Why is my furnace blowing cold air?
By Amelia Devall•Last updated Sep 14, 2023
Last updated Sep 14, 2023
The arrival of cool weather may mean enjoying hot drinks and comfy knitwear — but it most definitely doesn’t include enjoying your furnace blowing cold air.
So, why is your heating system not creating a warm and toasty home? Here are the most likely reasons this happens and how you can fix it.
1. Thermostat is set incorrectly or malfunctioning
Whether your thermostat is digital or mechanical, you should always check the fan setting, no matter what temperature you select. Make sure the fan is set to AUTO instead of ON.
When the fan is on, it keeps blowing air even after the room reaches the set temperature. That air may not be warmed, so what seems like a furnace blowing cold air may actually be a thermostat setting issue.
There are other thermostat problems you may be able to fix yourself. For example:
- Batteries may be dead and need to be replaced.
- The thermostat may have been bumped and need to be leveled again.
- Some thermostat components may be dirty or stuck. Follow the disassembly instructions in your manual.
Pro tip: Before you rule out thermostat problems, make sure it’s placed away from drafts or direct sunlight and that no furniture is blocking it.
2. Faulty electronic controls
Newer gas furnaces have electronic control panels that let you reset the system if it malfunctions.
If resetting doesn’t solve the problem, look for other signs of faulty electronics, such as LED warning lights. The blinking pattern works like a visual error code and can help you diagnose issues shown on your furnace schematic, such as “low voltage.”
Problems with electronic controls are usually caused by wiring that has come loose from vibrations or by 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 small flame is still burning, since gusts of air can easily blow it out.
If the pilot light is sheltered from drafts and this keeps happening, the problem may be a faulty thermocouple.
While you can relight the pilot light yourself by carefully following the manufacturer’s instructions, it’s usually best to play it safe and contact a professional. 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
Dirty or clogged air filters can restrict airflow in your home and even cause overheating that shuts the furnace down and stops warm air from blowing. This is called short cycling.
The furnace filter is usually located 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 help prevent early failure of your equipment.
If your furnace doesn’t restart and blow warm air after you replace the air filter, contact an HVAC technician. They may need to check the high-limit switch and reset it.
Pro tip: If overheating caused the furnace malfunction, you must wait for it to cool down before it can function again and blow warm air.
5. Damaged air ducts
Air ducts are usually made of sheet metal, which isn’t exactly hole-proof, especially if rodents are around. Damage to ductwork can also happen because of misalignment or repeated exposure to extreme temperatures. If there’s a leak in your ductwork, it can affect a room’s temperature even if the furnace itself is heating the air properly.
To improve the efficiency of your gas furnace, inspect the air ducts end to end and seal any holes and cracks. Use mastic sealant or metal tape. Never use duct tape, since it will come undone easily.
Duct cleaning is also something you can add to your maintenance checklist. However, consistent structural damage that leads to heat loss will need to be addressed by an expert in furnace repair.
6. Gas supply problems
Aside from thermostat and pilot light issues, a furnace that only blows cold air could indicate a problem with the gas supply line.
Whether the gas valve is stuck or there’s a more serious issue, an inadequate supply may cause the furnace to shut down to prevent hazardous situations.
Make sure the gas valve is open and clean, and check whether 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 suspect a leak, evacuate your home immediately and call your local emergency services.
7. Clogged condensate lines
High-efficiency furnaces often have condensate drain lines that help remove moisture. These are usually PVC pipes that can get clogged with ice, mold, or dirt, which causes water to back up and shuts the furnace down to prevent damage. If you notice standing water around the furnace unit, clogged condensate lines are likely the cause.
The short-term solution is to turn off the power and remove the water in the drain pan with a vacuum. Then follow the drainpipe to the exit and use your vacuum on that opening to clear the clog.
The long-term solution is to make sure drainage is set up properly. For example:
- Make sure the drainpipe slopes forward.
- Check that all condensate-producing components are properly sloped toward the drain.
- To help prevent ice blockages, make sure the pipes don’t run through areas exposed to freezing conditions.
Regular furnace maintenance is key
Routine HVAC maintenance and cleaning can help prevent furnace issues. This good habit can keep cold-air problems at bay and help prevent costly repairs later.
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 every year, you can help reduce the chance of breakdowns, lower your heating bills, and extend the life 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 having with your furnace and ask for price quotes.
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.
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