Why Is My Furnace Blowing Cold Air? Causes and Safe Checks

When your furnace runs but the air from the vents feels cool, the cause is often simple. Sometimes it is a thermostat setting. Other times the furnace has shut off its burners to protect itself. In a Minnesota winter, you want to find out which one quickly. This guide covers the common causes, the checks you can safely do yourself, and the signs that mean you should stop and call for service.

Common causes of a furnace blowing cold air

• The thermostat fan is set to ON. In the ON position, the blower runs all the time, even when the burners are off. Between heating cycles, the air coming out of the vents will feel cool.
• The furnace just started. Many furnaces wait a short time after the burners light before the blower starts, and the first air through the ducts may be cool. If warm air follows within a minute or two, the furnace is working normally.
• A dirty air filter. A clogged filter restricts airflow. The heat exchanger can overheat, and a safety switch called the high limit switch turns the burners off while the blower keeps running.
• A dirty or failed flame sensor. The flame sensor confirms the burners are lit. If it is coated with residue, the furnace may light, then shut the gas off within seconds and try again.
• A failed igniter. Most modern furnaces use a hot surface igniter. When it cracks or burns out, the burners cannot light.
• Blocked intake or exhaust pipes. High-efficiency furnaces vent through plastic pipes on an outside wall or the roof. Snow, ice or frost buildup can block them, and the furnace will shut down its burners.
• A clogged or frozen condensate drain. High-efficiency furnaces produce water. If the drain line or trap clogs, or if a line runs through a cold space and freezes, a safety switch can stop the burners.
• A gas supply problem. A closed gas valve or an interruption in gas service will leave the blower running with no heat.
• Leaky or disconnected ductwork. If cold air comes from only one or two vents, a duct may have separated in an attic, crawl space or garage.

What you can safely check yourself

These checks do not require tools or opening the furnace's burner compartment:

1. Check the thermostat. Make sure it is set to HEAT, the fan is set to AUTO, and the set temperature is a few degrees above the room temperature. Replace the batteries if the display is blank or dim.
2. Replace the air filter. Slide out the filter and hold it up to a light. If you cannot see light through it, replace it with the same size, which is printed on the filter frame. ENERGY STAR's maintenance checklist recommends checking your filter monthly.
3. Check the power. Make sure the furnace's service switch, which usually looks like a light switch on or near the furnace, is on. Check your electrical panel for a tripped breaker. If a breaker trips again after you reset it, leave it off and call for service.
4. Clear the vent pipes outside. If you have a high-efficiency furnace, find the two plastic pipes outside. Gently remove snow and ice from around them by hand or with a broom. Do not pound on the pipes or use a tool that could crack them.
5. Look at the condensate drain. Check whether water is pooling around the furnace or whether the drain line looks kinked or disconnected.
6. Read the status light. Many furnaces have a small LED on the control board that you can see through a window in the lower panel. Count the blinks and match them to the code chart on the inside of the panel door. Write the code down for your technician.
7. Restart once. After a fix such as a new filter or cleared vent, turn the furnace off at the thermostat for a few minutes, then turn it back on.

What not to do yourself

• Do not remove the burner compartment cover or try to light the burners by hand.
• Do not bypass or jumper a limit switch, pressure switch or rollout switch. These switches shut the furnace down to prevent fire and carbon monoxide hazards.
• Do not reset the furnace again and again. Repeated resets can hide a problem that needs repair.
• Do not handle wiring inside the cabinet. The furnace carries household voltage even when it is not heating.

Gas and carbon monoxide warning signs

If you smell gas, do not try to diagnose the furnace. Follow CenterPoint Energy's gas safety steps: leave the house right away on foot, do not use light switches, phones, lighters or anything that could cause a spark, and call 911 and your gas utility once you are safely away.

If your carbon monoxide alarm sounds, or if people in the home have headaches, dizziness, nausea or confusion, move everyone to fresh air and call 911. The U.S. Consumer Product Safety Commission's carbon monoxide guidance recommends CO alarms on every level of the home and outside sleeping areas.

How to prevent it next winter

Many cold-air calls trace back to a dirty filter, a dirty flame sensor or a clogged condensate drain. A fall tune-up gives a technician the chance to clean the flame sensor, check the igniter, flush the condensate drain and inspect the venting before the coldest weeks. A furnace maintenance visit each fall is the simplest way to catch these problems early.

When to call a professional

Call for service if the furnace still blows cold air after you have checked the thermostat, filter, power and vent pipes. Call right away if the furnace lights and shuts off repeatedly, if the status light shows a fault code, if a breaker trips again, or if the house is losing heat during sub-zero weather. Pipes can freeze when a home goes without heat in a Minnesota cold snap. Krinkie's offers furnace repair with 24-hour emergency service.

Common questions

Sources

• ENERGY STAR — Maintenance Checklist
• CenterPoint Energy Minnesota — Stay Safe (natural gas leaks)
• U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center