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Why Is My Furnace Blowing Cold Air? A Buffalo Tech Answers

Feeling colder air than normal coming out of the vents?

We know this is a frustrating problem, especially on a freezing day.

The first step to troubleshooting the problem is to determine when cold air is coming from your vents:

  • Do you feel cool air constantly after the furnace turns on? If so, the thermostat could be set incorrectly or your air ducts could be leaky.
  • Do you only feel cold air for a short time right after you turn on your furnace? If so, the furnace may have a faulty limit switch.
  • Does the furnace start off blowing warm air, then suddenly start blowing colder air? If so, your furnace may have an overheating issue. If your furnace is overheating, you’ll also notice that the furnace shuts off quickly after starting.

Scroll to the scenario that best describes your situation for more information about how to fix each problem.

Want a Buffalo professional to repair your furnace?

Call Roy’s for a worry-free repair. Our technicians can fix any furnace model or brand, and they will provide upfront, honest pricing before any work begins.

Problem #1: The furnace blows cool air constantly

If the furnace blows cold air from the moment you turn it on, the first thing you’ll want to do is check your thermostat.

Make sure the thermostat is set to HEAT. If the thermostat is set to COOL, your AC could kick on and start cooling your home. Also check the set temperature to make sure it is 5 or more degrees higher than the room temperature.

The HEAT and COOL settings on a thermostat

The HEAT and COOL settings on a thermostat

If your thermostat is set correctly but your furnace seems to be blowing cold air non-stop, then the cold-air problem could be caused by leaky ducts.

You see, leaks allow hot air to escape out of the ducts into unconditioned spaces, such as the attic. If the leaks are severe, the airflow will be noticeably weaker, which means the air coming out of your vents may feel colder.

Diagram of a duct leakage

When hot air leaks out of the ducts into the attic, the air coming out of the vents will feel weaker

Other signs that indicate your ducts have severe leaks include:

  • Uneven temperatures in certain areas
  • Increased dust inside your home

If you notice these signs, you’ll need to have a professional inspect your ducts and recommend any needed repairs.

Problem #2: The furnace blows cold air right after it starts, but then blows warm air

If your furnace blows cold air right after it starts, the fan limit switch could be faulty.

We’ll explain what a limit switch is.

Every furnace has a fan, which is responsible for pulling in cold air to be heated. Once the air is heated, the fan pushes the warm air back into your home. The part that tells the fan when to start is called the limit switch.

If the limit switch is malfunctioning, it may send an incorrect signal to the fan, telling it to turn on before the burners have had time to adequately warm up the heat exchanger (the part that heats your home’s air). If this happens, the fan will push cooler air into your home for a short time.

In normal operation, you should hear the fan come on anywhere from 1.5 to 3 minutes after the thermostat calls for heat. If you hear it come on sooner than that, your fan limit switch may be set to start the fan too early or may be malfunctioning altogether.

A malfunctioning limit switch can also prevent your furnace from turning on or off. If you’ve noticed those kinds of issues in addition to the cold-air problem, you’ll need to contact an HVAC professional to check the limit switch and replace it if it is faulty.

Problem #3: The furnace starts off blowing warm air, then suddenly switches to cold air

Does this describe your situation?

If so, the most likely culprit is an overheating issue.

If your furnace overheats for any reason, a safety switch will trigger and automatically shut down the system to prevent damage to expensive components such as the heat exchanger (the part that actually heats your home’s air).

After the burners or ignition system turn off, the fan will continue to blow cold air over the heat exchanger to cool it down and protect it from cracking. This could explain why you felt warm air for some time, then you noticed a drop in air temperature after a while.

What makes a furnace overheat in the first place?

A furnace can overheat if...

  • The air filter is dirty
  • The return or supply vents are obstructed or closed

Let’s look at both of these overheating issues in more detail.

Overheating issue #1: A dirty air filter

All the air that goes into your furnace passes through an air filter. The air filter catches dust and debris so that it doesn’t circulate throughout your home’s airstream.

Over time, the filter gets clogged with debris and needs to be replaced. If left dirty, the collected debris will block cool air from coming into the furnace, which can cause the heat exchanger to overheat.

Clean vs dirty air filter

Clean vs dirty air filter

Solution: Check your air filter and replace it if it is dirty.

Overheating issue #2: The return or supply vents are obstructed or closed

Cool air enters your furnace to be heated via the return vent (your home may have more than one). Return vents are usually found on walls.

Return vent (with dirty air filter)

Return vent (with dirty air filter)

After the air is heated, it gets pushed back into your home via supply vents, which are the smaller vents found high up on walls or on ceilings.

Location of a supply air vent

A supply air vent

Similar to a dirty air filter, if either type of vent is closed or obstructed by furniture or curtains, then your furnace will struggle to pull in enough cold air and it may overheat.

Solution: Check the vents throughout your house and make sure they are open and unobstructed. Be sure to leave vents open (even in rooms you don’t occupy often) to allow for proper airflow.

If you’ve checked your air filter and vents, there could be another issue with your furnace, such as a mechanical failure or dirty components. To fix these overheating problems, you’ll need to contact a professional HVAC technician.

Want a Buffalo technician to repair your furnace?

Call Roy’s for a worry-free repair. Our technicians can fix any furnace model or brand, and they will provide upfront, honest pricing before any work begins.

When you hire Roy’s to repair your furnace, you can count on first-class customer service and high-quality repair work. If a Roy’s technician fixes your furnace, it will be done right—the first time.