Furnace Blowing Cold Air Only At Night – What To Do?

A furnace is an appliance meant to keep you and your family warm, but you might have noticed it blows cold air at night. When this happens, you might be unsure of what to do. We asked HVAC experts, and here's what they advised. 

Your furnace is blowing cold air only at night because there are issues with either one or all of the following compartments:

  • Thermostat
  • Circuit control
  • Furnace burner
  • Flame sensors

What is required for you to do is to troubleshoot to find out which of these compartments is why your furnace blows cold air only at night. Then, remedy it by cleaning, repairing/resetting, or replacing the part.

Keep reading to learn more about how a furnace functions, how these compartments work, and how you can troubleshoot your furnace to identify and solve this problem. 

A home high energy efficient furnace in a basement, Furnace Blowing Cold Air Only At Night - What To Do?

How Does a Furnace Work? 

A furnace is a system designed to keep a residence of an area warm. A furnace works by retrieving cold air from the environment, heating this air up, and releasing heated air to the residence.

A home furnance in a basement color gray

This furnace activity only occurs by certain necessary operations performed within its system. They are as follows: 

  1. The operations start with the thermostat, which signals the furnace to begin functioning. 
  2. The operations continue to the burner, where propane or natural gas is ignited. 
  3. Next, the furnace's internal sensors need to ascertain that all burners have been ignited. 
  4. The burners now produce heat to the heat exchanger.
  5. The heat exchanger receives this heat, combines it with cold air taken from the surrounding environment into its chambers, and produces warm air. 
  6. Once the air is warm enough, the blower then begins to circulate this warm air throughout a residence while retrieving cold air from the residence's atmosphere back into the furnace through networks of passages. 

Why Is My Furnace Blowing Cold Air Only at Night? 

There are several reasons why your furnace is blowing cold air and only at night. The problem could originate from your thermostat, circuit control, furnace burner, and flame sensors. 


A thermostat, which is one of the essential regulatory components in a furnace, maintains a steady balance of the residence's temperature by detecting changes in the temperature within its environment.

When the temperature of a room drops or surpasses a particular level, the thermostat signals the blower to start functioning. Your furnace may be blowing cold air because your thermostat has been set to "ON" and not "AUTO." Thus, your blower keeps blowing air, even when your furnace has not been switched on.

On the other hand, your furnace may be blowing cold air because your thermostat has been set to cool. It is possible that you left the settings on cool since the last summer or the last heat period. 

Circuit Control

Circuit control of a furnace is very similar to a processing unit. It receives information from the thermostat and signals the correct information to the furnace. A bad circuit control could switch on the blowers of a furnace without activating the furnace's burners.

Furnace Burner

Your furnace may be blowing cold air if your burner is clogged with debris. This is because the thick substances prevent the burner from being ignited. If the burner gets clogged from time to time, your furnace could blow hot at certain times and cold at other times.  

Flame Sensor

A flame sensor is an essential component in a furnace. Issues with the flame sensor can interfere with the furnace's proper functioning. If your flame sensor is not in its proper state, the furnace may not produce heat to its coils.

Soon, the burner turns off after a while because the flame sensors are not functioning properly. Thus, the furnace works for a short while before stopping (blows hot air during the day and cold air at night). 

How to Stop My Furnace From Blowing Cold Air At Night

A person changing an air filter on a high efficiency furnace

Although issues with flame sensors are the usual causes of furnaces blowing hot and cold, however, it is not the only reason.

The best way to address this problem is by troubleshooting and then providing a solution to any specific problems identified. Here is a step by step process for troubleshooting your furnace:

Switch on Power to The Furnace

It is vital that you examine if the furnace is connected to the electrical supply. You would need to examine the power connection to ensure electricity is present.

Inspect the circuit control to ascertain its functioning and review the circuit breaker box to ensure it has not tripped off. You should connect your furnace to electricity if it is not connected. Repair or replace bad circuit controls and switch the circuit breaker back on if it has tripped off.


The next step is to examine the thermostat to see if it is set according to your needs. Your thermostat should be on the warm switch and on your preferred temperature.

Also, make sure that your thermostat is set on AUTO and not ON or OFF. If the thermostat appears to cause further problems, you should endeavor to correct or change it. 

Furnace Blower

You should examine your furnace blower to see if it functions when your thermostat turns on. If the furnace blower is not coming up, remove it for further examination. If it appears damaged, the services of a professional would be needed.

Air Filter

Examine the air filter to see if it is functioning properly. You may need to detach the air filter to examine it properly. To be maintained, an air filter would need to be cleaned from time to time and replaced every three months for the furnace's proper functioning.

How Do I Reset My Furnace? 

Technician doing some maintenance check on gas furnace

Resetting a furnace is much easier today because a reset button is attached in most cases. But the process of resetting a furnace is a bit more than just hitting the reset button. Below is a step by step procedure to reset your furnace: 

  1. Ensure you have the pilot light switched off. 
  2. While you should keep the pilot gas supply switched on, switch off the central gas supply passage to your furnace. 
  3. Reduce the thermostat's temperature setting to as low as it can be. 
  4. Turn off the circuit breaker from its box to cut off the electric supply to the furnace. 
  5. Light a match and hold it for the opening of the pilot flame jet to reignite the pilot light. 
  6. Switch the circuit breaker back on and reconnect the main gas supply. 
  7. Click the reset button located in the blower section and hold it for a while; at least 40 seconds would do. Then, set your thermostat to your desired temperature. 

Why Is My Furnace Not Getting Up To Temperature? 

Your furnace is not getting up to temperature because of your thermostat or flame sensors issues. 


Your thermostat is the regulatory component of your furnace, ensuring balancing in temperature levels. If your furnace is not getting up to temperature, your thermostat may be either dirty, malfunctioning, or not appropriately positioned. 

If your thermostat is dirty, you should clean it gently, as it can be easily subject to damage due to its sensitivity. Where it has been damaged, it should be replaced with a new one. 

Finally, if your furnace uses a mechanical thermostat, you should ensure that you position it properly and not against the wall. This is so that it reads the temperature in your room properly.  

Flame/Furnace Sensors

If your furnace is not getting up to temperature, it may be due to your flame sensors being dirty or bad. You may need to clean it properly if it is dirty. If it is malfunctioning because of damage, it is more beneficial to replace it. 

Where is a Furnace Sensor Located? 

Technician repairing the combi gas boiler with Screwdriver

It is important to know where a furnace sensor is located. You would need this information in troubleshooting and safety. Your furnace sensor is a slim bent metal situated on the assembly of the burner ahead of the pilot frame. 

How Do I Clean My Furnace Flame Sensor? 

Here is a detailed procedure of how you should clean a furnace flame sensor:

  1. Disconnect your furnace from the supply of gas and electricity.
  2. Locate the flame sensor on the burner assembly ahead of the pilot frame and remove it.
  3. Remove debris on the flame sensors.
  4. This could be done with the use of sandpapers or other abrasives. 
  5. Put the flame sensor back to its position and switch the furnace back on.


A home high energy efficient furnace in a basement

There are different possible reasons a furnace does not reach the right temperature or blows cold air at night. While there may be several causes, the major ones include issues with the thermostat and the flame sensors.

To learn more about furnaces, here are some related articles you should check out: 

Do Propane Furnaces Have a Pilot Light? 

Furnace Pilot Light Keeps Going Out - What To Do? 

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