Common Furnace Problems and How to Fix Them

Heating your home efficiently is crucial during the colder months, yet furnaces, like any mechanical system, are prone to breakdowns. Did you know that nearly 75% of no-heat furnace calls are due to preventable issues, many of which could be solved with simple DIY maintenance? 

Whether it’s a lack of heat, strange noises, or a furnace that just won’t turn on, these issues can often be resolved by homeowners with a little troubleshooting. 

In this comprehensive guide, we’ll explore the most common furnace problems, practical solutions, and preventative measures to keep your heating system running smoothly.

Table of Contents

Furnace Not Producing Heat

Few things are as frustrating as turning on your furnace only to find it’s not producing heat. Several factors could be behind this issue, but here are the most common culprits:

Possible Causes

  • Thermostat Issues: If your furnace isn’t heating, the first place to check is your thermostat. Sometimes, the settings may be wrong, especially if it was accidentally set to “cool” or if the temperature is set lower than the room temperature.
  • Pilot Light or Ignition Problems: In older gas furnaces, the pilot light can go out due to drafts, dirt buildup, or a faulty thermocouple. Modern furnaces use an electronic ignition system, which could fail due to electrical issues.
  • Power Supply Failures: Check whether the furnace is receiving power. A tripped circuit breaker or a blown fuse may prevent it from turning on.

DIY Solutions

  • Thermostat Troubleshooting: Make sure the thermostat is on the “heat” setting and that it’s set higher than the room temperature. For smart thermostats, check if they’ve lost connection to the network. Resetting your thermostat may solve the issue.
  • Relighting the Pilot Light: For older furnaces with a pilot light, follow the manufacturer’s instructions to safely relight it. If you notice frequent outages, the thermocouple may need replacement.
  • Check Power Supply: Ensure the furnace’s breaker hasn’t tripped. If it has, reset it. Check the furnace’s on/off switch (typically located near the unit) to ensure it hasn’t been accidentally turned off.

Advanced Troubleshooting: In some cases, the issue may be related to the gas flow. Ensure the gas valve to the furnace is open. If you’re still not getting heat and smell gas, turn off the furnace and contact a professional immediately, as this could indicate a serious leak.

Furnace Blowing Cold Air

A furnace that’s running but blowing cold air is a common issue. This problem can stem from several causes, each with varying levels of complexity.

Common Reasons

  • Clogged Air Filter: A dirty air filter restricts airflow, causing the furnace to overheat. In response, the safety switch shuts down the heating element while the blower continues to run, which results in cold air blowing.
  • Incorrect Thermostat Settings: If the thermostat’s fan setting is set to “on” instead of “auto,” the fan will blow continuously, even when the furnace isn’t actively heating the air.
  • Leaky Ducts: Air leaks in the duct system can pull cold air from attics, crawl spaces, or basements into the airflow, which cools the air before it reaches your living space.

How to Fix

  • Replace Air Filters: Check the furnace’s air filter, and if it’s clogged with dirt and debris, replace it. Furnace filters should be replaced every 1-3 months depending on the type of filter and the air quality in your home.
  • Thermostat Settings: Switch the fan setting from “on” to “auto” to ensure the fan only runs when the furnace is actively heating.
  • Duct Inspection: Inspect your ducts for leaks or damage. Seal any visible leaks with duct tape or, ideally, mastic sealant, which is more durable.

Choosing the Right Air Filter: Furnace filters come in different MERV ratings, which measure a filter’s ability to capture particles. A higher MERV rating (like HEPA filters) can trap finer particles but may also restrict airflow if not compatible with your furnace. 

Choose a filter that balances filtration efficiency with airflow, typically a MERV 8 to 12 for most homes.

Strange Noises from the Furnace

If your furnace is making strange noises, it’s trying to tell you something. Different sounds can signal different problems, and identifying them can save you from costly repairs later.

Types of Furnace Noises and Their Causes

  • Banging or Popping: This sound usually indicates expanding and contracting air ducts or a delayed ignition, which occurs when gas builds up before igniting.
  • Squealing or Whistling: Often caused by worn-out fan belts, dry motor bearings, or restricted airflow.
  • Rattling: Loose components, such as screws or bolts, can cause rattling noises inside the furnace or ductwork.

Fix

  • Tighten Components: Check the blower motor and other moving parts. Tighten any loose screws, bolts, or panel covers.
  • Lubricate Motor Bearings: If the motor is squealing, lubricating the bearings with a few drops of motor oil can reduce the noise.
  • Clear Air Ducts: If airflow is restricted, clear the ducts of any obstructions. In some cases, professional duct cleaning may be necessary to remove debris buildup.

When to Call a Professional: If the banging noise is due to a delayed ignition or there’s a burning smell, contact an HVAC technician immediately, as this could indicate a dangerous gas buildup or a failing motor.

Furnace Cycling On and Off

A furnace that cycles on and off frequently, also known as “short cycling,” is another sign that something isn’t quite right. This behavior can reduce efficiency and strain your system.

Causes

  • Dirty Air Filter: A clogged filter restricts airflow, causing the furnace to overheat and shut off to prevent damage.
  • Thermostat Issues: If the thermostat is malfunctioning or placed in a location where it reads the wrong temperature, it may cause the furnace to turn on and off too quickly.
  • Airflow Restrictions: Blocked vents, closed dampers, or undersized ducts can cause airflow problems that trigger short cycling.

