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A thermometer reads the temperature coming from a furnace

Are You Having Furnace Problems?

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Tim H.Verified

Lead Building Engineer

Residential and Commercial HVAC

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Types of Furnace Problems

Generally, furnace problems fall into three camps: thermostat malfunctioning, furnace not producing heat, and furnace not pushing out heat.

A furnace may look like a simple heating system, however, there are many components involved that could cause problems. A faulty gas furnace could lead to a carbon monoxide leak; whereas a malfunctioning electric furnace can put off a high-voltage shock. Some furnaces are used for heat in the winter and air conditioning in the summer. These will require more maintenance and upkeep as the furnace encounters heavier use. Knowing how a furnace works or operates can help you solve the problems you are having.

How a furnace works

Regardless of what fuel a furnace uses they all basically work in the same way. The furnace uses forced air to transfer heat through the ductwork, out the vents and then throughout the house. Propane and natural gas furnaces use a pilot light to ignite burners inside the combustion chamber. From there the heat exchanger holds the hot air until the temperature is at the desired degree selected on the thermostat. This triggers the blowers to turn on and send the hot air through the ductwork and into the house.

a diagram of how a furnace typically works

An electric furnace uses an electrical ignition and heating elements with conductive coils to begin the heating process. The temperature of the thermostat will depend on how many elements are activated. Once the desired temperature is reached, the blower forces air through the ductwork.

Common furnace problems

Most furnace problems are centered around not enough heat and ignition or blower problems. However, these issues are not usually noticed until it is time to turn on the heat. All types of furnaces can potentially have the same or similar issues as the ones listed below.

Dirty air filter

Clogged filters can cause the furnace to work harder and produce less heat. Air filters are found inside the return vents in your home. These returns might be on the ceiling, wall, or in the system itself. Check the filters and change them when they are dirty or at least twice a year. Changing a dirty air filter regularly will prove to be cost effective and save money over time.

Will not light

A thermocouple is used on a standing pilot furnace and controls the flow of gas from the gas valve. A faulty electronic ignition or a clogged thermocouple for the pilot light can prevent the furnace from heating.

A diagram of a furnace gas valve

Thermostat issues

Make sure the thermostat has new batteries. If it is hard-wired, check electrical connections for any bad spots or loose wires. A bad thermostat or dead batteries can keep the furnace blower from turning on.

Look for broken or cracked ductwork

Check the ductwork for any cracks or breaks. This can prevent hot air from entering the house or building. It can also cause the blower motor to constantly turn on and off.

Test for a bad limit switch

The limit switch turns the blower on and off when the desired temperature is reached. If the limit switch is bad, then the blower will continue running.

Listen for furnace noises

Mechanical problems with the furnace can cause it to be noisy or squeaky. This can be from a lack of maintenance. However not all noises are problematic, even though they can be quite annoying.

Furnace not providing enough heat

If the furnace is too small for the building, home or area it will not be able to produce enough heat. It will also have to run more often to try to reach the temperature the thermostat is set at.

Old furnace

An older furnace can begin to show its age by having cracked ductwork, creaks, and noises, poor air, and other mechanical problems due to age, wear and tear.

An old furnace in a basement

Furnace troubleshooting tips

Before calling an HVAC Expert or a repair person, there are some basic tasks you can perform yourself. Take all precautions necessary when performing any electric or gas furnace troubleshooting.

Check for power

Is the power switch on? This is one of the most common causes of a furnace not working. Most systems have a power switch (looks similar to a light switch) down near the unit. Sometimes this will get switched off.

Clogged or dirty air filter

Check for a dirty filter, if it is dirty, replace it. The air filters should be changed once a month while the furnace is in use. This should be done year-round when the furnace is also used for air conditioning.

Press the reset button after changing the air filters. You may have to do this a couple times.

Blowing cold air

Check the thermostat and ensure it is on the ‘heat’ setting. Set the thermostat 3 - 5 degrees above the actual room temperature.

No display on the thermostat

Make sure the thermostat batteries are new and that it is working when you turn it on and change the settings. If new batteries do not fix it, then turn the power off, and pull off the front section of the thermostat, and look at the wiring. Find a terminal marked R and one marked W and take the wires off, and twist the ends together, then power back on. If it now works, you have a bad thermostat and it will need replacement.

Blower fan problems

If the furnace will light, but not  blowing air, check for a blower wheel that is difficult to turn (indicating a bad motor) or on older furnaces a loose or broken belt. Correct these problems first.

If the blower is running constantly, check the thermostat and ensure it is on ‘Auto’ instead of on ‘On.’ If this is not the problem, then check the filters.

