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common reasons why furnace is blowing cold air

When the winter season arrives, the furnace becomes the most essential and heavily used heating system in many households. It keeps the indoor temperature warm and comfortable, making winter days and nights bearable, if not enjoyable. As homeowners, having a properly functioning furnace is vital not only for comfort but also for safety. However, what happens when a furnace starts blowing cold air instead of warm air?

This article will discuss some common reasons why a furnace may blow cold air and what homeowners can do to fix and prevent these issues.

Thermostat Issues

The thermostat plays a critical role in the operation of the furnace. When the temperature in the room drops below the set temperature, the thermostat sends a signal to the furnace to turn on. Once the furnace starts running, it continues to run until the temperature reaches the set point, at which point the thermostat signals the furnace to switch off.

Even a slight problem with the thermostat can cause significant issues with the furnace. Here are some of the common reasons why a faulty thermostat may cause the furnace to blow cold air:

Incorrect Temperature Setting

If the thermostat is set to the wrong temperature, the furnace may not operate as intended. For instance, if the thermostat is set to ‘cool’ instead of ‘heat,’ the furnace will blow cold air since it’s trying to function as an air conditioner. In this case, you only need to adjust the thermostat to the correct setting.

Dirt or Dust Accumulation

Dirt or dust accumulation can cause the thermostat to malfunction. It may cause incorrect temperature readings, signalling the furnace to blow cold air. Regular cleaning of the thermostat can help prevent this issue.

Sensor Placement

In some cases, the thermostat sensor may be placed in an incorrect location, causing it to give inaccurate temperature readings. For example, if the sensor is placed too close to a window or a heat source, it may detect the temperature incorrectly and signal the furnace to blow cold air. In this case, the sensor must be appropriately placed in a location that accurately reflects the room’s temperature.

Clogged Air Filter

Air filters serve an essential function in the operation of a furnace by trapping airborne particles like dust, pollen, and pet dander. The furnace’s blower motor forces air through the filter, which traps these particles before they can circulate through the rest of the house. Air filters help improve indoor air quality and promote better respiratory health for home residents.

When an air filter becomes dirty, it can restrict airflow to the furnace, causing it to overheat. As a safety mechanism, the furnace’s heat exchanger gets too hot, forcing the burner to shut off. When this happens, the furnace will continue to blow air, but it will be cold because the burner is no longer on.

If a homeowner neglects to replace or clean the air filter regularly, they can become clogged with dirt and debris, causing airflow restriction. It can potentially damage a furnace’s heat exchanger, leading to expensive repairs or premature replacement of the furnace.

Pilot Light Issues

Furnaces with standing pilot lights have a small flame that stays lit continuously. This flame serves as an ignition source for the furnace’s gas burner. When the thermostat signals for heat, the furnace’s gas valve opens, and gas flows into the burner, where the pilot light’s flame ignites it. The heat produced by the burning gas is then distributed to the home through ductwork.

If the pilot light goes out, the gas burner will not ignite, causing the furnace to blow cold air. Several factors can cause a pilot light to go out, including:

  • A faulty or dirty thermocouple
  • Problems with the gas supply
  • Strong drafts in the home

Faulty Igniter

Newer furnaces use electronic ignition systems instead of standing pilot lights for fuel ignition. This system consists of an igniter and an electrode that senses the flame from the ignited gas and signals for the furnace’s main burner to come on. If either the igniter or electrode fails, it can prevent the main burner from coming on, resulting in cold air from the furnace.

A faulty igniter or electrode can cause several issues that may prevent your furnace from producing heat:

  • The most common issue is that it prevents the gas valve from opening and allowing gas into the burner, which limits airflow through the furnace and affects its ability to produce heat.
  • A faulty igniter or electrode may also send inaccurate readings to the thermostat, which can cause temperature fluctuations or inaccurate readings in general.
  • There may be insufficient power reaching the unit, preventing it from operating correctly.

Dirty Flame Sensor

The flame sensor is an essential safety feature in modern furnaces with standing pilot lights or electronic ignition systems. As its name implies, it senses when a flame is present in the furnace’s gas burner and then sends a signal to the gas valve to keep it open while it operates. It prevents any dangerous buildup of unburned fuel inside your home, potentially leading to an explosion or fire hazard if left unchecked.

A dirty flame sensor cannot accurately sense when there’s a flame in the gas burner, which causes it to mistakenly send a signal for the gas valve to close before all the fuel has burned off, effectively shutting down your furnace prematurely as soon as it turns on! It can leave you with cold air coming out of your vents instead of warm air from your heater due to insufficient heat produced by your furnace.


All of the issues discussed above can lead to cold air coming from your furnace instead of warm air, and it’s essential to resolve them as soon as possible. If you’re experiencing any of these problems with your furnace, contact a qualified technician immediately for proper diagnosis and repair. Taking care of the issue now will help prevent further damage down the line and ensure that your furnace is operating safely and efficiently.

Your family’s comfort is essential, so don’t delay any longer when resolving furnace issues. Contact a professional technician today for help!

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