7 Most Common Furnace Problems

Furnace issues can be frustrating, as it can be challenging to determine the cause of the problem. With so many parts, various issues can cause the furnace to underperform or break down. However, understanding common furnace problems can help you identify and fix the issues. Here are the seven most common furnace problems.

Blower Runs Constantly

Generally, when the burner turns off, it is not supposed to keep running. Several factors can cause the blower to run constantly, but one main reason is a problem with the control board. If the relays get stuck in a closed position, the blower gets a constant supply of electricity, which keeps it running. To resolve this issue, the control board may have to be replaced.

Short Cycling

It is normal for a furnace to cycle off and on a few times every hour, but if the cycling is more frequent, it can indicate a problem. This is called short cycling. When a furnace is short-cycling, it underperforms and consumes more energy. There can be several causes of this problem. For example, a clogged air filter, thermostat problems, and poor insulation can all lead to short cycling. You may require the help of a professional to diagnose the issue.

Inadequate Air Flow

Another common problem with a furnace is reduced or no air flow. This can be caused by a broken fan belt, but there could be other reasons. The fan belt can break when it does not have the right tension. This problem can also be caused by damaged ductwork, which allows air to escape.

Unusual Noises Coming from the Furnace

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If unusual sounds come from the furnace, they could indicate an underlying problem. For example, if there is a shrieking or squealing noise, it could be a sign of a damaged belt. Rumbling or rattling sounds typically indicate that something is loose inside the furnace. Although most furnaces create a slight humming sound during operation, if the humming is loud, it suggests an issue with the capacitor or the blower fan. In that case, you may need to get a Houston furnace repair service to fix the problem.

Furnace Blowing Cold Air

Another common problem is the furnace blowing cold air. Although there can be several causes, one cause can be a dirty flame sensor. When the sensor is dirty and fails to detect the flame’s heat, it can disrupt the heating cycle. The flame sensor can be cleaned with a cloth to restore the proper functioning of the furnace. Other reasons why a furnace blows cold air can include dirty air filters, clogged condensate lines, inadequate power or gas supply, or incorrect fan settings.

Odd Smells From the Furnace

If odd smells are coming from the furnace, it could indicate that the heat exchanger is damaged. Gas leaks, mold, and contamination in the vent can also cause the furnace to produce odd smells. If there is a burning smell coming from the furnace, it could indicate that it is overheating. Ignoring such problems could lead to a major safety hazard.

The Furnace Won’t Switch On

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The furnace not turning on is another common problem. It could be a result of different issues, like a problem with the circuit board. An issue with the wiring inside the furnace can also cause the circuit breaker to trip even after resetting it.

Cycling On and Off Frequently

Frequent cycling, also known as short cycling, can be frustrating and inefficient. It often indicates an underlying problem within the furnace. Possible causes include a malfunctioning thermostat, restricted airflow, or a faulty heat exchanger. To address this issue, check if the air filter is clean and replace it if necessary. Ensure that the thermostat is set to the correct temperature and placed away from heat sources or drafts. If the problem persists, it is advisable to contact a professional HVAC technician for a thorough inspection.

High Energy Bills

Surprisingly high energy bills can be a sign of an inefficient furnace. Factors such as poor insulation, leaky ducts, or a piece in need of maintenance can contribute to excessive energy consumption. Begin by inspecting the ductwork for any leaks or gaps and seal them properly. Consider improving insulation in your home to retain heat more effectively. Regular maintenance, including cleaning and tuning up your furnace, can also optimize its efficiency and reduce energy costs.

Cracked Heat Exchanger

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A cracked heat exchanger poses a serious risk, as it can lead to carbon monoxide leaks. Carbon monoxide is a colorless and odorless gas that can be harmful or even fatal. If you suspect a cracked heat exchanger, take immediate action. Look for signs such as soot or rust accumulation around the furnace, visible cracks, or a yellow or flickering pilot light. If you notice any of these signs, turn off your furnace immediately, evacuate the premises, and contact a professional HVAC technician for inspection and repairs.

Pilot Light Problems

The pilot light is responsible for igniting the burners in older gas types. If the pilot light keeps going out or does not stay lit, it could be due to a variety of issues. Common causes include a faulty thermocouple, a clogged pilot orifice, or a gas supply problem. Start by carefully following the manufacturer’s instructions for relighting the pilot light. If it continues to go out, a thermocouple replacement may be necessary. However, for safety reasons, it is recommended to consult a qualified HVAC technician to diagnose and resolve pilot light issues.


Maintaining a well-functioning furnace is vital to ensure warmth, comfort, and energy efficiency during the colder months. By understanding common heat problems and their troubleshooting methods, you can take proactive steps to address issues promptly. Remember to perform regular maintenance, including filter cleaning or replacement, and consider professional inspections to ensure the safety and efficiency of your furnace. If you encounter persistent or complex problems, don’t hesitate to reach out to a qualified HVAC technician for assistance. Stay warm, cozy, and worry-free with a properly functioning one that keeps your home comfortable throughout the winter season.