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Solving Common Furnace Problems: Troubleshooting Guide for Start-up and Cold Air Issues

Title: Troubleshooting Common Furnace Issues: Won’t Start and Blowing Cold AirImagine coming home on a freezing winter day, eagerly waiting for the comforting warmth of your furnace, only to find that it won’t start. Or worse, it starts but blows cold air instead.

These frustrating situations can quickly turn your home into an icy haven. But fear not! In this article, we will explore troubleshooting steps to help you diagnose and fix these common furnace issues.

Furnace Won’t Start

When your furnace refuses to ignite, it’s crucial to investigate and determine the underlying problem. Let’s delve into the potential causes and troubleshooting steps.

Basic Checks to Perform

1. Check the furnace switch and circuit breaker: Ensure the furnace switch is turned on and that the circuit breaker hasn’t tripped.

2. Adjust the thermostat setting: Make sure the thermostat is set to the desired temperature and mode (heating).

3. Inspect the wiring connections: Verify that all wiring connections are secure and undamaged.

4. Examine the filter: A clogged or dirty filter can impede airflow, causing your furnace to shut off.

Clean or replace the filter if necessary.

Advanced Checks to Consider

1. Check the thermostat batteries: Replace the batteries if they are low or dead, as this can prevent the furnace from starting.

2. Inspect the main circuit board/control board: Look for any burned-out components or loose connections.

Consider calling a professional if you lack experience with electrical troubleshooting.

Furnace Blowing Cold Air

If your furnace is up and running, but it’s blowing cold air instead of the desired warmth, don’t panic. Follow these troubleshooting steps to resolve the issue.

Potential Causes and Solutions

1. Check the gas cock or gas control valve: Ensure the gas supply is turned on.

If the valve is already open, try closing and reopening it to ensure it is fully operational. 2.

Examine the ignitor: If the ignitor is faulty or worn out, it may not generate the necessary heat to ignite the gas. Consider replacing it after disconnecting the power.

3. Verify the draft motor and fresh-air intake: A malfunctioning draft motor or restricted fresh-air intake can prevent the furnace from igniting.

Clean or replace if necessary.

Additional Factors to Check

1. Inspect the draft motor: A faulty draft motor can disrupt the combustion process, leading to cold air blowing through the vents.

Consider professional assistance if you suspect a motor issue. 2.

Assess the capacitor: A defective or weak capacitor can hinder the blower motor’s functioning. Replace it cautiously only if you have experience with electrical components.

By following these troubleshooting steps, you can swiftly diagnose and address common furnace issues. Remember, if you’re unsure or uncomfortable with any step, it’s always best to seek professional assistance to avoid causing further damage.


A malfunctioning furnace can quickly turn your cozy home into a chilly refuge. However, armed with the right knowledge and troubleshooting steps, you can resolve many common furnace issues yourself.

Just remember to perform the basic checks first, such as inspecting the switch, thermostat, wiring connections, and filters. If the problem persists, delve into the advanced checks, like examining the circuit board or changing thermostat batteries.

Furthermore, when your furnace blows cold air, consider investigating the gas cock, ignitor, draft motor, and fresh-air intake. Lastly, if in doubt, always reach out to a professional who can provide expert guidance and assistance.

Stay warm, stay safe!

Draft Motor Runs, Furnace Won’t Start

Potential Causes and Solutions

When your draft motor runs but your furnace still won’t start, it’s essential to explore possible issues that might be hindering its operation. 1.

Motor Failure: If your draft motor is running but the furnace won’t start, the motor itself might be faulty. Excessive wear and tear or a burned-out motor can prevent the necessary air circulation required for the furnace ignition.

In such cases, it’s advisable to seek professional assistance as replacing the motor requires technical expertise. 2.

Blocked Air Intake: Insufficient air intake can also hinder the furnace’s startup process. Check the air intake vent and ensure it is clear of debris, dust, or any obstructions.

Additionally, make sure there are no objects blocking the supply or return vents. A blocked air intake restricts airflow, preventing the furnace from starting.

3. Blocked Exhaust Chimney: Similar to the air intake, the exhaust chimney can become obstructed due to debris, bird nests, or accumulated soot.

A blocked exhaust impedes the expulsion of combustion gases, triggering a safety shut-off mechanism that prevents the furnace from igniting. In such cases, it is important to have a professional chimney sweep clear out any obstructions and ensure proper venting.

