Gas Furnace Blowing Cold Air and Won’t Shut Off

It’s a singingly cold Saturday morning around Christmas. You’re off work for the holidays. So, you’re spending time at home with the family. Unfortunately, the thermostat doesn’t seem to put out heat when you turn it on.

 You’re tried switching it off and back on but, still nothing. It only gives off cold air! More worryingly, it has now refused to go off. So, it continues to put out cold air but never seems to go off. What could be wrong, and what can you do about it?

Let’s find out.

The Basics 

The first thing you need to understand is how the furnace works, from when it gets gas to when it puts out and circulates heat throughout your home. This will help us understand what may cause the furnace to blow cold air, why it may refuse to shut off, and why you may sometimes experience the two issues simultaneously. Assuming that the unit is plugged on, you have power, and the gas supply is good, the heating process begins when you switch on the furnace by pressing the press button.

Immediately, the furnace’s control board will engage. Then the next thing that happens is thermostat engagement. The thermostat will call for heating if the current temperature is below your setting (most people set the thermostat to 68°-72°). Otherwise, nothing will happen. Suppose it calls for heating, the furnace checks that the furnace’s controls are in order before engaging the pressure inducer motor (exhaust fan).

The exhaust fan will run for up to a whole minute or longer, depending on the furnace, to clear exhaust gases out of the furnace. Once the control board determines that all is fine, it engages the ignition mechanism. Ignition systems vary from one gas furnace to the next. However, the process generally involves lighting a pilot light that ignites the furnace’s main burners.

 Remember that standing pilot systems don’t need ignition as they remain lit throughout. A flame sensor helps keep an eye on the flame within the burner and is designed to discontinue gas supply immediately if the flame goes out. It’s an essential safety precaution. Another critical safety mechanism on modern gas furnaces is the limit switch. Limit switches guard against overheating.

They immediately open the furnace’s circuitry to discontinue heating if overheating is detected. However, suppose everything is fine and the flame stable. In that case, the resulting heat enters a heat exchanger that safely and efficiently transfers heat from the flame to air from the furnace’s return plenum, also known as return air.

Return air typically goes through a filter to trap most airborne particles, then flows across the heat exchanger where it absorbs heat from the burners. A second fan system, the furnace blower, is strategically positioned just after the filter to help draw cold air into the furnace, pull the air through the filter, force it past the heat exchanger, and drive warm air out of the furnace via the supply air plenum. From the supply plenum, the warm air enters your home’s ducting system and, soon after, comes out through the vents in the individual rooms. 

Why The Furnace is Blowing Cold Air

Let’s now look at common reasons why your furnace may blow cold or lukewarm air instead of warm air.

  • The thermostat is set wrongly or broken

The furnace relies on the thermostat for heating directions as we saw earlier. It only heats when the thermostat says so. Therefore, the thermostat is the first place to check if your furnace is generating cold air or not working at all. First, make sure the furnace is set to “heat” and the setting is higher than the outdoor temperature.

 Then make sure it’s “on” or on “auto” – not “fan” mode. Alternatively, maybe the thermostat is broken, thus continuing to request heating even after your home is warm enough.

  • The pilot light is out

The furnace’s burners which produce the heat necessary to keep you warm can only light if the pilot light is working. Otherwise, the burners will remain off, and you won’t get any heat. Common reasons the pilot light may fail include a draft, a faulty valve, dirt buildup, and age. Pilot systems in modern furnaces can also fail once the ignitor loses sensitivity.

  • The gas supply is weak

The amount of heat you ultimately get in your rooms directly depends on the strength of the flame in the burner unit. A stronger flame means more heat, while a weak, feeble flame typically means less heat. One of the leading causes of a weak burner flame is a weak gas supply. This can result from a gas leak or partially blocked gas pipes or valves.

  • A defective flame sensor

As we saw earlier, the flame sensor has a significant role in furnace operation. It instantly cuts operation if it can no longer detect the burner flame. Now, imagine if the sensor is dirty, damaged, or malfunctioned! It may fail to sense a flame even if one is present. Thus, it will shut down the heating process even if the burner works properly.

  • The filter is dirty 

A dirty filter can cause all sorts of problems, including health-related issues. However, one of the biggest dangers of filthy filters is overheating. Dirty filters restrict airflow into the furnace, meaning that the furnace won’t have enough cold air flowing through it. This can quickly result in overheating inside the furnace, tripping the limit switch, and effectively cutting the heating process.