Fix

  • Replace the Air Filter: As mentioned earlier, a clean air filter ensures proper airflow and prevents overheating.
  • Recalibrate or Replace the Thermostat: Ensure the thermostat is in a location where it can read the room’s actual temperature, away from direct sunlight, drafts, or appliances that generate heat.
  • Improve Ventilation: Make sure all supply and return vents are unobstructed. If the cycling persists, a professional may need to evaluate your duct system for airflow issues.

Pilot Light or Ignition Issues

For gas furnaces, problems with the pilot light or ignition system can lead to a no-heat situation. Addressing these issues promptly is essential for safety and efficiency.

Causes

  • Dirty Flame Sensor: A dirty flame sensor can cause the pilot light to go out or prevent the furnace from igniting. This component needs to sense the flame, and if it’s covered in soot, it won’t function correctly.
  • Drafts: In older homes, drafts may blow out the pilot light. If your furnace frequently experiences pilot outages, check for air leaks near the furnace.

DIY Fix

  • Clean the Flame Sensor: Turn off the furnace’s power and remove the flame sensor. Use a fine abrasive pad or steel wool to clean off any carbon buildup. Reinstall it and turn the furnace back on to see if the issue is resolved.
  • Check for Drafts: Seal drafts near the furnace to prevent the pilot light from going out. In some cases, you may need to install a draft shield around the furnace.

When to Contact a Professional: If cleaning the sensor or sealing drafts doesn’t solve the problem, you may have a faulty thermocouple or ignition system that requires a professional’s attention. Ignition systems in modern furnaces are complex and may need replacement.

Furnace Not Turning On

If your furnace refuses to turn on, the problem could be something as simple as a blown fuse or as complicated as a malfunctioning safety switch.

Causes

  • Faulty Thermostat: If the thermostat isn’t communicating with the furnace, it may not signal the system to turn on.
  • Tripped Circuit Breaker: Power outages or surges can trip the furnace’s circuit breaker.
  • Safety Switch Problems: Many furnaces have a safety switch on the blower door that prevents the furnace from running if the door isn’t securely closed.

Troubleshooting

  • Check the Thermostat: Raise the thermostat’s temperature setting and see if the furnace kicks on. Replace the thermostat batteries if necessary.
  • Reset the Breaker: Head to the breaker panel and reset any tripped breakers. If the furnace continues to trip the breaker, consult an electrician.
  • Check the Blower Door: Make sure the blower door is securely closed, which will engage the safety switch.

High Energy Bills

If your heating bills are higher than usual, it could indicate your furnace is working harder than necessary due to inefficiency.

Causes

  • Dirty Air Filters: Clogged filters reduce airflow and make the furnace work harder, increasing energy consumption.
  • Aging Components: As furnaces age, parts like the blower motor or heat exchanger wear out, causing the system to use more energy.
  • Poor Insulation: If your home is poorly insulated, the furnace has to run longer to maintain a comfortable temperature.

How to Improve Efficiency

  • Regular Filter Replacement: As always, keeping the filter clean is one of the simplest ways to ensure your furnace runs efficiently.
  • Consider an HVAC Tune-Up: Schedule an annual furnace tune-up with an HVAC professional. They will clean and lubricate components, inspect the heat exchanger for cracks, and ensure the system is running efficiently.
  • Upgrade Insulation: Improve the insulation in your attic, walls, and around windows to prevent heat loss. This reduces the workload on your furnace, leading to lower energy bills.

When to Call a Professional

While many furnace issues have DIY solutions, some problems are best left to professionals. Here’s when to call an expert:

  • Electrical Issues: If your furnace is experiencing power issues that can’t be solved by resetting the breaker, you’ll need an HVAC technician to check the wiring and electrical components.
  • Persistent Pilot Light Problems: If relighting the pilot or cleaning the flame sensor doesn’t solve ignition issues, there could be a deeper issue with the gas line or ignition system.
  • Unusual Smells: Burning or gas odors from your furnace indicate serious problems, such as a gas leak or electrical fire risk. Shut off the system and call a professional immediately.

Finding the Right Professional: When looking for an HVAC technician, make sure they’re certified, licensed, and insured. Ask for references or check reviews online. Some certifications to look for include NATE (North American Technician Excellence) certification and EPA 608 certification for handling refrigerants.

Conclusion

Addressing common furnace issues as soon as they arise can save you money, improve energy efficiency, and ensure your home remains warm throughout the winter. While many problems, like replacing air filters or troubleshooting a thermostat, can be handled by homeowners, more complex issues may require professional assistance. 

Stay proactive with regular maintenance and inspections to avoid costly breakdowns and extend the life of your furnace.

FAQs

What is the typical lifespan of a furnace?
A well-maintained furnace can last 15 to 20 years, though efficiency may decline over time. Regular maintenance can help extend its lifespan.

What time of year is cheapest to replace a furnace?
Spring and fall are typically the best times to replace a furnace, as HVAC companies tend to offer discounts during these off-peak seasons.

How much should I expect to spend on a new furnace?
The cost of a new furnace, including installation, can range from $2,500 to $7,000, depending on the type, size, and energy efficiency of the system.

How long does it take to replace an old furnace with a new one?
A typical furnace replacement takes between 4 to 8 hours, depending on the complexity of the installation and whether any modifications are needed.

What furnace size do I need for my home?
The furnace size is determined by the square footage of your home, insulation, and climate. Consult an HVAC technician to calculate the correct size, ensuring efficiency and proper heating.

Sources

Additional Furnace Resources