4. Gas Issues: If your furnace is operated by natural gas or propane, there could potentially be a gas-related issue causing the furnace to not start.

Inspect the gas supply valve to ensure it is open and functioning correctly. Additionally, check for any gas leaks or gas pressure issues, as these can prevent the furnace from igniting.

If you suspect a gas problem, it is crucial to contact a professional immediately to address and resolve the issue safely.

Capacitor Failure

A faulty capacitor can also play a role in preventing your furnace from starting, even if the draft motor is running. 1.

Capacitor Failure: The capacitor is responsible for providing an electrical boost to start the blower motor. If it fails, the blower motor may not receive sufficient power, resulting in the furnace not starting.

A capacitor can fail due to age, electrical issues, or manufacturing defects. 2.

Troubleshooting and Replacement: To diagnose a failed capacitor, check for visual signs of swelling or leakage. If you suspect a capacitor issue, it’s essential to turn off the power to your furnace before attempting any repairs.

Capacitors store electrical energy and can cause injury if mishandled. It is highly recommended to consult a professional to test and replace a faulty capacitor safely.

Draft Motor Won’t Start

Troubleshooting Steps

If your draft motor is not running, it could indicate a problem either with the motor itself or with its associated components. 1.

Check for Power: Ensure that the furnace is receiving power by checking the furnace switch and circuit breaker. If the switch is off or the circuit breaker is tripped, reset them accordingly.

2. Motor Issues: If the draft motor won’t start, it could be due to a defective motor.

Conduct a visual inspection of the motor for any signs of wear, damage, or loose connections. Consider seeking professional assistance to replace the motor if necessary.

3. Capacitor Problems: Inspect the capacitor associated with the draft motor.

Look for signs of damage, such as bulging or leaking. Test the capacitor’s functionality using a multimeter or consult a professional for accurate evaluation and replacement.

4. Electrical Checks: Ensure all wiring connections related to the draft motor, including control panels and safety switches, are secure and correctly installed.

Loose or damaged wiring can prevent the motor from starting. Conclusion:

In troubleshooting furnace issues, it’s important to understand the various components involved and their respective roles.

When the draft motor runs but the furnace won’t start, potential causes include motor failure, blocked air intake or exhaust chimneys, and gas issues. Seek professional assistance for motor replacement or chimney cleaning to avoid further damage.

Additionally, a faulty capacitor can hinder furnace ignition, even if the draft motor is operational. Always exercise caution when handling capacitors and consult a professional for proper testing and replacement.

If the draft motor won’t start at all, check the power supply, inspect the motor and wiring connections, and troubleshoot the associated capacitor. By following these troubleshooting steps and seeking professional help when needed, you can effectively diagnose and address common draft motor-related furnace issues.

Stay warm and cozy!

Flame Starts, Furnace Shuts Down

Flame Sensor Corrosion

One of the common reasons for a furnace to start and then shut down shortly after is an issue with the flame sensor. The flame sensor is a vital safety component that detects the presence of a flame.

If it becomes dirty or corroded, it may fail to detect the flame accurately, causing the furnace to shut down as a safety precaution. 1.

Identifying Corrosion: A corroded flame sensor can often be identified by a buildup of debris or a white, powdery substance on its surface. This corrosion creates a barrier between the sensor and the flame, preventing it from accurately detecting the flame’s presence.

2. Sanding the Flame Sensor: To remedy corrosion on the flame sensor, you can use fine-grit sandpaper (200 to 400 grit) to gently sand the sensor’s surface.

This will remove any buildup and restore its functionality. Be cautious not to sand too aggressively, as this can damage the sensor.

3. Sensor Replacement: In some cases, the corrosion may be severe, or the sensor may be damaged beyond repair.

In these instances, it is recommended to replace the flame sensor entirely. Be sure to consult the manufacturer’s guidelines or seek professional assistance for proper sensor selection and installation.

Blower Won’t Shut Off

Possible Causes and Solutions

If your furnace blower won’t shut off, there are a few potential causes to consider, along with corresponding troubleshooting steps. 1.

Fan Switch on Thermostat: Check the fan switch on your thermostat. If it is set to “On” instead of “Auto,” the blower will continuously run, even when the furnace is not heating.

Adjust the fan switch to “Auto” to ensure the blower operates only when needed. 2.

