Furnace Blowing Cold Air? Causes and Fixes

By Ali Manafi
Retired air conditioning technician · 20 years’ experience · former NATE-certified technician

A furnace that runs but blows cold air is one of the most common winter calls we get in Las Vegas. Sometimes it’s a setting you can fix in seconds; sometimes it’s a part that needs a technician. Here are the usual causes, how to tell them apart and what to do about each one.

First: is it really a problem?

Two situations look like a broken furnace but are completely normal:

• The first minute of a heating cycle. Many furnaces push out a little cool air from the ducts before the warm air arrives, especially when the ducts run through a cold attic. Give it two or three minutes.
• A heat pump in defrost mode. On cold mornings a heat pump periodically reverses to melt frost off the outdoor unit. You may feel cooler air and see steam outside for a few minutes. That’s normal.

Nine common reasons a furnace blows cold air

1. The thermostat fan is set to “ON”

With the fan on ON, the blower runs all the time, including between heating cycles, so you feel room-temperature (cool) air from the vents. Switch it to AUTO. Also confirm the thermostat is set to HEAT and is configured for your type of system: a thermostat set up for a heat pump can’t run a gas furnace properly, and vice versa.

2. A clogged air filter is overheating the furnace

A dirty filter chokes the airflow. The heat exchanger overheats, the high-limit switch shuts the burners off to protect it, and the blower keeps running to cool things down, blowing cool air at you. Replace the filter and give the furnace a few minutes. If it keeps happening with a clean filter, call us: a weak blower or blocked ductwork can cause the same thing (see why a furnace keeps shutting off). (Which filter to buy.)

3. The furnace isn’t lighting: ignitor or flame sensor

Modern furnaces light with a hot-surface ignitor, and a flame sensor confirms the burners lit. If the ignitor is cracked, or the flame sensor is dirty, the furnace tries to light, gives up, and may keep the blower running. You might hear clicking or see the burners light briefly and go out. Both are common, relatively inexpensive repairs, usually at the lower end of our repair range.

4. The pilot light is out (older furnaces)

Older natural-draft furnaces in Las Vegas homes built before the 1990s may still use a standing pilot. If it’s out, there’s no flame to light the burners. If you’re comfortable following the relighting instructions on the furnace label, you can try once. If it won’t stay lit, the thermocouple is usually the culprit. Call us.

5. No gas reaching the furnace

Check that the gas valve on the line to the furnace is open (the handle runs parallel to the pipe) and that other gas appliances, like the stove or water heater, work. If nothing gas-powered works, contact your gas utility. If only the furnace is affected, its own gas valve may have failed.

6. A clogged condensate drain on a 90% furnace

High-efficiency (90%+) furnaces make water as they run and drain it through a small line. If that line or its trap clogs (algae and debris are common), a safety switch stops the burners while the blower may keep running. Signs include water near the furnace or an error code. A technician can clear the drain and check the pressure switch.

7. A tripped flame rollout or limit switch

Rollout switches shut the furnace down if flames escape where they shouldn’t, often because of a blocked flue or a cracked heat exchanger. Don’t keep resetting a rollout switch. If it trips, something is wrong, and it’s a safety issue for a technician.

8. Leaky or disconnected ductwork in the attic

If warm air comes out of some vents but others blow cool, a duct may be crushed, torn or disconnected in the attic, pulling in cold attic air or dumping warm air where it does no good. Las Vegas attics see extreme summer heat, which is hard on flex duct over the years.

9. Heat pump problems

If you have a heat pump rather than a gas furnace, cool air outside of defrost mode can mean low refrigerant, a stuck reversing valve, a failed defrost control, or backup (auxiliary) heat that isn’t coming on. These all need a technician with the right gauges and tools.

Read the furnace’s error code

Most furnaces made in the last 20 years have a small LED on the control board, often visible through a little window in the lower door. It blinks a pattern (for example, 3 blinks, a pause, 3 blinks) that matches a code printed on a label inside the door. Write it down before you call. It helps our technician arrive with the right part.

When to stop and call a professional

• You smell gas (leave first, then call the gas company from outside)
• The carbon monoxide alarm sounds (get everyone out, pets too, and call 911)
• A rollout switch has tripped, or you see scorch marks around the burners
• The furnace still blows cold after the checks at the top of this page
• You hear loud banging when the burners light (delayed ignition)

We’re open 24/7, holidays included, with an average emergency response under one hour. The $75 diagnostic is free when you go ahead with the repair, and most repairs run $250 to $1,000. You get the exact price before any work starts. See furnace repair in Las Vegas.

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