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How to Fix Your Furnace When it's Not Working


There are many reasons your furnace may be blowing cold air into your home. In most cases, it can be a quick fix, and in other cases, you will need the help of a professional.

In this guide, we will cover the main reasons your furnace is not working or is blowing cold air, how to fix it, and when you may need the help of a professional.

The main reasons your furnace is not working properly

Before you panic and worry something is wrong with your heating system, take a deep breath and notice if your heating system is in between cycles.

The most common reason your heater is blowing cold air is that it is in between cycles—your HVAC systems cycles when it reaches certain temperatures.

Your thermostat reads the temperature and notifies your system to turn on and off accordingly.

If your fan is on, you may feel cold air coming from your system even though it is working correctly.

If your system cycles often, it may be because you have an oversized HVAC system, which is an entirely different topic.

If your furnace is still blowing cold air or not working correctly, it can be for the following reasons:

  1. The thermostat fan setting is on and not the heat setting

  2. The thermostat is not working correctly

  3. The air filter is clogged

  4. Faulty ducting and air supply issues

  5. Blockage in the condensate drain line

  6. The furnace control panel is faulty

  7. The pilot light is not working

  8. Faulty capacitor

  9. There is a gas supply issue

  10. Safety triggers

  11. Clogged evaporator coil

  12. Conclusion

We will dive deep into each of these reasons to understand the root causes and see how they can be fixed or avoided.

We always recommend you get professional help with HVAC-related issues as it is a complex system that involves mechanical and electrical work that can be dangerous.

Plus, you never want to create more harm to your system, costing you more.

Your thermostat fan setting is on, not the heat setting

This one is an easy fix. Often, your heat setting is not on.

It is easy to confuse the fan setting with the heat setting.

Ensuring that your thermostat heat setting is on is the first thing you should check when your heating system is blowing cold air into your home.

It can take up to 10-15 minutes to start feeling the warm air flowing through your home, so give it a few minutes before jumping to conclusions.

Your thermostat is not working correctly.

If your heat setting is on, you have waited a few minutes, and still, your heater is blowing cold air, it could be that your thermostat is not working correctly.

A faulty thermostat does not always mean the thermostat itself is defective; it could be that the communication wiring is loose or not correctly wired.

If all the wiring is in place and wired correctly it can still be a faulty thermostat issue.

Many times power surges and outages can cause a thermostat to go bad.

Your Air Filter Is Clogged

A common reason for your heater blowing cold air is airflow issues. One of the most common reasons for air restriction is dirty air filters.

A clogged air filter will limit the airflow to the system. Limited airflow will cause the heat exchanger to heat up too much.

This will trigger the high limit switch which shuts off the system.

Replacing your air filter is an easy fix, and you can do this yourself.

Faulty ductwork and Air Supply Issues

In continuation to the previous point, airflow issues can arise for several reasons.

Faulty ductwork occurs for many reasons; it could be that your ducting is old, and there are holes in your ducting leading to air leakage, causing the hot air to escape before entering your home.

Damaged ducting can also result from people working in your attic or crawlspace and accidentally cutting or piercing the ductwork.

Duct cleaning can also cause damage to your ductwork if not done carefully.

Another more severe issue could be that the ducting in your home is designed poorly. Poorly designed ductwork can cause a variety of problems with your HVAC system.

To check if your ducting and airflow issues are working properly you will need to do a static pressure test. This will help determine where the issue is within the system.

If you have a blocked supply or return air vents it can also cause airflow issues.

You always want to make sure that both supply vents (the vents that blow air into your home) and the return vents (the vents that suck the air back into your HVAC system) are not blocked by any objects or furniture.

Blocked supply or return vents cause airflow issues.

Blockage in Your Condensation Drain Line

If you have a high-efficiency furnace, it generates condensation when operating. The condensation needs to be drained to ensure no water enters your furnace.

If you have a blocked condensation line, water will build up, and your furnace will trigger a safety switch to ensure there is no damage to your home or your system.

Typically a condensation safety switch is for the air conditioning system however in high-efficiency furnaces it serves both purposes.

Check out this guide on how to clean your condensate drain line yourself. Or contact a professional for help.

Your Furnace Has a Faulty Control Panel

Your furnace control panel is the brains of your furnace or heater. It is what commands your furnace to turn on or off, among other things.

There are a few reasons your control panel may be faulty. If it gets dirty or dusty, it can cause issues. And if it gets wet due to your furnace leaking, it can cause permanent damage to your furnace.

A control panel should not be tampered with, and we recommend consulting with a professional.

The only way to fix a faulty control panel is to replace it.

