Why Is My Furnace Blowing Cold Air?

You turn up the heat, the blower kicks on, and you put your hand over the vent. The air is cold, or barely lukewarm.

The good news: a furnace blowing cold air is one of the most common heating calls, and the cause is often simple. Sometimes it is not a problem at all. But sometimes cold air is the furnace’s way of telling you a safety switch has shut the burners off. It is worth knowing the difference.

This guide covers every common cause, what you can safely check yourself, and when to call a technician.

First, is it actually a problem?

Before you worry, rule out three normal situations.

1. The first minute of a heating cycle. When your furnace starts, the burners light first and then the blower waits to turn on until the heat exchanger warms up. On some systems, especially older ones or those with long duct runs, the first puff of air is cool because it is air that was sitting in the ducts. It should warm up within a minute or two.

2. The end of a heating cycle. When the thermostat is satisfied, the burners shut off, but the blower keeps running for a minute or two to pull the leftover heat out of the furnace. That air gets cooler as it goes. This is normal and protects the equipment.

3. You have a heat pump, not a furnace. Heat pump air is warm, but not hot. A gas furnace often sends out air that feels hot to the touch. A heat pump usually sends out air in the 85°F to 100°F range at the vents. That is warmer than the room, but it is cooler than your skin, so it can feel cool on your hand. If you are not sure what you have, look outside. If your “AC unit” runs in winter, you have a heat pump. Our comparison of a gas furnace and a heat pump in Denver’s cold weather explains more.

If the air stays cold for more than a few minutes while the thermostat is calling for heat, keep reading.

The 9 most common causes of a furnace blowing cold air

1. The thermostat fan is set to “On”

With the fan set to On, the blower runs all the time, even between heating cycles when the burners are off. You will feel room-temperature air coming out of the vents, and it will feel cool.

Fix: Set the fan to Auto. The blower will then only run when the furnace is heating.

We cover this and other whole-house causes in our post on why a furnace runs but the house stays cold.

2. The thermostat is set to the wrong mode, or its batteries are dying

If the thermostat is set to Cool or Off with the fan on, you will get unheated air. Weak batteries can also cause some thermostats to behave oddly before the screen goes blank.

Fix: Confirm the mode is Heat, the setpoint is a few degrees above room temperature, and replace the batteries.

3. The furnace overheated and a safety switch shut the burners off

This is one of the most common real causes. Every gas furnace has a high-limit switch. If the inside of the furnace gets too hot, the switch turns the burners off. The blower keeps running to cool things down. So you get cold or cool air.

The furnace overheats because not enough air is moving through it. Common reasons:

• A dirty or clogged air filter (the number one cause)
• Closed or blocked vents, or too many closed rooms
• Undersized return ducts or blocked return grilles
• A failing blower motor or a weak capacitor

What you will notice: The furnace heats for a few minutes, then the air turns cool while the blower keeps running. After a while it may heat again, then repeat. Over time this can lead to a full lockout.

Fix: Replace the filter and open all vents and return grilles. If the problem continues, call a technician. Repeated overheating is hard on the heat exchanger. Over years, it is one of the things that can lead to cracks. Our post on whether closing vents saves energy or hurts your system explains why closing vents backfires.

4. A dirty or failing flame sensor

The flame sensor is a thin metal rod that sits in the burner flame. Its job is to confirm that the gas actually lit. If it is coated with dust or residue, it cannot “see” the flame, so the furnace shuts the gas off within a few seconds for safety.

What you will notice: You may hear the burners light, then click off a few seconds later. The blower may still run for a bit, pushing cool air. The furnace may try three times, then lock out.

Fix: A technician cleans or replaces the sensor. It is one of the most common and least expensive furnace repairs, and it is part of a proper fall tune-up.

5. A failed igniter

Most furnaces made after the mid-1990s use a hot surface igniter, a small part that glows bright orange to light the gas. They crack and burn out over time. When the igniter fails, the gas never lights.

What you will notice: You hear the small inducer fan start, then nothing. Some furnaces will run the blower anyway during the retry sequence.

Fix: Replace the igniter. Do not touch it; they are fragile and skin oils can shorten their life.

6. A blocked condensate drain or blocked vent pipes (high-efficiency furnaces)

If your furnace has two white PVC pipes, it is a high-efficiency (condensing) furnace. These furnaces produce water as they run, and they breathe through pipes that go outside.