Stuck Relays in Control Board: The control board in your furnace contains relays that control various functions, including the blower motor. If a relay becomes stuck in the “ON” position, it can cause the blower to run continuously.

Carefully inspect the control board for any visible signs of damage or malfunctioning relays. If you suspect a stuck relay, it is best to consult a professional for proper diagnosis and repair.

3. Worn Thermostat Contacts: Over time, the electrical contacts within your thermostat can become worn.

This wear can lead to the blower not receiving the correct signals to shut off. Consider replacing the thermostat if you notice signs of wear or if it is an older model.

Ensure you follow all safety guidelines and hire a professional if you are unsure about thermostat replacement.

Blower Motor and Control Board Issues

If the blower continues to run even after troubleshooting the thermostat, it may indicate a problem with the blower motor or the control board. 1.

Blower Motor: Check the blower motor for signs of damage, such as burning smells, unusual noises, or excessive vibration. A faulty blower motor can cause the fan to run continuously.

If you suspect a motor issue, it is advisable to contact a professional for inspection and potential replacement. 2.

Furnace Fan Switch: The blower motor is connected to a fan switch, which signals the motor when to turn on and off. If the fan switch becomes faulty, it may fail to shut off the blower motor as intended.

Replacing the fan switch requires experience and knowledge, so it is recommended to seek professional assistance. 3.

Control Board: Issues with the control board can contribute to blower motor malfunctions. Visually inspect the control board for any signs of damage, loose connections, or burnt-out components.

However, troubleshooting and repairing the control board should be done by a qualified HVAC technician. Remember, improper handling or repairs of electrical components can be dangerous.

It is crucial to prioritize safety and seek professional assistance when dealing with blower motor or control board issues. In conclusion, troubleshooting furnace-related issues is essential for maintaining a comfortable home environment.

When facing flame sensor corrosion, gentle sanding or sensor replacement can easily resolve the problem. If the blower won’t shut off, check the thermostat fan switch setting, inspect the control board for stuck relays, and consider the condition of the thermostat contacts.

Additionally, examine the blower motor and fan switch for any faults, and inspect the control board for damage or loose connections. Professional assistance should be sought for complex repairs or unfamiliar electrical components.

By systematically addressing these issues, you can restore your furnace’s functionality and enjoy a warm and cozy home. Furnace Keeps Shutting Off

Possible Causes and Solutions

If your furnace repeatedly shuts off, there are a few common culprits to investigate and potential solutions to consider. 1.

Dirty Filter: A dirty or clogged filter can obstruct airflow, triggering the furnace’s limit switch to shut it off for safety reasons. Regular filter maintenance is necessary to prevent this issue.

Clean or replace the filter according to the manufacturer’s guidelines, typically every one to three months, or as needed based on your home’s air quality and usage. 2.

Flame Sensor Failure: The flame sensor is responsible for detecting the presence of the burner flame. If it becomes dirty or damaged, it may not function properly, leading to frequent shutdowns.

Gently clean the flame sensor with fine-grit sandpaper (200 to 400 grit) to remove any debris or corrosion. If cleaning doesn’t restore proper functioning, consult a professional to inspect and potentially replace the flame sensor.

3. Thermostat Failure: A malfunctioning thermostat can also result in frequent furnace shutdowns.

Check the thermostat settings to ensure they are correct for your desired comfort level. Consider replacing the batteries if they are low or dead.

If your thermostat is outdated or consistently inaccurate, it may be time for a complete replacement. Consult a professional for proper installation and optimal thermostat selection.

Furnace Short-Cycling

Furnace short-cycling refers to a scenario where the furnace repeatedly turns on and off in short cycles, not allowing it to operate consistently. Several factors can contribute to this issue.

1. Overheating: A dirty filter, blocked vents, or excessive debris around the furnace can lead to overheating.

When the furnace reaches unsafe temperatures, the limit switch will shut it off to prevent damage. Clear any obstructions and ensure proper airflow to resolve overheating-related short-cycling.

2. Incorrect Thermostat Placement: If the thermostat is exposed to direct heat or cold drafts, it may affect its temperature readings, causing the furnace to short-cycle.

Relocate the thermostat to a more suitable location, away from direct sunlight, heat sources, or drafts. 3.

