough? Check for these issues:

  • A dirty or clogged filter. Clean or replace.
  • The wrong filter. If your filter is the wrong type for your furnace, it may restrict airflow, and you’ll need to replace it.
  • An incorrectly installed filter also may restrict airflow. Look for the arrows on the filter frame that indicate proper installation direction and adjust it as necessary.
  • Leaking air ducts. If you hear whistling noises, you may need to seal your air ducts.

7. Noisy Furnace

Rattling, rumbling, and whistling may be caused by:

  • A dirty or clogged filter. Clean or replace.
  • Loose panels in the furnace cabinet. Tighten panel screws.
  • Leaky ductwork near the furnace that must be tightened or sealed.
  • A faulty bearing in the draft motor or the blower motor that must be replaced.

Gas Furnace Troubleshooting

If you find your gas heater not working even after you’ve checked the thermostat, here are some specific tips for troubleshooting gas furnaces:

1. Pilot Light Out

If you discover your gas heater is not working, check that the pilot light is lit. Follow the instructions in your user manual to relight the pilot light.

2. Pilot Light Won’t Stay Lit

If your pilot light shuts off, you may have a clogged pilot opening. Turn off the power and gas and poke a thin wire into the opening to free it of any debris. Otherwise, your pilot flame may be set too low. Adjust the level by turning the flame adjustment screw until you achieve a steady 1.5- to 2-inch flame with no yellow in it. If the pilot still shuts off, you may have to replace a faulty thermocouple or safety cutoff valve. 

3. Faulty Burner Flames

Your burner flames should burn blue and evenly. If you notice yellow or uneven flames, your burners are likely dirty. Turn off the power and gas and clear out all dust and debris with a vacuum cleaner.

Furnace Problems: When to Seek the Help of a Professional

Though several simple problems result in your furnace not heating, more complicated issues require a repair professional. 

If you suspect a gas leak or smell the odor of gas in your home, exit immediately (leaving the front door open, if possible) and call an experienced professional for emergency repair. 

You should also let a professional handle complex repairs and replacements, such as issues with the furnace’s primary circuit or control board, draft and blower motors, gas valves, limit switches, and thermocouples. An experienced contractor will know how to choose and install the correct replacement parts, so your furnace runs efficiently and safely.

Choose Valley Service

If you haven’t resolved the problem with these troubleshooting tips, contact Valley Service. Our expert team of HVAC service technicians have been servicing furnaces in the Fargo area for over 40 years. We’ll have your furnace up and running—and your home toasty—in no time!