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Furnaces, Troubleshooting

Why Is My Furnace Blowing Cold Air?

Last Updated: January 12, 2022

In the dead of winter, there’s nothing quite like the instant relief of holding a cold hand over a vent blowing hot air. There’s also nothing like the disappointment of your furnace blowing cold air inside when it’s turning cold outside. Before you hit the panic button because your trusty old heater has picked a terrible time to go into hibernation, there are some legitimate reasons why normally functioning furnaces can give you the cold shoulder. Some of these you might be able to fix yourself (if you like that sort of thing) to get the warm air flowing.

As an example, one of the more common reasons your furnace may be blowing cold air is when the thermostat is set to “on” instead of “auto”. In this case, the fan will run constantly, even when the furnace isn’t heating. Simply re-setting the thermostat to auto should resolve your issue, if the system is working properly. Of course, more complicated issues often require furnace service. If you end up needing help, you can connect with a local Carrier HVAC expert to schedule a furnace repair.

In the sections that follow we will explore thermostat settings and other common causes for "Why my furnace is blowing cold air”, and will offer both DIY and “call for help” solutions.

furnace blowing cold air on woman at table

Common Causes of Heaters Blowing Cold Air

INCORRECT THERMOSTAT SETTINGS

Symptoms

If a thermostat is set incorrectly, you might notice your furnace blowing cold air from the heat registers, a lower-than-expected temperature reading on the thermostat, or you may even hear your air conditioner running outside.

Details

There are a lot of options when it comes to thermostats – programmable models, smart thermostats and more -- but they all control both heating and cooling. With all of these choices, there are a number of scenarios involving your thermostat that can result in your furnace blowing cold air.

Solution

If you experience any of these symptoms, the first step is to check the thermostat and look for the obvious answers.

  • Is it set for air conditioning instead of heating? If it is, simply re-set the thermostat to heat your home. Or, re-set the thermostat to auto. When it’s set to auto, it will automatically change between heating and cooling as temperatures change.
  • Is the fan set to run continuously? If so, your system may be circulating cool air between heating cycles. Changing the fan setting to “auto” will turn off the blower when the system is not actively heating and turn it on when it's time to heat your home.
  • Check the temperature setting to make sure somebody else didn’t change it to a lower setting.
  • If you have a programmable thermostat, review your “comfort schedule” for any issues, and make sure your programmed settings haven’t shifted times due to daylight savings time.

Of course, if your thermostat is not working or the furnace has a mechanical issue, these adjustments likely won’t resolve the issue of your furnace blowing cold air. If you have made some of the described adjustments and your system does not respond, it may be time to contact your local Carrier HVAC technician for a professional analysis of your heating system.

THE HEATER HASN’T WARMED UP YET

Symptoms

You hear your furnace kick on but there is no airflow, or you find the heater blowing cold air.

Details

While your first instinct might be to call your HVAC technician, your furnace might be operating normally. Many furnace models include a fan limit switch that allows the furnace to warm up the air before the blower pushes it out through the air ducts and into your living areas.

Solution

Check the owner’s information for your furnace to see if there should be a blower delay after the gas burner kicks on. If, after a few minutes you still experience the furnace blowing cold air, you may need to contact your HVAC contractor.

THE PILOT LIGHT IS OUT

Symptoms

The furnace is blowing cold air, or there is no heat in the house.

Details

Pilot lights used to be a standard component on gas furnaces before the 1990s. When the pilot light is out, you’ll find an otherwise perfectly functioning furnace blowing cold air. Because a pilot light is designed to burn continuously, it also uses more of your gas supply, which can be reflected in higher utility bills. So, if you have an older model furnace, your home is too cold and the furnace hasn’t turned on, your pilot light may be out.

Solution

Start by checking that your thermostat is set properly. Next, locate the pilot light assembly on your furnace and see if there is a flame. If the pilot light is out, follow the manufacturer’s directions on how to relight it. If it won’t stay lit, it may be time to call a professional. When your pilot light is damaged your system won’t operate efficiently, if it runs at all. Sometimes the issue may be as simple as cleaning the pilot’s orifice or the flame sensor. Even if the pilot assembly needs to be replaced, it’s usually not an expensive repair. Finally, if your furnace does have a pilot light, it may be time to consider upgrading to a new, higher efficiency pilotless gas furnace.

THE FAN LIMIT SWITCH HAS MALFUNCTIONED

Symptoms

If your furnace is blowing cold air, or is not operating at all, a component called the fan limit switch may have malfunctioned. You may also see an error message on your thermostat, or a blinking LED light on the furnace control board.

Details

The fan limit switch measures the air temperature inside the furnace or hot air supply plenum and controls blower motor operation. If the fan limit switch malfunctions, the blower might continue to operate even when the furnace is not heating the air, resulting in your furnace blowing cold air from your registers. Or, it may prevent the furnace from operating at all.

Solution

Check your air filter, and clean or replace it if needed. A dirty air filter can restrict airflow, causing the heat exchanger to get too hot which triggers the fan limit switch to shut down the furnace. If cleaning or replacing the filter doesn’t solve the problem, it’s probably best to contact your local HVAC professional.

THE FLAME SENSOR HAS MALFUNCTIONED

Symptoms

If the temperature in your home is cooler than expected, or if you hear your furnace start up and shut down quickly over and over, you might suspect a flame sensor has malfunctioned. If this is the case, you may also see an error message on your thermostat, or a flashing LED light on the furnace control board.

Details

The flame sensor’s job is to detect a flame from the gas burner. A properly working flame sensor will shut down the furnace if it does not detect a flame while the gas valve is operating. This helps prevent the flow of natural gas (or propane) from a gas furnace into the home’s air supply. A malfunctioning flame sensor shuts down the furnace when the burner is working correctly (flames are present) because it does not sense the heat.