Malfunctioning Components: Faulty components, such as the limit switch, thermostats, or control board, can contribute to short-cycling. Inspect these components for visible signs of damage or consult a professional to diagnose and resolve any component-related issues.

Furnace Runs But Can’t Keep House Warm

Filter-related Issues

Even if your furnace appears to be running, it may struggle to maintain warmth due to filter-related problems. 1.

Dirty Filter: A dirty filter restricts airflow, making it difficult for the furnace to distribute warm air throughout your home effectively. Regularly clean or replace the filter to ensure proper airflow and efficient heating.

2. Wrong Filter Type: Using the wrong type of filter for your furnace can also impede its efficiency.

Consult your furnace’s manual or contact a professional to determine the appropriate filter type and size for your specific system. 3.

Backwards Filter Installation: Installing the filter incorrectly, particularly putting it in backward, obstructs airflow and reduces heating performance. Make sure to follow the manufacturer’s instructions when installing a new filter and check that the airflow arrow aligns with the correct direction.

Leaking Air Ducts

Leaking air ducts can lead to heat loss and reduced efficiency, making it challenging for your furnace to adequately warm your home. 1.

Inspect Air Ducts: Visually inspect the air ducts for any visible signs of leaks, such as gaps, loose connections, or damaged sections. Addressing minor leaks yourself, such as sealing gaps with duct tape or using mastic sealant, may help improve air circulation.

However, for more significant duct leaks, it’s best to consult a professional for thorough duct assessment and repair. 2.

Professional Duct Testing: To thoroughly evaluate the condition of your air ducts, consider hiring a professional for a duct leakage test. This test quantifies the amount of air leakage in your duct system and allows for targeted repairs or sealing of problem areas.


Frequent furnace shutdowns can be attributed to various factors including a dirty filter, flame sensor failure, or thermostat malfunction. Ensure regular maintenance of your filter, clean flame sensors, and consider replacing thermostats that are outdated or malfunctioning.

Short-cycling can be caused by overheating, incorrect thermostat placement, or malfunctions in various furnace components. Resolving these issues often requires clearing obstructions, adjusting thermostat placement, or repairing faulty components.

When your furnace struggles to keep your home warm, consider filter-related issues such as dirt accumulation, incorrect filter type, or improper installation. Additionally, leaking air ducts can result in heat loss, leading to reduced heating efficiency.

Regular inspection and addressing leaks through self-repair or professional assistance can help optimize heating performance. By addressing these common issues, your furnace will operate more effectively, keeping your home warm and comfortable.

Furnace Makes Loud Noises

Possible Causes and Solutions

When your furnace makes loud noises, it can be an indication of underlying issues that require prompt attention. 1.

Single-Stage Furnace: A single-stage furnace is known for making louder operational noises compared to newer, multi-stage models. The sudden rush of air when the furnace starts or stops may produce noticeable sounds.

While these noises are normal for single-stage furnaces, consider scheduling routine maintenance to ensure the system is operating optimally. 2.

Bad Bearing: If you hear a high-pitched squealing or grinding noise, it may indicate a faulty blower motor bearing. Over time, the bearing wears out, causing friction and noise.

Professional assistance is recommended for diagnosing and replacing a bad motor bearing. 3.

Leaky Furnace Cabinet, Plenum, or Ductwork: Loose connections or cracks in the furnace cabinet, plenum, or ductwork can create rattling or vibrating noises. Inspect these components, tightening any loose screws or connections.

For cracks or significant leaks, it’s best to seek professional help to seal or repair the affected areas.

Dirty Filter

A dirty filter can contribute to unusual noises coming from your furnace. 1.

Whistling or Whirling Noise: A dirty filter can obstruct airflow, causing an increase in pressure within the system. This increase in pressure can result in a whistling or whirling noise as air passes through narrower openings.

Clean or replace the filter regularly to ensure proper airflow and reduce noise. Furnace Leaks When AC is Running

Possible Causes and Solutions

If you notice water leakage when your AC is running, it may indicate an issue with the drain pan or drain line. 1.

Clogged Drain Pan: The drain pan is designed to catch condensation that forms on the evaporator coil. Over time, dirt, debris, or mold can accumulate in the drain pan, leading to clogs.

Regularly inspect and clean the drain pan, ensuring it is free from blockages and able to remove condensation effectively. 2.

Clogged Drain Line: The drain line carries the condensation from the drain pan to the outside of your home. A clogged drain line can cause water buildup, leading to leaks.