  • Condensate/drainage problems 

Finally, the heating process in your furnace can also go off, resulting in cold air if you run into condensate or drainage issues. This applies to high-efficiency furnaces that produce lots of water. Blocked condensate lines or a filled drain pan can cause water to back up into the furnace and potentially damage internal components.

 Furnaces have a drain pan switch that opens and stops the heating process when the fan is full.

Why it Won’t Shut Off

The furnace may refuse to shut down for many reasons. the following are just a few issues to watch out for;

  • The thermostat is set wrongly or broken 

Before you look anywhere else, you want to make sure that the thermostat is set to “AUTO” and not “ON.” Why? Because setting the thermostat “ON” is essentially telling the blower to run nonstop as long as the appliance is switched on, even when there’s no heat. However, when you set it to “AUTO,” the blower will only run when the furnace is running and producing heat.

So, it will automatically stop when the furnace stops producing heat. Alternatively, the thermostat could be broken.

  • The filter is blocked 

The furnace may also run nonstop if the filter is too dirty or otherwise blocked. You’re probably wondering how. The reason is that a partially or fully blocked furnace filter means that the furnace will only produce very little heat at the supply end. However, remember that the thermostat requires the furnace to heat your rooms to a specific temperature (typically 68°F to 72°F) before cycling off.

It means that a furnace putting out only a tiny amount of heat may stay a long time in the ON cycle.

  • Defective blower fan motor

The constant humming even when your furnace cycles off could also point to a broken blower motor. Blower motors automatically come on when furnace temperatures reach a specified level and automatically go off when the internal temperatures drop below a given setting. However, the fan can no longer follow these requirements if broken.

Maybe it’s stuck on. If so, it will run nonstop until you shut down the furnace at the start switch.

  • Leaky ductwork 

Finally, the furnace may run longer than usual if you have leaky ductwork. Leaky ductwork means that only a tiny fraction of warm air from the furnace reaches the designated areas, i.e., your rooms. Thus, the furnace must run longer to supply the volume of warm air required to raise your indoor temperatures to the thermostat setting.

Why is the Gas Furnace Blowing Cold Air and Won’t Shut Off?

From the above discussion, it’s easy to identify why the furnace may blow cold air while running incessantly.

  1. Thermostat is broken

The first thing you to check is whether your thermostat is broken. This is critical as the thermostat tells the furnace what to do. A broken thermostat may run nonstop even when the furnace cycles off. Thus, the furnace will hum continuously while producing cold air.

Solution: Ensure the thermostat is correctly installed and displays steady values on the screen. If the screen is black or the values keep jumping, you must repair or ideally replace the thermostat. Also, make sure the thermostat has fresh batteries.

2. The thermostat is wrongly set

Setting the thermostat rarely seems a big challenge – and it isn’t. Unfortunately, mix-ups aren’t too uncommon. A common mistake that could cause the furnace to run incessantly while only producing cold air is if you set it to “ON” and never bother to adjust the settings as appropriate throughout the day or heating season.

Solution: The best thermostat setting is “AUTO.” This way, the blower fun only engages when the furnace is running and generating heat. The fan will go off as soon as the heating stops.

3. Blocked/dirty filters 

Finally, you may also experience nonstop furnace operation with cold air output if the filters are clogged or otherwise blocked. Of course, the resulting overheating may ultimately cause the heater to shut off. However, until that happens, the furnace will run nonstop and only put out lukewarm air.

Solution: The best solution here is regular maintenance. Make sure to clean or replace the filter as recommended by the manufacturer. Typically, you need to wash the filter once a week (for washable models) or replace it every three weeks at most for both washable and single-use models.


Why would a gas furnace not shut off? The gas furnace can refuse to shut off for many reasons. However, the most common reasons are a wrongly set or broken thermostat, blocked/dirty filters, a broken blower motor, and leaky ducts.

What do I do when the furnace won’t shut off? Try to “reboot” it by turning it off and then back on. This should help resolve internal issues. If nothing happens, call a professional.

How do you reset a furnace that blows cold air? A manual reset can help here too. However, you’ll often need to diagnose and fix the underlying problem. So, you need to call a professional.


There you go! Now you know common reasons why your furnace may be running nonstop and only putting out cold or lukewarm air. Even better, you now know how to fix the potential issues.