Why is my furnace blowing cold air?

1. Thermostat is set incorrectly or malfunctioning.

Whether your thermostat is digital or mechanical (and regardless of the temperature you select on it), you must always pay attention to the fan setting. Specifically, make sure the fan is set to AUTO instead of ON. 

When the fan is on, it continues to blow air even after the room reaches the right temperature. That air may not be warmed. So what seems to be a faulty furnace blowing out cold air is, in fact, a thermostat fan setting that increases your discomfort.

There are other possible issues with the thermostat that you may be able to easily fix on your own. For example:

• Batteries have run out and need to be replaced.
• The thermostat was accidentally nudged and needs to be leveled again.
• Some thermostat components may have gotten dirty or stuck. Follow the disassembly instructions in your manual.

Pro tip: Before you completely rule out thermostat problems, make sure it's placed away from drafts or direct sunlight and that no furniture blocks it.

2. Faulty electronic controls.

Newer gas furnaces have electronic control panels that allow you to reset them in case there's a malfunction.

If resetting doesn't solve your problem, check for other signs that might indicate faulty electronics, such as LED warning lights. The light blinking pattern is like a visual error code that can help you diagnose the issues presented in your furnace schematic, such as "low voltage."

It's worth noting that problems with electronic controls are usually caused by wiring coming loose due to vibrations or fuses that are too small or have become dirty.

3. Pilot light is out.

Older gas furnace models have a constantly burning pilot light, also known as a "standing pilot." If you have an older furnace, check that this tiny flame is still burning, as gusts of air can easily blow it out. 

If the pilot lights are sheltered from drafts and this is a recurring problem, the culprit may very well be a faulty thermocouple. 

While you can relight the pilot light yourself (by carefully following the manufacturer's instructions), it is usually best to play it safe and contact a professional about it. They may need to replace the pilot light. Or, you may want to upgrade to a newer furnace without a pilot light.

4. Dirty or clogged air filter.

Clogged and dirty air filters can restrict airflow in your home and even cause overheating to the point where it shuts down and stops blowing warm air. This occurrence is called short cycling.

The furnace filter is usually situated behind the blower, and it can be easily replaced with a new one. Make a habit of checking it every month during winter and summer. Clean or replace it at least every three months to prevent early failure of your equipment.

If your furnace doesn't restart and blow out warm air after replacing the air filter, contact an HVAC technician. They may need to take a look at the high-limit switch and reset it.

Pro tip: If overheating was the cause of the furnace malfunction, you must wait for it to cool down before it can once again be functional and blow warm air.

5. Damaged air ducts.

Air ducts are usually made of sheet metal, which isn't exactly hole-proof — especially if there are rodents in the area. Damage to the ductwork can also happen because of misalignment or repeated exposure to extreme temperatures. If there's a leak in your ductwork, it will affect a room's temperature, even if the furnace itself is doing its part and properly heating the air.

To increase the efficiency of your gas furnace, perform an end-to-end inspection of the air ducts and seal all the holes and cracks. Use mastic sealant or metal tape. Never use duct tape, as it will come undone easily.

Duct cleaning is also something you can put on your maintenance checklist. However, consistent structural damage leading to heat loss will have to be addressed by an expert in furnace repair.

6. Gas supply problems.

Thermostat and pilot light aside, the fact that your furnace is only blowing cold air could indicate an issue with the gas supply line.

Whether the gas valve is stuck or there's a more serious problem, an inadequate supply might make the furnace shut down to prevent hazardous situations.

Make sure the gas valve is open and clean, and check if other gas appliances in your home (such as stoves or water heaters) are working properly. You may need to call a professional to inspect and fix your gas supply issues.

Note: If you smell gas or otherwise suspect a leak, evacuate your home immediately and call your local emergency services.

7. Clogged condensate lines.

High-efficiency furnaces tend to come with condensate drain lines that help remove moisture. These are usually PVC pipes that can get clogged with ice, mold, or dirt, which causes a water buildup that shuts down the furnace to prevent damage. If you notice standing water around the furnace unit, clogged condensate lines are likely the culprit.

The short-term solution involves turning off the power and removing the water in the drain pan with a vacuum. You should then follow the drainpipe to the exit, and use your vacuum on that opening to unclog it.

The long-term solution is ensuring proper drainage. For example, take these steps:

• Ensure the drainpipe itself is sloped forward.
• Check that all condensate-producing components are properly sloped toward the drain.
• To prevent ice blockages, make sure the pipes don't run through areas exposed to freezing conditions.

Related: Here's your HVAC troubleshooting checklist.

Regular furnace maintenance is key.

Routine HVAC maintenance and cleaning can help prevent furnace issues. This healthy practice will not only keep the cold air inconvenience at bay but will also prevent costly repairs further down the line.

When cleaning your furnace and HVAC system, make sure you address the following:

• Ducts and vents
• Air filters
• Blower fan and motor
• Flame sensor
• Combustion chamber

By checking your entire venting system and the integrity of the heat exchanger on an annual basis, you can help reduce the chance of breakdowns, lower your heating bills, and extend the lifespan of your system.

Find pros who can repair your furnace.

Is your furnace still blowing cold air? Do you need someone to inspect, maintain, and clean your HVAC system? Thumbtack pros can help.

Download the Thumbtack app today to see a list of top-rated HVAC pros near you. Describe the problems you're experiencing with your furnace, and ask for price quotes.

Related: 4 common AC problems (and how to troubleshoot them).

A note about project costs included or linked in this article: Figures represent national average cost estimates using data provided by Thumbtack pros and additional research. These figures are provided for educational purposes only and are subject to change at any time due to various factors. Details about your specific project and local rates can impact costs.