Your Furnace Pilot Light is Not Working

The furnace pilot light ignites your furnace burners. The burners then heat the heat exchanger, which facilities the heat transfer into your home.

If your furnace pilot light fails to ignite your burners, the flame sensor will not sense any heat and turn off.‍

There can be two reasons your pilot light isn't working, the first is that it is simply faulty, and you will need to replace it.

The other is that the furnace control panel is faulty and isn't signaling the furnace pilot light to ignite.

For this, you will need an HVAC professional to diagnose the problem.

We advise you not to tamper with the internal mechanisms of your furnace as it can get quite complex.

Always consult with a professional if you believe your furnace pilot light is not working.

Faulty Capacitor

Many blower wheels in a gas furnace have a capacitor to help run the motor.

If the capacitor goes bad then the blower wheel will not start, preventing your system from blowing any air at all.

A faulty capacitor should be diagnosed and replaced by an HVAC professional.

There is a gas supply issue

Similarly to your pilot light not working, a gas supply issue will also prevent the burners from working, causing your furnace to blow cold air.

A gas supply issue can occur because of an issue with the supply of gas to your home or a problem with the gas valve.

A gas valve may be faulty in the sense that it is getting the command to release gas however the internal components of the valve are not working. Or it may not get the signal from the control board.

You will need the help of an HVAC professional to diagnose the problem and replace the correct component.

Safety Triggers

Your furnace has a few safety triggers to ensure your furnace operates safely.

We briefly discussed what may cause some of them to trigger. Let's discuss the safety switches and why they are in place.

The flame sensor ensures that the flame is lit when the furnace is supplied with gas.

This safety trigger ensures that your home doesn't get filled with dangerous flammable gas.

And to prevent a fire hazard as the area around your furnace could get filled with gas if this were not in place.

If the flame sensor is dirty or has corrosion it won't sense any heat and as a result the furnace will shut off.

This is the most common reason why your furnace won't heat.

The high limit switch safety switch ensures that your heat exchanger doesn't get too hot. If your heat exchanger gets too hot, it can crack, leading to a carbon monoxide leak and an expensive repair.

Your heat exchanger getting too hot can mean that your blower fan is not working and can be a reason your HVAC system is not blowing any air at all, or you have an air supply issue.

If your blower motor is not working then you will not feel any air flowing out of your vents at all. It can either be due to an electrical problem or that the motor went out completely.

You should consult with an HVAC professional to get an accurate diagnosis.

If you do feel air coming out of your system but it is not hot. It could be airflow issues that will need a static pressure test to determine.

The pressure switch ensures that your inducer motor is working properly and that the flames from the furnace get sucked into the heat exchanger and the fumes will exhaust from your home.

The main reason a pressure switch would cause your furnace to not ignite is because of blockage in the FLUE vent pipe.

A blockage will not allow the motor to suck the air properly therefore not creating the pressure needed to let the switch know the system is working properly.

In other cases the switch may have gone bad, you will need an HVAC professional to diagnose the problem and switch the switch if necessary.

The rollout safety switch cuts the gas supply to the system. The purpose of this switch is to ensure the flames don't burn in the opposite direction.

When the burners are on, they should burn towards the heat exchanger and not the opposite way.

The rollout switch will trigger shutting off the gas supply to prevent carbon monoxide leakage and potential fire hazards.

It is rare that a rollout switch will get triggered because the pressure switch is a pre safety that should prevent a rollout in the first place but often times it happens.

A rollout switch oftentimes simply needs to be reset, but in other cases, they may need to be replaced entirely.


If your heating system is not working because of a safety trigger, you should contact an HVAC professional.

Clogged Evaporator Coil

If you have a split system, there is an evaporator coil on top of your furnace before your supply duct. A clogged evaporator coil can limit the airflow.

As we have discussed above, this can cause your furnace to not work properly.

An evaporator coil can become clogged because of dirt and debris. Dirt and debris can build up in your system because of inefficient or dirty filters.

That is why it is essential to have a good air filter. It will protect your lungs, and your system and help you avoid costly repairs.

For help on how to choose an air filter, see this guide.

If it is dirty, you will need to have your evaporator coil cleaned.


There are many reasons your heater may be blowing cold air into your home. You can start and try and solve some of these problems yourself.

However, we always recommend you consult with a professional, especially if you are dealing with electrical or mechanical systems.

Sometimes messing with the HVAC system yourself will cause more harm than good and it may void any manufacture or labor warranties.

If you maintain and take care of your system regularly, you can avoid many of these problems and ensure your HVAC system will cool or heat your home when you need it most.

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