Appointment Lookup
802-552-4364
Discover what the Seasonal Energy Efficiency Ratio (SEER) is and why it's crucial for assessing air conditioner efficiency.
Furnace Blowing Cold Air
Common reasons for your furnace blowing cold air
The most common reasons for your furnace blowing cold air are a dirty air filter, faulty safety interlock switch or ignition system failures.
28% of the time it's the Dirty Air Filter
24% of the time it's the Failed Safety Interlock Switch
19% of the time it's the Ignition System Failure
29% of the time it's the Something else
Schedule your furnace repair now!
We can help! Our certified technicians repair all brands and models of furnaces. We can fix yours no matter where you bought it.
Chat
Loading scheduler options...
Furnace Blowing Cold Air — How to Troubleshoot and Fix It
When your gas furnace is blowing cold air instead of heat, it usually means one of three key components isn’t working as it should. The most common causes are:
- A dirty air filter
- A failed interlock switch
- A bad igniter
Let’s go through each issue and how to troubleshoot it safely.
1. Dirty Air Filter
A clogged air filter is one of the most common reasons a furnace stops heating.
The filter traps dust and debris to protect your blower and heat exchanger. When it becomes dirty, airflow through the system drops dramatically.
Why a dirty air filter stops your furnace from heating
- Restricted airflow causes the heat exchanger to overheat.
- When that happens, the furnace’s limit switch shuts off the burners to protect the system.
- Once the burners shut down, the blower continues running, sending cool air through your vents.
How to check and replace the furnace air filter
- Turn off power to the furnace at the switch or breaker.
- Locate the filter—usually behind a panel near the blower compartment or in the return air vent.
- Slide out the old filter and check it against a light source. If you can’t see light through it, it’s too dirty.
- Replace it with a new filter of the same size and airflow rating.
- Restore power and turn the furnace back on.
A clean filter allows proper airflow and ensures your furnace can heat efficiently again.
2. Failed Interlock Switch
Modern furnaces include several interlock or safety switches that prevent unsafe operation. Common examples include:
- Blower door switch — ensures the access panel is properly closed before the furnace starts.
- Pressure switch — verifies proper venting and airflow through the combustion system.
- Limit switch — shuts off burners if the furnace overheats.
Why a failed interlock switch prevents heating
If any interlock switch fails or doesn’t close properly, the furnace’s control board won’t allow the ignition sequence to start. The blower may still run, but the burners won’t light—resulting in cold air from your vents.
A technician can use a multimeter to test each switch for continuity and replace any that have failed.
3. Bad Igniter
The igniter is what lights the gas burners. Most modern furnaces use a hot surface igniter that glows red-hot to ignite the gas as it flows into the burner assembly.
Why a failed igniter stops the furnace from heating
If the igniter is cracked, weak, or has lost electrical continuity, it won’t get hot enough to light the gas.
When the control board detects this failure, it shuts off the gas valve to prevent unburned gas from building up.
The result: no heat.
A trained technician can test the igniter for resistance and replace it if it’s defective.
Why Choose Sears Home Services for Furnace Repair
When your furnace blows cold air, Sears Home Services is your trusted source for professional repair.
Our licensed technicians service all major furnace brands and carry the parts and expertise to fix heating problems fast.
We provide nationwide coverage, with local HVAC repair experts in major metro areas including:
- Chicago
- Dallas–Fort Worth
- Houston
- Los Angeles
- Miami
- Atlanta
- Denver
- Phoenix
- Seattle
- Minneapolis–St. Paul
- Washington, D.C.
- Philadelphia
- New York City
Whether you’re dealing with a dirty filter, a safety switch failure, or a bad igniter, our experts can diagnose and repair your furnace quickly—keeping your home comfortable and your system running safely.
Schedule Furnace Repair Today
Don’t wait for the temperature to drop further. Schedule furnace repair with Sears Home Services today and get your heating system working right again.
Sears Home Services is the trusted nationwide service provider for reliable furnace repair, maintenance, and home comfort solutions.
Frequently Asked Questions
Why is my furnace blowing cold air instead of heat?
A furnace that blows cold air usually has one of three problems: a dirty air filter restricting airflow, a failed interlock switch preventing ignition, or a bad igniter that can’t light the burners. Each of these issues can stop your furnace from heating properly.
How can a dirty air filter stop my furnace from heating?
When the air filter is clogged with dust, it restricts airflow through the furnace. This causes the heat exchanger to overheat, triggering a safety limit switch that shuts off the burners—leaving only the blower running and pushing out cold air.
What does an interlock switch do in a furnace?
Interlock switches are safety devices that ensure proper operation. They verify that doors are closed, airflow is correct, and temperatures are safe. If an interlock switch fails, the furnace’s control system won’t allow the burners to ignite, preventing the furnace from heating.
Why should I choose Sears Home Services for furnace repair?
Sears Home Services offers expert furnace repair nationwide with certified technicians who service all major brands. With coverage in major metro areas like Chicago, Dallas, Los Angeles, and New York, Sears delivers fast, reliable heating repair when you need it most.
Repair HVAC Resources
Learn how an air filter works in your HVAC system and how to choose the right filter.
Improve indoor air quality and protect your family's health with HVAC maintenance, air purifiers, and natural solutions like plants and proper ventilation.
Wondering what it costs to replace an air conditioner in 2026? This guide explains average replacement pricing, SEER2 efficiency costs, and how to decide whether to repair or replace your AC system.
Glossary Terms
The 608 Certification, mandated by the Environmental Protection Agency (EPA), is required for HVAC technicians to legally handle refrigerants. It ensures technicians understand refrigerant types, environmental impact, and proper handling techniques.
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume, essential in various systems including refrigerators, air conditioners, and HVAC units for cooling and refrigeration processes.
A condenser is a component of HVAC and refrigeration systems, responsible for releasing absorbed heat from the refrigerant into the outside air or a water line, thus facilitating the cooling process.
A capacitor in an AC unit is an electrical component that stores and releases electrical energy to help start the motor and stabilize the voltage during operation.