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No warm air

Furnace Blowing Cold Air from the Vents

The blower is moving air, but it feels cool instead of warm. That usually means the fan is working and the burners are not.

Quick answer

A furnace blowing cold air often has the thermostat fan set to ON, so air keeps moving between heating cycles. Other common causes are a clogged filter tripping a safety limit, ignition or flame sensor failure, a gas supply problem, or leaky ducts in a cold crawlspace. If you notice a gas smell, leave the home and call the gas utility or 911 from outside.

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Technician using a flashlight to inspect an indoor HVAC system

What this symptom may mean

In a gas furnace, the blower and burners are separate, and the blower keeps going for a while after the burners stop. If the burners never light, or light and quickly go out, the blower just circulates cool house air.

A brief puff of cool air at the start of a cycle is normal while the heat exchanger warms. Cold air lasting more than a few minutes is not.

Possible causes

  1. Fan set to ON

    The blower runs nonstop and pushes room-temperature air between cycles, which feels cold.

  2. Overheated limit switch

    A dirty filter causes overheating, so the burners shut off while the fan keeps running.

  3. Ignition failure

    When the igniter or flame sensor fails, the burners will not stay lit.

  4. Duct leaks in unheated space

    Disconnected ducts in a crawlspace or attic let warm air escape and pull cold air in.

  5. Condensate problem on high-efficiency units

    Some high-efficiency furnaces stop the burners when their condensate drain backs up.

Safe checks you can make

  • Set the thermostat fan to AUTO, then confirm the mode is HEAT and the setpoint is above room temperature.
  • Replace a dirty air filter and give the furnace a few minutes to restart its sequence.
  • On high-efficiency furnaces, look at the plastic condensate line for kinks or standing water without disconnecting it.
  • Feel the registers after ten minutes. Warm air in some rooms and cold in others points to ducts rather than the furnace.

What not to do

  • Don't ignore a rotten-egg odor. Get everyone outside first and call the gas utility or 911 from there.
  • Don't open the burner compartment or try to relight anything by hand.
  • Don't keep cycling the power to force repeated ignition attempts.
  • Don't crawl into tight attic or crawlspace areas to reconnect ductwork yourself.

When professional evaluation may be appropriate

If the fan setting and filter are correct and the air stays cold, the burners are not lighting or not staying lit. That points to ignition, gas valve, pressure switch or control problems that need a technician.

Call to be connected with a local provider for furnace repair. Cold air in only certain rooms may call for ductwork repair instead.

  • Cold air continues with the fan on AUTO and a new filter
  • The furnace clicks repeatedly without lighting
  • Any rotten-egg smell near the equipment (leave, then call)

What an HVAC provider may inspect

  • Burner operation and ignition parts
  • Limit switch and airflow across the heat exchanger
  • Gas valve and supply pressure
  • Duct connections in attics and crawlspaces

Keeping it from happening again

Keep the fan on AUTO during heating season and change the filter before winter. Older homes in the region with crawlspace ducts benefit from an occasional duct inspection, since a disconnected run often goes unnoticed until the first cold snap.

Where to go next

Related serviceFurnace Repair

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Questions homeowners ask

Furnace Blowing Cold Air: questions homeowners ask

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Why does my furnace blow cold air before it gets warm?

At the start of a cycle, the burners need a moment to warm the heat exchanger. Many furnaces delay the blower for that reason, but a little cool air can still come through. If warm air follows within a minute or two, that is normal operation.

Should my thermostat fan be on AUTO or ON in winter?

AUTO is usually the right choice. It runs the blower only while the furnace is heating, so the air from the vents feels warm. ON keeps the blower running constantly, which can feel drafty and cool between heating cycles.

Why are some rooms warm and others cold?

Uneven heating often comes from duct issues such as disconnected runs in a crawlspace, closed dampers or blocked registers. Poor insulation in certain rooms can add to it. A provider can check duct connections and the airflow reaching each room.

Does a high-efficiency furnace have a drain?

Yes. High-efficiency gas furnaces produce water as they burn fuel, and it drains through a small plastic line. If that line clogs, a safety switch can stop the burners while the fan keeps running, which feels like cold air from the vents.

Can a furnace blow cold air because of the gas supply?

It can. A closed gas valve, a service interruption or a faulty valve inside the furnace will keep the burners from lighting. If you suspect a supply problem and smell gas at all, leave the home and call the gas utility or 911 from outside.

Is it dangerous if my furnace keeps blowing cold air?

Cold air by itself is usually not dangerous, but the cause may be a safety device shutting down the burners. Do not force repeated restarts. Make sure carbon monoxide alarms work, and call a provider if simple checks do not bring back warm air.