Solution

If you can see the burner in your furnace, you may be able to visually inspect the flame sensor for cracks in its porcelain base, or soot build up on its tip. Either of these would be a clue that a malfunctioning flame sensor could be causing your furnace to shut down. Because a functioning flame sensor and a malfunctioning flame sensor can both result in a furnace shutting down, it may be best to contact an expert to determine why your system isn’t functioning properly.

A DIRTY AIR FILTER CAUSED THE FURNACE TO OVERHEAT

Symptoms

If you notice the air temperature in your home is cooler than normal, if the airflow from your heat vents seems to be weaker than usual, and/or if you hear the furnace running for short periods of time before shutting down (short cycling), you may have a dirty air filter to blame.

Details

Lack of filter maintenance can lead to build-up of dirt, dust and other airborne pollutants that can clog your air filter, restrict airflow, cause discomfort, or in more extreme cases, cause system failure.

Solution

Before checking your heater filter, be sure the thermostat is set for heating and that the temperature setting is correct. If you still suspect the problem is a dirty air filter, consult your owner’s manual for instructions on removing and cleaning or replacing your air filter. If cleaning or replacing the filter doesn’t solve the problem, contact your local Carrier dealer for assistance.

TOO MANY CLOSED SUPPLY VENTS CAUSED THE FURNACE TO OVERHEAT

Symptoms

If you notice a room is too cold or hot, inconsistent temperatures in different parts of the house, the actual room temperature and your thermostat’s setting don’t match, or if your furnace is constantly turning on and then back off after a short time (short cycling), you may have too many closed vents.

Details

Some homeowners close the heat vents in rooms that go unused in the winter. Closing too many vents may cause the furnace to adjust by cycling on and off more frequently, which can reduce overall comfort in the house and can shorten the life of the furnace.

Solution

Try opening all of your heat vents and see if that solves the problem. If it does, and you still want to close a few vents, try closing fewer vents this time. If opening all of the vents doesn’t solve the problem, you may want to contact a local HVAC professional to evaluate your heating system and ductwork.

AIR DUCTS ARE LEAKING WARM AIR

Symptoms

If you notice your heater blowing cold air, an increase in your utility bills, or can actually feel warm air coming from seals or joints in your ductwork, it’s time to investigate.

Details

Leaky ductwork not only leads to reduced comfort, it can result in higher utility bills because of the amount of heat lost due to the leakage.

Solution

If you have exposed ductwork, checking for leaks can be relatively easy. Simply turn your system on, then hold your hand over joints, connections, or anywhere you can visibly see a seal in the ductwork. The average homeowner can make simple repairs by straightening sheet metal and securing sheet metal joints with screws, appropriately rated tape product or sealant. If you continue to have problems associated with duct leakage or your heater not blowing hot air, an HVAC professional can provide a more sophisticated analysis and duct sealing options for ducts that are not easily accessible.

THERE IS A PROBLEM WITH THE GAS OR OIL SUPPLY

Symptoms

A furnace blowing cold air, turning on and then back off, struggling to keep its pilot light on, or the furnace not turning on at all may indicate a fuel supply issue.

Details

Fuel supply issues can plague any type of combustion furnace – natural gas, propane or oil. For example, with a natural gas furnace an issue with the supply line can cause the pilot light to keep going out. A propane furnace or oil furnace blowing cold air might indicate a low or empty fuel supply. And, a clogged oil filter can also cause an oil furnace to turn on and blow cold air.

Solution

If you suspect a fuel supply issue with your natural gas furnace, it’s best to contact a qualified professional. Sometimes your fuel supply problem is actually a faulty gas valve or gas burner in the furnace itself. For an oil or propane furnace, having your supply tank refilled might alleviate your issue as long as no other underlying issues exist with your furnace, tank and supply lines. If you are a do-it-yourself type, replacing a clogged oil filter is relatively simple and requires only basic tools.

THERE IS A PROBLEM WITH THE COMPUTERIZED CONTROLS

Symptoms

Most modern furnaces follow a specific sequence of operations managed by an electronic control board. However, if find yourself asking why my heater is blowing cold air, why does the temperature keep fluctuating, or why isn't my furnace running the way it usually does, you may have an issue with the computerized control.

Details

If your furnace is running but indoor temperatures are either too hot or too cold, this may indicate a faulty control board. Another sign is when the usual sequence of operation seems different. Can you hear the igniter turning on? Do you hear the “whoosh” of the gas burner activating? Is the blower fan running long after the rest of the heating operation has shut down? And, if you remove the cabinet door and can see blinking LED lights on the board, there may be an issue. Many furnaces have diagnostics built in and can signal potential trouble by blinking in distinct patterns.

Solution

If you have a control board going out, it is best to contact your local Carrier expert for assistance. If the LEDs on the control board are blinking, you may be able to relay that information to your HVAC dealer in advance of the service call. Some Carrier furnaces can be also be diagnosed remotely over the internet by your local Carrier dealer when set up with this function.

SO, WHY IS MY FURNACE BLOWING COLD AIR?

Regardless of why it's happening, having a furnace blowing cold air can make your life uncomfortable. From a dirty air filter to incorrect thermostat settings or too many vents closed, some of the solutions can be easily solved with minimal technical skills. Unfortunately, a number of issues will require a higher level of expertise – the kind you can get from your local Carrier dealer. A qualified HVAC technician has the tools, the training and the experience to take on the bigger challenges associated with electrical, mechanical or fuel supply issues that can leave you in the cold. Need help today? Find a Carrier expert in your area using our dealer locator.

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