older man feeling vent and wondering Why Is My Furnace Blowing Cold Air

A furnace blowing cold air indicates that something is amiss. The furnace might have overheated, or there could be an issue with the thermostat, air filter, pilot light, flame sensor, ducts, condensate line, or gas supply.

Furnaces are designed to heat homes. If your furnace is not producing warm air, something is wrong. You could call a local HVAC service to schedule a furnace inspection, but you’ll have to fit it into your busy schedule, pay service fees, and endure a cold house in the meantime. Wouldn’t it be great if you could fix the problem yourself?

Fortunately, furnaces are relatively uncomplicated appliances. With some basic knowledge of how furnaces work and an understanding of the issues that cause furnaces to blow cold air, you can resolve the problem yourself. Your furnace could be back to heating your home in less time than it takes to call the professionals.

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How does a furnace work?

Forced-air heating systems are the most common type of furnace. The furnace burns fuel and transfers the heat to the air. A fan blows the warm air through the home’s ductwork and heating vents. Return ducts channel cold air back to the furnace to be reheated.

Furnaces are categorized by the fuel that powers them: natural gas, propane, oil, electric, or wood. Natural gas and liquid propane gas (LPG) are two common types of furnaces that operate similarly. Gas is piped to the home from the local natural gas line network or a liquid propane storage tank.

A thermostat inside the house turns on the furnace when the home’s temperature drops below the setting. The furnace’s gas valve opens, and the burner expels gas through a small outlet. The igniter creates a spark or hot surface to light the gas. The burning gas warms the heat exchanger. The exhaust produced from the combustion process is vented outside the house through the exhaust pipe.

how a gas furnace works illustration

Cold air from the house is channeled to the furnace through the return ducts. The air travels through a filter to remove dust and other airborne particulates. The furnace’s heat exchanger warms the air, and the blower fan pushes the air through the ducts and vents. The warm air blowing from vents increases the home’s temperature. The thermostat switches the furnace off when the temperature setting is reached.

Is Your Furnace Blowing Cold Air? Here’s Why:

A furnace blowing cold air is no reason to panic. The furnace might need more time to heat up, especially if it hasn’t been used since last season. Give it time. If the air is still cold after fifteen minutes, there might be a furnace issue.

Many issues that cause cold air to blow from the furnace can be easily resolved. The remedy might be as simple as flipping a switch or swapping out a filter. Complex issues require professional expertise. Regardless of the issue’s complexity, hire an HVAC professional if working on your furnace makes you uncomfortable.

8 Causes of Cold Air From Your Furnace:

  1. Thermostat issues
  2. Dirty air filter
  3. Pilot light is out
  4. Faulty flame sensor
  5. Furnace has overheated
  6. Leaky ducts
  7. Clogged condensate line
  8. Low gas supply

Thermostat Issues

The thermostat sends signals to your furnace to turn on and off. Incorrect thermostat settings and wiring cause cold air to blow from the vents.

Incorrect Setting

Ensure the thermostat is set to “heat” and not “cool.” Checking the thermostat might seem obvious, but someone in the home might have changed the setting.

The fan is another setting to check. The fan constantly runs when set to “on.” Unheated air blows from the vents between heating cycles. Change the fan setting to “auto” so it will only run during the heating cycles.

Incorrect Wiring

A recently installed thermostat might be wired incorrectly or incompatible with the system. These issues can cause incorrect signals to be sent to the furnace. Call a professional HVAC technician to check the wiring and ensure the thermostat is compatible.

woman in a winter hat and coat pressing buttons on home thermostat because furnace blowing cold air
dirty furnace air filter

Dirty Air Filter

A dirty or clogged air filter obstructs airflow through the furnace, resulting in an insufficient supply of cool air to heat. Restricted airflow can also cause the temperature to rise inside the furnace, forcing the limit switch to shut down the heating cycle.

How often you change your air filter depends on a few essential factors – number of pets and household members in a household, location, size of your home, time of the year, allergy sensitivity, and type of filter.

Replace the dirty filter with a new one. The furnace should restart after it has cooled down.

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Pilot Light Is Out

Older furnaces manufactured before 2010 have gas pilot lights that can be extinguished, whereas modern furnaces have an electric furnace ignition. The furnace pilot light is a small blue flame that burns constantly and ignites burners at the beginning of a heating cycle. If the pilot light is out, the furnace won’t heat up.

Consult your owner’s manual to find the pilot’s location, and visually check if it’s lit. If you're comfortable doing so, you can try relighting the pilot following your manual's instructions, but bear in mind that gas is highly flammable. Call a professional HVAC technician if you are not comfortable performing this task or if the pilot won’t stay lit.

pilot light with orange flame on gas furnace

Relighting the pilot generally involves these steps:

  1. Turn off the power to the furnace by flipping the toggle resembling a light switch on the furnace or shutting off the breaker at the circuit breaker box.
  2. Locate the pilot light assembly. Turn the gas valve to “off.” Wait ten to fifteen minutes for the gas that escaped the pilot to clear. Gas is highly flammable!
  3. Turn the gas valve to “pilot.” Press and hold the reset button, which is located near the gas valve. While holding the reset button, press the igniter to light the gas coming out of the pilot. Release the ignitor when the pilot lights. A BBQ lighter or a long match can also be used to light the pilot. Continue to hold the reset button for 20-30 seconds.
  4. Turn the gas valve to “on.” Turn on the power to the furnace.

