Video: Understanding and Resolving the Issue of Furnace Blowing Cold Air and Won't Shut off
Figure 1: Furnace fan blower
Winters bring a sense of coziness as we rely on our furnaces to keep our homes warm and comfortable. However,
discovering that your furnace is blowing cold air and refusing to shut off can be a cause for concern.
In this in-depth guide, we'll delve into the potential reasons behind this issue and explore effective solutions to
get your furnace back to optimal functioning.
1. 11 Possible Reasons for a Furnace Blowing Cold Air and Won't Shut off and What to Do
1.1 Incorrect Thermostat Settings
One of the most common reasons for a furnace blowing cold air and not shutting off is incorrect thermostat settings.
The thermostat acts as the control center for your heating system, and if it's not set correctly, it can lead to
undesirable outcomes.
Solution:
Check thermostat settings: Ensure that your thermostat is set to the desired temperature for heating. Verify that
the mode is set to "heat" rather than "cool" or "fan only." If the thermostat is programmable, review the programmed
settings to rule out any conflicting instructions.
1.2 Faulty Thermostat
A malfunctioning thermostat can cause the furnace to behave erratically, including blowing cold air when it should
be providing warmth.
Solution:
Thermostat calibration or replacement: If you suspect an issue with your thermostat, try recalibrating it according
to the manufacturer's instructions. If problems persist, consider replacing the thermostat with a new, compatible
model.
Figure 2: Digital programmable thermostat.
1.3 Clogged Air Filter
A clogged or dirty air filter can restrict airflow, causing the furnace to overheat and potentially blow cold air.
Solution:
Regularly replace the air filter: Check the condition of your furnace's air filter and replace it if it's dirty or
clogged. A clean filter ensures proper airflow, preventing the system from overworking and blowing cold air.
1.4 Malfunctioning Limit Switch
The limit switch is a safety feature that regulates the temperature inside the furnace. If it malfunctions, it may
fail to shut off the furnace at the appropriate temperature.
Solution:
Professional inspection and replacement: A malfunctioning limit switch requires the expertise of an HVAC
professional. They can assess the switch's condition and replace it if necessary to restore proper furnace function.
1.5 Pilot Light or Ignition Issues
If your furnace relies on a pilot light or ignition system, problems with these components can lead to cold air
blowing from the vents.
Solution:
Check the pilot light or ignition system: Ensure that the pilot light is lit, or if your furnace has an electronic
ignition system, check for error codes. If you're unsure or encounter issues, consult the furnace's manual or seek
professional assistance.
Figure 3: Man inspecting the furnace.
1.6 Faulty Thermocouple
The thermocouple is a safety device that detects the pilot light's presence. If it's faulty, the furnace may not
stay on, leading to cold air circulation.
Solution:
Replace the thermocouple: A malfunctioning thermocouple should be replaced. This is a relatively simple task, but if
you're not comfortable doing it yourself, seek assistance from a qualified technician.
1.7 Issues with the Gas Supply
In gas furnaces, issues with the gas supply, such as a closed gas valve or a gas leak, can result in the furnace
blowing cold air and not shutting off.
Solution:
Check the gas supply: Ensure that the gas valve supplying the furnace is open. If you suspect a gas leak, evacuate
your home immediately and contact your gas utility provider and emergency services.
1.8 Electronic Control Board Malfunction
The electronic control board governs the furnace's operation. If it malfunctions, it can lead to erratic behavior,
including the continuous blowing of cold air.
Solution:
Professional inspection and replacement: An HVAC professional should inspect the electronic control board for any
signs of malfunction. If necessary, they can replace the board to restore proper functionality.
Figure 4: Furnace circuit control board.
1.9 Thermostat Wiring Failure
Inspect the Thermostat Wiring: Examine the thermostat wiring to ensure there are no loose or disconnected wires. If you find any issues, consider consulting the thermostat manual or seeking professional help.
1.10 Flame Sensor Failure
Inspect the Flame Sensor: If you have a gas furnace, the flame sensor might be dirty or malfunctioning. Clean the flame sensor with a fine abrasive pad or replace it if necessary.
1.11 Ductwork Leaks
Check the Ductwork: Inspect the ductwork for leaks, loose connections, or blockages. Seal any leaks with duct tape or mastic and ensure that the ducts are properly connected.
Figure 5: Gas furnace in a basement.
2. Conclusion
A furnace blowing cold air and refusing to shut off can be a complex issue with various potential causes. While some
troubleshooting can be done by homeowners, it's essential to know your limits and seek professional assistance when
needed.
Regular maintenance and timely repairs are key to ensuring your furnace operates efficiently, keeping you warm and
comfortable throughout the winter months. If you encounter persistent issues, don't hesitate to reach out to a
qualified HVAC technician to diagnose and address the problem effectively.
