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  • Cozy Heating Inc.

Why is my furnace blowing cold air?

Why is my furnace blowing cold air when it is set to heat?

Does this mean it’s time to replace my furnace?

What should be done to get my furnace to blow hot air again?


There are a few different reasons your furnace is blowing cold air. When your furnace is blowing cold air it could be a result of an improper thermostat setting, a dirty air filter, a dirty or malfunctioning ignition system, or something else entirely. Some units blow cold air in a call for heat to tell you it has a fault code stored and needs attention, in which you would want to hire a professional HVAC technician to come out and diagnose.

To figure out what’s causing the cold air, you need to determine when  cold air is blowing. Here are some things to watch for:

  • Does it happen immediately after starting up and then gradually get warmer? 

  • Does it blow cold air sometimes and warm air at other times?

  • Does it blow cold air consistently?

Let’s look into a few possible causes and solutions for each of these situations….

A FURNACE THAT BLOWS COLD AIR BUT EVENTUALLY WARMS UP The most common cause of this scenarios would be a delayed ignition - What is delayed ignition? Your furnace has either a hot surface ignitor or spark ignitor. If they do not work correctly, the gas does not ignite. It may be taking multiple tries to light the gas to produce heat. An ignitor can be worn out and not reach proper ignition temperatures or be a dirty spark rod that cannot ignite the gas at the proper points. Even dirty burners can cause an interruption in air flow.

Do this: Call a HVAC technician, who will test and replace it if necessary.



The most common cause of this would be that the thermostat fan being set to ON - Furnaces run in heating cycles and only produce hot air for short bursts of time. When your fan setting on your thermostat is set to AUTO, the fan only blows when the furnace is heating the air.

If your thermostat is set to ON instead of AUTO, the blower motor will run constantly, even when the furnace isn’t actively heating the air. 

Do this: Check your thermostat setting. If it is set to ON, switch it to AUTO. 

POSSIBLE CAUSE #2: FURNACE OVERHEATING If your thermostat is set to AUTO and your furnace is blowing hot and cold air intermittently, your furnace is most likely overheating - When a furnace overheats, a safety switch shuts off the heat gas and the fan continues to blow to cool it down. As the heat exchanger is cooling down, you may feel cool air coming from your vents. 

Lack of airflow is the most common cause of an overheating furnace - When there is an insufficient amount of cool air blowing over the hot heat exchanger, the temperature of the heat exchanger will rise. When it reaches unsafe temperatures, a safety feature triggers the furnace to shut down the gas burners that warm up the heat exchanger. Low airflow in a furnace is caused by:

  • Dirty air filters. An air filter that is clogged with dust restricts airflow to the furnace.  Be sure to check your air filter monthly and replace it if it’s dirty.

  • Dirty components. If your air filter is clogged with dirt, there’s a good chance that the furnace components are dirty, as well. Dirty components (clogged ducts, dirty fan blades, dirty blower motor, etc.) restrict airflow to the heat exchanger and can also cause the motors driving the furnace to overwork and eventually overheat, all of which can lead to mechanical failure. 

  • Closed or blocked vents. To make sure airflow to your furnace is unrestricted, keep the vents throughout your home open (even in rooms you don’t use) and unobstructed by furniture or other items. 

  • Mechanical failure. Regular use will eventually wear down older components. Blower motor bearings and the Capacitors that start them are the main reasons for no airflow. Occasionally, a main control board can fail. That’s why it’s important to have an HVAC technician inspect and clean your furnace annually.

Do this: If you suspect your furnace is overheating, you will need to check your air filter and replace it if it’s dirty. Open any closed vents and remove obstructions from blocked vents. 

If your air filter is clean and your vents are open and unobstructed, contact an HVAC technician to diagnose your problem. 


It can be frustrating to have a furnace that is blowing cold air and never warms up, but don’t worry. This is most likely an easy fix. 

Possible cause #1: An improper thermostat setting

Furnace that consistently blows cold air with a strong/normal airflow - This is a common mistake homeowners make when the seasons change. If the homeowner hasn’t switched the thermostat to heat or raised the desired temperature of the home, the HVAC system may actually be triggered to cool the home and blow cool air (depending on the outdoor temperature and the set temperature on the thermostat).

Do this: If your thermostat is set to COOL, switch it to HEAT. 

Possible cause #2: Leaky ducts.

If you’re getting weak but cool airflow from your furnace and your thermostat is set to HEAT- It’s not uncommon for some warm air from the furnace to escape from the ducts into unheated areas of the home like the attic. However, if you feel a weak airflow that feels colder than it should coming from your furnace, you could have a major leak in your ductwork. When heated air from the furnace leaks into the attic, airflow from your vents is reduced and it might feel colder than it really is, even though the furnace is working fine.  

Do this: If you suspect you have leaky ducts, contact an HVAC technician who will inspect your ductwork and make a recommendation to seal or replace leaky ducts. 


A tripped breaker - A furnace breaker may trip if the furnace is overworked.


Do this: At the electrical panel, find the breaker marked HVAC or furnace and firmly press it off, then firmly press it on. 

If your breaker trips again, call an HVAC technician to diagnose the issue. A furnace breaker that keeps tripping is a signal of a larger issue with the electrical components. 

Other issues that will prevent your furnace from turning on:

  • The furnace is switched off. There is a power switch on your furnace that may be switched off. 

  • No power to the thermostat. If your thermostat is not receiving power, it cannot signal the furnace to turn on.

  • A triggered safety switch. Safety features signal your furnace to turn off when there’s a malfunction. 

***If you’re struggling with a furnace that won’t turn on, contact an HVAC professional to help resolve the problem. 

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