Furnace Blowing Cold Air? Troubleshoot the Flame Sensor

The flame sensor is a crucial component in a furnace that detects the presence of a flame in the burner. It is designed to shut down the furnace if it does not detect a flame while the gas valve is operating, thereby preventing the flow of natural gas or propane into the home’s air supply. A malfunctioning flame sensor can cause the furnace to shut down even when the burner is working correctly, resulting in cold air blowing from the vents.

Inspect the Flame Sensor

One way to determine if a malfunctioning flame sensor is causing the furnace to blow cold air is to visually inspect the flame sensor for cracks in its porcelain base or soot buildup on its tip. A clean flame sensor with no cracks or soot buildup can still be malfunctioning, and it may be best to contact an expert to determine the cause of the problem.

Flame Sensor Inspection Checklist:

1. Locate the Flame Sensor: The flame sensor is typically located near the burner assembly and is easily accessible.
2. Inspect the Porcelain Base: Check the porcelain base of the flame sensor for any cracks or chips. Cracks can prevent the sensor from making proper contact with the flame, leading to a malfunction.
3. Check for Soot Buildup: Examine the tip of the flame sensor for any soot or carbon buildup. This can insulate the sensor from the flame, preventing it from detecting the presence of the flame.
4. Measure Resistance: Use a multimeter to measure the resistance of the flame sensor. The resistance should be within the manufacturer’s specified range, typically between 10-50 ohms.
5. Test the Flame Sensor: Use a flame sensor tester or a multimeter to test the flame sensor for proper operation. The flame sensor should generate a small electrical current when exposed to a flame.

If the flame sensor is not functioning properly, it may need to be cleaned or replaced.

Maintain the Air Filter

A dirty air filter can also cause the furnace to blow cold air. Lack of filter maintenance can lead to build-up of dirt, dust, and other airborne pollutants that can clog the air filter, restrict airflow, cause discomfort, or in more extreme cases, cause system failure. Replacing a clogged air filter is a relatively simple task that requires only basic tools.

Air Filter Maintenance Recommendations:

1. Check the Air Filter Regularly: Inspect the air filter every 1-3 months, depending on the manufacturer’s recommendations and the level of dust and debris in your home.
2. Replace the Air Filter: Replace the air filter when it appears dirty or clogged. Use the correct size and type of filter recommended by the furnace manufacturer.
3. Clean the Filter: If the filter is reusable, clean it according to the manufacturer’s instructions. This may involve vacuuming, washing, or tapping the filter to remove accumulated dirt and debris.
4. Ensure Proper Airflow: Make sure the new or cleaned air filter is installed correctly, with the airflow arrow pointing in the right direction.

Maintaining a clean air filter can help ensure proper airflow and prevent the furnace from blowing cold air.

Troubleshoot the Furnace Controls

In some cases, the problem may be with the computerized controls of the furnace. Most modern furnaces follow a specific sequence of operations, and a problem with the controls can cause the furnace to blow cold air.

Furnace Control Troubleshooting Steps:

1. Check the Thermostat: Ensure that the thermostat is set to the correct temperature and that it is calling for heat.
2. Inspect the Control Board: Examine the furnace’s control board for any signs of damage, such as burnt components or loose connections.
3. Test the Limit Switch: The limit switch is a safety device that shuts off the furnace if the heat exchanger becomes too hot. Test the limit switch for proper operation.
4. Verify the Sequence of Operations: Review the furnace’s sequence of operations, as outlined in the manufacturer’s instructions, to ensure that each component is functioning as intended.
5. Consult the Manufacturer: If you are unable to identify the problem with the furnace controls, contact the manufacturer or a qualified HVAC technician for assistance.

Addressing any issues with the furnace’s computerized controls can help resolve the problem of the furnace blowing cold air.

Perform Routine Maintenance

When faced with a furnace blowing cold air, it is essential to perform routine maintenance, including cleaning the flame sensor, burner, and grounds, inspecting the heat exchanger and the plate around the induced draft motor for any holes, and taking data readings such as Delta T, P gas pressure, and amp draws. These steps can help identify the problem and prevent future issues.

Furnace Maintenance Checklist:

1. Clean the Flame Sensor: Use a soft cloth or brush to remove any dirt or debris from the sensing element. Avoid using abrasive materials or solvents that can damage the sensing element.
2. Clean the Burner: Remove any dirt, dust, or debris from the burner assembly to ensure proper combustion.
3. Check the Grounds: Inspect the furnace’s electrical grounds for any signs of corrosion or damage.
4. Inspect the Heat Exchanger: Look for any cracks, holes, or other damage in the heat exchanger that could be causing the furnace to blow cold air.
5. Check the Induced Draft Motor: Examine the plate around the induced draft motor for any holes or damage that could be affecting the airflow.
6. Take Data Readings: Measure the Delta T (temperature difference between supply and return air), P gas pressure, and amp draws to identify any potential issues with the furnace’s operation.

By performing these routine maintenance tasks, you can help ensure that your furnace is operating efficiently and safely, and prevent future problems with the flame sensor or other components.

In summary, a malfunctioning flame sensor, a dirty air filter, and problems with the computerized controls can cause a furnace to blow cold air. Regular maintenance and inspections, including cleaning the flame sensor, replacing the air filter, and troubleshooting the furnace controls, can help identify and resolve these issues. By following the steps outlined in this guide, you can keep your furnace running smoothly and ensure that your home stays warm and comfortable.

References:
– DIY Stackexchange – Furnace Blowing Cold Air
– Carrier – Furnace Blowing Cold Air
– HVAC-Talk – Can this Flame Sensor be Bad?