Clear the clog by carefully using a wet/dry vacuum or by using a mixture of bleach and water to flush out the line. If the clog remains persistent, it’s advisable to seek professional assistance.

3. Professional Maintenance: Regular professional maintenance for your HVAC system, including cleaning and inspection, can help prevent clogs and address potential issues before they cause water leakage.

Consider scheduling annual HVAC maintenance to ensure optimal performance and avoid leaks. Conclusion:

When your furnace makes loud noises, it’s important to identify and address the underlying causes to prevent further damage or disruptions.

While some noise, like those from single-stage furnaces, may be normal, excessive noise or new sounds should prompt further investigation and potential repairs. Pay attention to components such as the blower motor bearing, furnace cabinet, plenum, and ductwork for any loose connections, cracks, or damage.

Cleaning or replacing dirty filters regularly can also help reduce noise. If your furnace leaks when the AC is running, inspect and clean the drain pan and drain line to ensure proper drainage and prevent water buildup.

Regular maintenance by professionals can help prevent clogs and address potential issues before they cause leaks. By taking appropriate steps to troubleshoot and resolve these issues, you can enjoy a quiet and efficient furnace along with a leak-free HVAC system.

Bad Smell Coming from Furnace

Possible Causes and Solutions

When a bad smell emanates from your furnace, it is crucial to identify the source and take appropriate action. 1.

Dirty Filter: A dirty filter can produce a musty or stale odor as dust, dirt, and debris accumulate over time. Regularly cleaning or replacing the filter can eliminate this issue and improve indoor air quality.

2. Dead Rodent or Bird: Unfortunately, small animals may find their way into your furnace system, resulting in an unpleasant odor.

If you suspect a dead rodent or bird, turn off your furnace immediately and contact a professional to safely remove the carcass and clean the affected area. Do not attempt to remove it yourself, as it can be hazardous.

Seeking Professional Help

In certain cases, it may be necessary to involve a professional furnace technician to address bad smells originating from the furnace. 1.

Professional Inspection: If you are unable to determine the source of the odor or if it persists even after filter replacement, it is recommended to contact a professional furnace technician. They can perform a thorough inspection to identify any underlying issues that may be causing the smell.

2. Gas Leaks: If you smell natural gas or a rotten egg odor near your furnace, it is essential to evacuate the building immediately and contact your gas company or emergency services.

Gas leaks can be extremely dangerous. Do not attempt to operate the furnace or investigate the source of the smell on your own.

CO Monitor Going Off

Steps to Take

If your carbon monoxide (CO) alarm goes off, it is vital to follow proper protocol to ensure your safety and well-being. 1.

Evacuation: Carbon monoxide is a colorless and odorless gas that can be extremely harmful or even deadly if inhaled in high concentrations. If your CO alarm sounds, evacuate the premises immediately, taking all household members and pets to a safe location outdoors.

Ensure you are a safe distance away from the building before taking further action. 2.

Turning Off the Furnace: Once everyone is safe, locate the main gas shut-off valve near your furnace and close it to stop the flow of gas. If you are unsure or unable to locate the gas shut-off valve, leave it to the professionals and focus on exiting the building safely.

3. Contacting Emergency Services: After evacuating the building, call emergency services or your local fire department to report the situation.

They will assess the situation, conduct necessary tests to determine CO levels, and provide guidance on when it is safe to re-enter the building. 4.

Professional Inspection: Following an incident where your CO alarm goes off, it is necessary to have a professional furnace technician inspect your system to identify and address any issues causing the carbon monoxide leak. Professional expertise is crucial in ensuring the safety and proper functioning of your furnace.


When a bad smell emanates from your furnace, it is important to investigate the cause to address any underlying issues. A dirty filter or the presence of a dead rodent or bird can be common sources of bad odors.

Regular filter maintenance and professional assistance may be required to resolve such issues. In more severe cases, a professional furnace technician should be involved to thoroughly inspect the system and identify any potential problems.

Similarly, in the event of a CO alarm going off, immediate evacuation is crucial along with turning off the furnace and contacting emergency services. Professional inspection and maintenance are necessary to prevent and address the occurrence of CO leaks.

Prioritizing safety and seeking professional assistance when dealing with furnace-related odors or carbon monoxide incidents is of utmost importance.

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