9 Common Furnace Problems and How to Fix Them

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 nine common furnace problems and what to do about them.

1. Furnace Won’t Turn On

A common complaint about furnaces is them not turning on when they’re supposed to. If your furnace isn’t running or blowing air, it could be a problem with your thermostat, power supply, or circuit breaker. It could also signal a more serious issue with your home’s electrical system. 

How to Fix It

First, check whether your thermostat is on and replace the batteries if needed. If it’s on and functioning normally, check whether 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. Malfunctioning Thermostat

Your thermostat detects the temperature in your home and tells your furnace when to turn on and off based on your desired temperature. But sometimes, the thermostat can malfunction and show the wrong temperature. As a result, your furnace won’t come on when it’s meant to, or might continue running when it should have stopped.

How to Fix It

Check your thermostat by setting the temperature 10 to 15 degrees higher than the current temperature in your home. If the furnace doesn’t turn on, your thermostat is faulty. Fixing a faulty thermostat isn’t a DIY-friendly job, so call a local furnace repair professional to check your thermostat and repair or replace it.

3. Faulty Limit Control Switch

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. The limit control switch is a safety control that senses the air temperature and shuts your HVAC system down when it overheats.

How to Fix It

Repairing or replacing a furnace’s limit control switch is not a suitable DIY project. Instead, you’ll need to call an HVAC pro to repair or replace this small but critical part. This type of furnace repair costs around $150 to $300.

4. Unusual Sounds

Some noise coming from your furnace and HVAC system—such as small clicks or the “whoosh” of warm air traveling through your ducts—is expected. Noises coming directly from your furnace, however, can indicate problems. The exact problem depends on the type of noise your furnace makes.

• Grinding or scraping suggests your ball bearings have worn out. 

• Squealing or shrieking indicates a slipped or worn-out blower belt. 

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

• A loud rumbling may be from fuel left over in the combustion chamber after your system has shut off.

How to Fix It

Turn off your furnace immediately if you suspect worn-out ball bearings, and call a local HVAC technician to diagnose and fix it for you properly. Blower belt repair can be a DIY fix if you can confidently disconnect the motor wires and adjust or replace the belt. However, most of the issues behind these noises are best addressed by a professional, so save yourself time and hassle by calling a qualified pro to diagnose and fix your furnace.

5. Pilot Light Issues

There are two potential causes for pilot light issues. A faulty flame sensor, also called a thermocouple, is a safety device that contains a metal rod signaling the furnace to shut down when the flame isn’t lighting. The second potential cause is a clogged burner, 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. Another sign of a clogged burner is a booming or rumbling noise coming from your furnace.

How to Fix It

You can attempt to adjust or relight the pilot light yourself, but remember 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. 

If you suspect that the flame sensor is dirty or malfunctioning, call a pro immediately to investigate the issue. Repairs may cost around $150 to $300. It’s also best to leave cleaning a clogged burner to a professional.

6. Dirty or Old Filters

Replacing furnace filters regularly is an essential 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.

How to Fix It

Change your furnace filter at the intervals recommended by your furnace manufacturer. Some filters may need to be replaced every 90 days, while some will last up to a year. Others may require monthly or biannual replacement.

7. Cracked Heat Exchanger

You may not recognize this problem immediately, but a cracked heat exchanger can be highly dangerous. The biggest signs include soot inside your furnace, a strong chemical odor, and water on the floor around the furnace. People in your home may also experience headaches, nausea, irritation, and flu-like symptoms.

How to Fix It

A cracked heat exchanger can leak deadly carbon monoxide gas, causing carbon monoxide poisoning. If you suspect this problem, immediately evacuate your home and call an HVAC expert.

8. Unusual Odors

An odd smell emanating from your furnace can indicate potential problems needing prompt attention. Here are some distinctive odors that can signal furnace issues.

• A burning or metallic odor is often associated with overheated electrical parts, a malfunctioning blower motor, or a clogged air filter. Ignoring this smell could lead to further damage or even a fire hazard.

• A rotten egg or sulfur odor indicates a gas leak from your furnace. Turn off your furnace, ventilate the area, and evacuate your home immediately.

• A musty or moldy odor may indicate moisture infiltrating your furnace or ductwork, impacting air quality. A professional should inspect your system to identify and rectify issues.

• A burning dust odor happens when turning on the furnace after a while of inactivity. If the smell persists beyond a brief period, change the air filter.

Regular maintenance and prompt attention to odors will keep your furnace running safely and your home comfortable.

How to Fix It

Regular maintenance and prompt attention to odors will keep your furnace running safely and your home comfortable. If you notice unusual smells coming from your furnace, contact your gas utility company and a licensed HVAC technician, who can address the issue and ensure your safety. 

9. Tripped Circuit Breaker

Even furnaces that run on gas or oil rely on electricity. When a furnace encounters issues or malfunctions, it can overload the electrical circuit, causing the circuit breaker to trip. This is a safety measure to prevent fires and damage. A tripped breaker could indicate dirty air filters, a system malfunction, electrical issues, or an inadequate circuit supply. 

How to Fix It

Try changing your furnace filter to see if that fixes the problem. If it doesn’t, your furnace may have an electrical issue. DIY electrical fixes can be extremely dangerous, so it’s best to contact a licensed electrician to diagnose and fix the problem. An electrician can also upgrade your circuit to handle your home’s electrical demands adequately.

When to Hire a Pro

Fixes for some common furnace problems, like changing a filter or replacing the thermostat batteries, are simple and generally don’t need a pro. However, more complex issues, especially those that require electrical work or furnace part replacement, are best left to an experienced HVAC professional.

A pro will first inspect your furnace to determine the cause of the problem. Then, they will make a recommendation for repair—or, in some cases, replacement. If the fix is expensive, it’s smart to shop around and get several quotes from HVAC repair companies before moving forward.

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.