6 Common Furnace Problems: Here’s How to Fix Them

Nick P. Cellucci
Written by Nick P. Cellucci
Updated December 22, 2021
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  • Repairing common furnace issues can cost anywhere from $150–$300.

  • A qualified technician can best address most furnace problems.

  • Improve efficiency by regularly cleaning your furnace filter.

  • Have a pro inspect your furnace annually to prevent issues.

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The last thing any homeowner wants is to lose heat in the dead of winter. When it happens, you may be stricken with anxiety over the potential cost of repairs and the time you might be without heat. If your furnace malfunctions, it’s important to stay calm and figure out the best course of action. Here are six common furnace problems and what to do about them.

1. Your Furnace Won’t Turn On

If your furnace isn’t running, the first thing to check is whether your thermostat is on. If it won’t turn on, replace the batteries. 

If your thermostat is on and functioning normally, check if your furnace is plugged in with the access panel fully closed. If so, check to see if the furnace circuit breaker has tripped. If it’s off, flip it on; if it’s between off and on, flip it to off and then to on. If it stays on, the tripped breaker was possibly a fluke. However, if it trips again, you may have an electrical issue that a licensed electrician near you should address.

2. Your Furnace Blower Fan Won’t Shut Off

If your thermostat is set to auto (instead of on), your system should turn off once your home reaches the desired temperature. If your furnace never shuts off and you can hear the blower fan running continuously, you may have a faulty limit control switch.

This important safety control senses the air temperature and shuts your HVAC system down when it overheats. You’ll need to call an HVAC pro to repair or replace this small but critical part. This type of furnace repair costs $150 to $400.

3. Your Furnace Is Making Unusual Sounds

Some noise is expected, such as small clicks or the “whoosh” of warm air traveling through your ducts. Noises coming directly from your furnace, however, can indicate problems. To adequately address the noise your furnace is making, first identify the type of sound:

  • Grinding or scraping: Your ball bearings may have worn out. Turn off your furnace immediately.

  • Squealing or shrieking: This indicates a slipped or worn-out blower belt. If you’re confident in your ability to disconnect the motor wires and adjust or replace the belt on your own, this can be a DIY fix.

  • Knocking or rattling: A part may have come loose and needs to be tightened or adjusted.

  • Loud rumbling: This may be from fuel left over in the combustion chamber after your system has shut off.

Most of the issues behind these noises are best addressed by a professional, so save yourself time and hassle by calling a qualified local HVAC technician to diagnose and fix it for you properly.

4. Your Pilot Light Is Too Small or Won’t Stay Lit

A professional repairing a furnace
Photo: Tomasz Zajda / Adobe Stock

There are several possible causes for pilot light issues. You can attempt to adjust or relight it yourself, but keep in mind that natural gas is highly flammable and dangerous. First, turn off your gas supply for several minutes to allow built-up gas to dissipate. Carefully follow your furnace owner’s manual if you go the DIY route. Other causes for pilot light issues may include:

Faulty Flame Sensor

Also called a thermocouple, this safety device contains a metal rod signaling the furnace to shut down when the flame isn’t lighting. If you suspect that it’s dirty or malfunctioning, call a pro immediately to investigate the issue. Repairs may cost $150 to $300.

Clogged Burner

This is the part of your system emitting natural gas to light your pilot. Check the color of your pilot flame. A healthy burner produces blue flames, while yellow or orange ones indicate your burner is clogged with debris and may require professional cleaning. If the burner is clogged, you may also notice a booming or rumbling noise coming from your furnace. 

5. Your Filters Are Dirty

It’s important to replace furnace filters regularly as part of a routine inspection. Clogged air filters restrict airflow and make your furnace work harder. At best, you’ll end up paying more for heat with an inefficient furnace. And at worst, your system can overheat and shut down, leaving you without heat completely.

6. Your Heat Exchanger Is Cracked

You may not recognize this problem right away, but a cracked heat exchanger can be highly dangerous and has several telltale signs:

  • Soot on the inside of your furnace

  • A strong, unpleasant chemical odor

  • Water on the floor around the furnace

  • People in your home experiencing headaches, nausea, irritation, and flu-like symptoms

All these are signs your heat exchanger is leaking deadly carbon monoxide gas, which can cause carbon monoxide poisoning. If you suspect this problem, immediately evacuate your home and call an HVAC expert.

Preventive Furnace Maintenance

If you haven’t yet experienced any of these common issues with your furnace, you can avoid them altogether with preventive maintenance. Have a professional HVAC technician conduct an annual furnace inspection and service before cold weather hits. This helps keep your furnace running efficiently throughout the winter and saves you money on energy costs in the long run.

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