Furnace will not turn on

If your furnace has an ‘on/off’ switch, make sure it is in the ‘On’ position.

If the furnace is still not working, make sure it is plugged into the outlet. Check the circuit breaker to ensure that it has not tripped. If it has tripped, turn the breaker off, then on, and try again.

If it is a newer furnace, there is a small motor near the top of the furnace called an inducer motor. Is it running? It should be with a call for heat. Try resetting the furnace by,  powering off and back on. If there is a fault, this will clear it and the furnace will try again to light.

Check the furnace board for a flash code. This can usually be seen through a small window in the panel where the furnace board is located. Check the number of flashes and refer to the flash code legend posted near the board refer to the number of flashes and what that means.

A homeowner bundles up next to a broken furnace

Not warm enough

If the thermostat is working but not meeting the desired temperature, check the vents throughout the house or building. If too many are closed it will prevent the hot air from entering.

Pilot light keeps going out

If you have an older furnace with a standing pilot, this could be out. Usually there are lighting instructions printed on a panel near the pilot, but if not, locate a component called the gas valve (follow the gas piping into this where this is). On the top of the valve will be a knob. Turn to the pilot position and push down and hold while reaching in with a butane match stick or long match and light the pilot. Continue holding down the knob for about a minute or more and then release. The pilot should stay lit. Once it is lit, turn the knob to “ON”.

Check for drafts, open windows or doors, that could cause the pilot light to go out.

When to call an HVAC professional

There are some repairs that require specialized training. Calling an HVAC professional will prevent any unnecessary repairs or serious injuries. Call if any of the following happens:

  • The pilot valve of your natural gas or propane furnace has malfunctioned or there is a problem with the gas pressure.

  • You experience electrical ignition control problems.

  • There’s mechanical wear and tear. Tackling these problems yourself could void the warranty.

  • You undergo yearly maintenance. Professionals are trained in what should be serviced each time and what furnace problems to check for.

Regular furnace maintenance

The maintenance of a furnace is just as important as your vehicle maintenance. Performing regular furnace maintenance will help find potential problems and prevent higher repair costs. When the furnace is running correctly it will also keep your heating bill from increasing dramatically.

You can clean and change the dirty filters yourself. Clean filters will prevent dirt from clogging up components inside the furnace and keep your energy bill lower. If you need help with this an expert can tell you how.

If your furnace is used for both heat and air conditioning, then preventative maintenance should be performed twice a year; once in the spring and once in the fall. If it is just used for heat, then annual maintenance or once in the fall should be enough. Maintenance should be performed by a professional.

The following is a list of furnace maintenance tasks that a professional will carry out:

  • Check thermostat settings to ensure it will keep a comfortable setting when you are home and energy saving when you are away.

  • Check all electrical connections, voltage and current.

  • Lubricate all necessary parts. This will prevent the furnace from working too hard.

  • Ensure the drain is clear, if not the humidity levels can increase and it can also cause damage to floors and walls.

  • Examine all controls to make sure they operate correctly.

  • Check all gas or oil connections to prevent fire and health hazards.

  • Check the burners and heat exchangers to make sure they are clean and free of cracks.

When should you consider replacing the furnace?

When all troubleshooting and repairs fail to make the furnace work properly then it may be time for a new one. The following should help you decide if you need a new one.

Age: If your furnace is more than 16-20 years old, it may be time to start shopping. Purchasing an energy-efficient furnace will take some time, so planning is a good idea.

Increased energy bill: Older or ill-maintained furnaces will run more and work harder to produce the desired temperature. The more it works, the more gas or electricity it will use.

An increase in repairs: As your furnace ages, repairs become more frequent and costly. Finding parts for older furnaces becomes harder as well. A well-maintained furnace can last for a few decades.

Constantly adjusting the thermostat: If your house is colder in one part and hotter in another, this is a sign that the furnace is not able to keep up with the desired air flow the thermostat is trying to produce.

Carbon monoxide poisoning: If you have a gas furnace and the flame is no longer blue, the furnace is not allowing the correct mixture of gas to enter or there is a crack in the heat exchanger. If you or your family complains of headaches or flu-like symptoms, the culprit may be carbon monoxide poisoning. Always keep a carbon monoxide detector running.

Lack of humidity: An older furnace may not be able to provide the correct amount of moisture, resulting in a very dry house. This can also lead to allergies, skin problems, and an accumulation of dust in the house.

A well-maintained furnace will provide you and your family with a comfortable home and can last for 20 years or more. When furnace problems arise, it is vital to find the issue and get it corrected immediately. Consult an HVAC expert who has the knowledge and experience needed to find and repair problems and keep your furnace well maintained.

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