Common Furnace Problems and How to Troubleshoot Those Problems

We all love a furnace — after all, they keep us warm during those frigid winter months! But sometimes they can be temperamental, run inefficiently or the furnace stopped working altogether. Many of the common furnace problems can be fixed by yourself, without paying (and waiting) for a technician to come out.

In this article, we’ll go over the most common furnace problems and how to troubleshoot them yourself.

1. Dirty Filters
2. Pilot Light Problems
3. Electric Ignition Problems
4. Malfunctioning Thermostat
5. Furnace Stopped Working Altogether
6. The Furnace Doesn't Heat Enough
7. Frequent Cycling
8. Blower Runs Nonstop
9. Furnace Is Too Noisy

Common Furnace Problems and How to Fix Them

One of the most common furnace problems is a dirty air filter. A dirty filter will reduce airflow, making your furnace have to work harder to produce heat. Additionally, as dust bunnies and other allergens build up in the filter, they will be spread throughout your house, so a clean air filter does wonders for the air quality inside your home.

If you have a fiberglass filter, you should change it every two months or so, and if you have a paper filter, change it every 3-6 months. Electrostatic filters, if cleaned regularly, can last for years. Follow the manufacturer’s instructions, or ask a furnace repair professional how to change it.

Some of the most common furnace problems are related to the pilot light. If your pilot light has gone out, you can relight it according to the manufacturer’s instructions. If it keeps going out regularly, there might be a problem with your system and you should contact a professional.

Even if your pilot is lit, it could be burning inefficiently. If the flame isn’t blue, it means it’s not burning properly due to a lack of oxygen. There could be dirt built up on the intake valve, preventing enough oxygen from feeding the flame. Getting annual maintenance on your system and regularly changing the air filter can prevent dirt buildup. If you suspect your pilot is burning inefficiently or if the flame is yellow or orange, contact a professional.

Electric ignitions are a replacement for the pilot light that runs on electricity, and while there is no flame, they can have problems of their own, including limited heat, constant starting and stopping of heating cycles, or an overactive blower. These problems could be caused by an old igniter, a faulty temperature limiting switch, the wrong igniter, or power surges.

While most of these issues should be solved by a professional, before calling you can do some troubleshooting on your own. First, check the breakers and make sure they aren’t tripped or blown. If your furnace is automatically tripping the reset fuses, call a professional. You can also try turning your furnace off and on again. Turn off your thermostat, and make sure the power switch is in the ON position. Then turn off the furnace’s supply of power and gas, and wait for 10 minutes. Then find the ignition switch and turn your furnace back on.

Additionally, before calling a professional, replace the filter if you haven’t recently, and make sure the gas line valve is completely open. If none of the above steps work, it’s time to call in a pro to fix the problem!

If you’ve ruled out dirty filters and ignition or pilot light problems, another common furnace problem is the thermostat not functioning properly. If your furnace is not producing heat, is producing erratic amounts of heat, or if the furnace is cycling on and off more than usual, you’ll want to check the thermostat.

First, check that the power switch for the furnace is on, as this typically powers the thermostat as well. Next, try replacing the backup batteries on the thermostat. Then, look for a tripped circuit breaker or a blown fuse and reset or replace it.

If that still doesn’t solve the problem and you have a multi-tester, you can check if the transformer is functioning properly. The transformer is usually inside the furnace service panel, and there will be wires running from the transformer to the thermostat. Turn the circuit off before testing it, or before replacing it if it’s faulty.

If you’re still having trouble with the thermostat, it could be another issue caused by improper components, faulty wiring, or dust. Call a professional to come diagnose the problem.

A furnace not producing any heat is usually related to thermostat issues. Refer to the above section, about malfunctioning thermostats for troubleshooting steps. If troubleshooting your thermostat doesn’t work, you’ll need to call an HVAC professional.

A furnace producing heat, but not enough heat, is typically an air filter problem, specifically, a dirty or clogged air filter. Check your manual and make sure you’re switching out the air filter per manufacturer instructions on a regular basis.

Typically, issues with the blower indicates a problem with the limit switch. The limit switch is what tells the furnace to turn on and off depending on the temperature inside the furnace. It will turn off if the temperature inside the furnace becomes dangerous, so if you’re having blower issues, for safety reasons it’s best to call a professional right away to replace your limit switch.

If your furnace is making a lot of noise, that’s not normal and is probably a sign that something is wrong, possibly a mechanical problem, reduced airflow, or a clogged burner.

If your furnace is making a loud noise right as it starts up, this could be caused by delayed ignition, occurring when oil builds up in the chamber and is ignited all at once. Needless to say, this can be extremely dangerous and you should call a professional immediately.

Ratting and vibrations could be caused by loose ducts or an unstable furnace. Make sure your ducts are screwed down properly, and add duct tape or more screws to secure them. If the furnace is unstable, add some pads underneath to level it.

Whistling can be caused by a leak in your ducts or a clogged filter. First, replace the filter with a new one and see if the whistling stops. If it doesn’t stop, your air ducts could be too small for your furnace.

If you hear scraping, thumping, squealing, rumbling, or banging — or if you hear a noise and aren’t quite sure what adjective it can be described by, call an HVAC professional.

If you’re tired of furnace problems and you think it’s time to replace your system, visit our guide on how to select a home heating system.