• If the condensate drain or trap clogs, a switch stops the burners from firing.
• If the intake or exhaust pipe is blocked by snow, ice, leaves, or a bird nest, a pressure switch stops the burners.

In both cases, you can end up with the blower running and no heat.

Denver tip: After a heavy snowstorm or during a long stretch of below-zero nights, check where these pipes exit your home. Wind can drift snow over sidewall vents, and frost can build up on the exhaust. Clear them gently. Never cover them.

7. A gas supply problem

No gas means no heat. Check whether your other gas appliances, like the stove or water heater, are working. Check that the gas valve on the furnace supply line is open (the handle runs parallel to the pipe).

If you smell gas, do not troubleshoot. Leave the home and call 911, then Xcel Energy’s gas emergency line at 800-895-2999.

8. Leaky or disconnected ducts

Sometimes the furnace is working fine, but the heated air leaks out before it reaches your rooms. If one or two rooms get cold air while others are warm, check any ducts you can see in the basement, crawlspace, or attic. A duct that has come loose can dump warm air into an unheated space and pull cold air in.

This is common in homes with ducts running through garages, attics, or crawlspaces, and in older homes that have been remodeled.

9. A control board or wiring problem

Less often, a failing control board or a loose thermostat wire runs the blower without telling the burners to fire. This needs a technician with a meter.

Quick diagnosis table

A special note on heat pumps and dual-fuel systems

More Denver homes now have heat pumps, often paired with a gas furnace as backup (called a dual-fuel system). Heat pumps have two normal behaviors that can feel like “cold air”:

• Lower supply temperature. As explained above, heat pump air is warm but not hot. That is by design. It runs longer and steadier than a furnace.
• Defrost mode. In cold weather, frost can build up on the outdoor coil. The heat pump briefly reverses to melt it. During defrost, you may feel cooler air for a few minutes, and you may see steam rising from the outdoor unit. That is normal. Most systems turn on backup heat during defrost to keep the air from feeling cold.

If your heat pump blows cool air for long periods, runs constantly without keeping up, or the outdoor unit is covered in thick ice, that is a service issue.

When cold air is a safety warning

Most cold-air problems are about comfort. But there are two situations where you should turn the furnace off and call right away:

• The flame changes when the blower starts. If the burner flames dance, bend, or lift when the blower kicks on, it can point to a crack in the heat exchanger. That is a carbon monoxide risk.
• The furnace keeps tripping its limit switch. Repeated overheating cycles put stress on the heat exchanger.

In either case, make sure your carbon monoxide alarms are working. Our guide on how to tell if your furnace is leaking carbon monoxide covers the warning signs.

What a technician will check

When you call us about a furnace blowing cold air, our technician will typically:

1. Confirm the thermostat is calling for heat and wired correctly
2. Watch a full heating cycle, from inducer start to ignition to blower
3. Read any blink codes on the control board
4. Test and clean the flame sensor
5. Test the igniter and gas valve
6. Check the pressure switch, venting, and condensate drain
7. Measure how hot the air leaving the furnace is compared with the air coming in (the “temperature rise”)
8. Check the filter, blower, and airflow

At Denver’s altitude, we also make sure the furnace is set up correctly for the thinner air. A furnace that is not adjusted for altitude can have ignition and combustion problems. See how Denver’s altitude affects your furnace.

How to prevent it next winter

• Change the filter on schedule. In Denver, check it monthly in winter. See our furnace filter guide.
• Get a fall tune-up. A tune-up cleans the flame sensor, tests the igniter, checks safety switches, and clears condensate drains before the first cold night. Read what a Denver furnace tune-up includes.
• Keep vents and returns open and clear of furniture and rugs.
• Check your sidewall vent pipes after big snowstorms.

Get warm air back

If you have checked the fan setting, the filter, and the vent pipes and the air is still cold, it is time for a professional. Our NATE-certified technicians repair all major furnace brands across Denver, Arvada, and the surrounding metro, and we offer 24/7 emergency service.

Call 303-645-4889 or schedule furnace repair. If your furnace is older and the repairs are adding up, we can also give you an honest repair-or-replace assessment.

Frequently asked questions