Why Furnace Blowing Cold Air? – Troubleshooting Guide

Cold air coming from the vents during winter is definitely a reason for concern. 

Your furnace can stop blowing hot air if the thermostat settings are incorrect, the filter or the condensate line is clogged, the flame sensor is dirty, the pilot light won’t stay lit, or if there is an issue with the gas supply. Leaky ducts will also significantly affect the furnace’s heating capacity. 

What exactly should you do if your heating unit is blowing cold air? Below you will find a simple step-by-step guide. 

Why Furnace Blowing Cold Air?

Your thermostat settings are incorrect

Make sure that the thermostat is set to ‘heat’ and that the fan setting is ‘auto’. If the fan is ‘on’, then the system will be circulating air even when the furnace is not working (which means that your rooms will be receiving cool air).

Read: How To Troubleshoot And Reset Furnace?

The filter is dirty

A clogged filter will restrict the airflow and you won’t be getting enough cooled air, even if the system is working perfectly fine. A dirty filter can also make your furnace overheat and shut down.

Dirty hvac filters

The condensate line is clogged

If you have a high-efficiency furnace, then it most likely has a condensate line. If the component gets clogged, it will prevent the unit from igniting. 

There’s an issue with the pilot light

Older models have a standing pilot light that might have to be manually relit if it ever goes out.

The flame detector is dirty

If your flame sensor is covered in grime, then it won’t be able to detect the flame and the system won’t heat up.

There’s an issue with the gas supply

Your heating system won’t be receiving any gas if there is a problem on the supplier’s end or if the gas valve is malfunctioning.

You have leaky ducts

Not all of the hot air will make it to your room if there are gaps in the ductwork.

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Why Is Furnace Running but No Heat?

If you have just turned your furnace on and you immediately felt a cold blast of air come out of the vents, then you have nothing to worry about. Your furnace needs about 10-15 minutes to warm up and begin the heating cycle. 

Read: How To Reset Furnace Control Board?

Furnace Blowing Cold Air and Won’t Shut Off

The most common reason for a heating unit blowing chilly air and not turning off is a thermostat that is set to ‘fan on’. Once your house has reached the set temperature, the furnace will stop heating the air, but the fan will keep going until you manually turn it off or set it to ‘auto’.

Furnace Blowing Cold Air Only at Night

The problem might, once again, be with the thermostat. If you have a programmable device, then it will be following a certain schedule – the thermostat might be programmed to lower the house temperature at night. In such a case, the furnace won’t turn on until the indoor temperature drops to a certain level (which can be a lot lower than that during the day).

Read: Should You Repair Or Replace A Furnace?

Furnace Blowing Cold Air After Power Outage

If the burners are not kicking in after a power outage, but the blower is still blowing air through the system, then you might have to reset the flame rollout switch.

Also, check all the switches at the circuit breaker. Sometimes, one heating system can have two breakers instead of one. If any of those had tripped, then a part of the unit won’t be working.

Why Is My Furnace Blowing Out Lukewarm Air?

Leaky ducts might be to blame. If there are holes in the ductwork, then you will be losing heated air and cool outside air will be able to make it into the ducts. As a result, you will have lukewarm air coming out of the vents.

What Temp Should Be Coming Out of Furnace Vents?

You shouldn’t expect the air coming out of your vents to be extremely hot. In fact, in the majority of cases, the air might feel just a tiny bit warm. That is because the human body’s temperature can be anywhere between 97 F and 99 F and the temperature of the air coming out of the vents during the heating season is usually 90-120 degrees.

Read: What Are Common Furnace Repair Problems?

What Is the Average Furnace Temperature?

To find out if your heating unit is actually blowing cold air or not, you would have to check the temperature of the return and the supply vent and then find the difference between the two.

In a properly working heating or cooling system, the temperature difference between the air that is being supplied to the system and the air that is being returned to the room should be 16-22 degrees Fahrenheit. 

Should I Turn My Furnace Off If Its Blowing Cold Air?

If your furnace continues blowing cold air even after you have given it enough time to warm up, then you might want to turn the system off before taking some basic troubleshooting steps.

The chances are high that either the thermostat settings are incorrect or the filter is clogged. 

If the problem persists, then your furnace might be overheating or there is an issue with the ignition system. In any case, shutting the system down is recommended as this will help prevent any further damage

Read: How Can I Avoid Furnace Repairs?

How Do You Fix a Furnace That Blows Cold Air?

  1. Check the thermostat settings – make sure that the device is set to ‘heat’ and that the fan is in ‘auto’ mode.
  2. Make sure that the registers are fully open and that nothing is blocking the vents.
  3. Change the filter, if it’s dirty.
  4. Check your home’s gas supply – if you have another appliance that runs on gas, try turning it on to see if it works.
  5. Unclog the condensate lines.
  6. Check the ductwork for any leaks.
  7. Clean the flame sensor.
  8. Call an HVAC professional. If you haven’t managed to find the root cause of the problem, then there might be something wrong with the ignition system – and you would definitely want to leave that to an expert. 
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How to Find Out If Furnace Ignitor Is Bad?

A furnace blowing cold air can be a sign that you have a malfunctioning ignitor. This element creates the spark that ignites the gas. If it stops working, the air circulating through the system won’t get heated up (or the unit will shut down altogether). 

Why Is My Pilot on But No Heat?

If your pilot light is on, but the furnace is failing to produce warm air, then there might be an issue with the gas valve or the gas supply.

How Often Should a Gas Furnace Be Cleaned?

If you never get your gas furnace cleaned, then:

  • The unit will struggle to reach the temperature that you want
  • There will be strange smells and noises coming from the furnace
  • The quality of the indoor air will get reduced
  • The furnace might start blowing cold air

To avoid these issues, make sure to get the unit cleaned by a professional at least once a year.

How Do You Test a Furnace Thermostat?

One of the easiest ways to fix a furnace that is blowing cold air is to check the thermostat settings. Once you have confirmed that the settings are correct and you have made sure that the device is not located close to another heat source, then you can test the device to ensure that it’s operating correctly.

Undo the terminals for the wires connecting the device to the furnace and pull the wires off. Twist the ends of the wires together and turn on the power to the furnace. If the blower starts and the burners come on, then the thermostat has gone bad.


There are plenty of things that can make your furnace start blowing cold air. The good news is that, in the majority of cases, you’ll be able to fix the issue by simply checking the thermostat settings and replacing the air filter.