Call a professional HVAC technician if you cannot light the pilot, or it won’t stay lit.

flame sensor in gas furnace

Faulty Flame Sensor

The furnace flame sensor is a thin, bent metal rod on the burner assembly in front of the pilot light. When the furnace turns on, gas spews from the gas valve into the combustion chamber, and the pilot or ignitor lights the burners. The flame sensor ensures flames are burning when the gas valve opens. If the sensor doesn’t detect flames when the gas is on, it shuts the system down. This process safeguards the home from filling with toxic, flammable gas if the furnace malfunctions.

The flame sensor regularly becomes coated in soot and carbon deposits from the furnace’s flames. A dirty or faulty flame sensor has difficulty detecting flames. It might shut the furnace down, and cold air will blow from the vents.

Test the flame sensor by turning off the furnace for 5 minutes. The flame sensor is dirty or faulty if the air is initially warm and becomes cold when you turn the furnace back on. The air changes from warm to cold because the heat exchanger initially heats up until the flame sensor cuts the burners off.

Clean or replace the flame sensor by following these steps:

  1. Turn off the power to the furnace by flipping the toggle resembling a light switch on the furnace or shutting off the breaker at the circuit breaker box.
  2. Remove the furnace’s access panel, and locate the flame sensor. It looks like a bent metal rod with white porcelain around the mounting end.
  3. Remove the sensor’s mounting screw and gently slide out the sensor. Carefully detach the sensor from the wire.
  4. If the sensor’s tip is black, it needs to be cleaned. Rub the metal part of the sensor with steel wool or scrape it with a straight-edged knife to remove the deposits. If the porcelain casing is cracked, the sensor needs to be replaced. Purchase a new sensor, and move on to steps 5 and 6.
  5. Reattach the wire to the sensor and gently slide it back into place. Tighten the screw. Replace the access panel.
  6. Turn on the power to the furnace.

System Has Overheated

If your furnace overheats, the high limit switch shuts the system down to prevent a fire, carbon monoxide leak, or an explosion. Cold air might blow from the vents temporarily until the entire system shuts down. Mechanical failure, dirt buildup, and old age can cause a furnace to overheat.

Mechanical Failure

One of the furnace's components or processes might have malfunctioned or failed. The fan limit switch, the safety feature that shuts an overheated furnace down, could be faulty. A cracked furnace heat exchanger, which poses the risk of carbon monoxide poisoning, can also cause a furnace to overheat.

Dirt Buildup

Restricted airflow due to a dirty, clogged filter can cause the furnace to overheat. Dirt and debris within the furnace can also cause it to overheat, so replacing the filter won't resolve the issue. Contact an HVAC company to clean your furnace professionally.

Old Age

Furnace performance declines over time, and they can overheat due to old age. A furnace should last 15 years, but it will break down sooner if it’s not maintained correctly. A furnace replacement might be your best option.

gas furnace overheating
older man fixing leaking ductwork with hand drill

Leaky Air Ducts

Warm air escapes through cracks or holes in the ducts. The air will feel less warm when it reaches the vents. Leaks are difficult to locate, so it’s best to call a professional to identify and seal the leaks.

Clogged Condensate Lines

High-efficiency gas furnaces create water, known as condensate, which drains into a PVC pipe called a condensate line. Water accumulates around the furnace if the line becomes clogged with dust, dirt, mold, or other debris. The furnace will shut down as a safety measure. The fan will continue to circulate unheated air.

Check for dampness or if your furnace is leaking water. You can unclog the line, but clearing a condensate line can be tricky. Call a professional if you don’t feel up to the task.

clogged condensate line furnace
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Unclog the condensate line by following these steps:

  1. Turn off the power to the furnace by flipping the toggle resembling a light switch on the furnace or shutting off the breaker at the circuit breaker box.
  2. Locate the condensate drain pan. Dry up the water in the pan with a wet/dry vacuum. Remove the pan and it with dish soap and water. Replace the pan.
  3. Follow the condensate line from the pan to where it ends. Typically, the line ends outside the house. Use a wet/dry vacuum to suck out clogs from the end of the line.
  4. Turn on the power to the furnace.

Low Gas Supply

Your gas furnace cannot operate without a gas supply. There could be an issue with the gas supply line or furnace gas valve. Call an HVAC technician to inspect cases involving the gas supply.

If your furnace uses propane, the tank could be empty. Call a company to refill your tank. If your furnace uses natural gas and you’ve missed payments, the gas company might have shut off your supply. Check if your bills have been paid.

Troubleshooting a Furnace Blowing Cold Air

When your furnace is not blowing hot air in the middle of winter, it's not ideal, but you can often avoid hiring a professional by resolving the issue yourself. You’ll save time and money, and your home will quickly return to a comfortable, cozy temperature.

Adjusting the thermostat settings, changing the air filter, and checking the gas supply are simple tasks that anyone can do. Relighting the pilot, cleaning the flame sensor, and unclogging the condensate line require more competence, but handy people should still be able to accomplish these tasks.

Always hire a professional if trouble-shooting measures don’t resolve the issue or if you feel uncomfortable working on your furnace. Cases involving electricity, the gas supply, or major repairs are best left to the professionals.

In the future, keep your furnace properly maintained and regularly inspected to keep it in good working order and prolong its lifespan.

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