The last thing you want on a cold morning is to turn on your furnace for the first time and realize it’s not working. When you feel cold air blowing from your furnace, it can be due to a number of issues: Some of these issues are an easy fix, but others you may need a professional’s help with.

Read on to find out some common reasons why your furnace may be blowing cold air! 


1. You feel a burst of cool air out of your vents as your furnace starts.

If you feel cool air coming from your vents only as the furnace kicks on, this can be a normal part of furnace operation. Your furnace is simply pushing out the cool air that was sitting in the vents before you start to feel warm air. 

2. The Fan Setting on Your Thermostat is set to “On.”

Check your thermostat settings. If the fan is set to “ON,” it will continuously blow air. When it is set to “AUTO,” your HVAC will only blow air when the furnace is on. You may feel cold air coming from your vents if the fan is on but your heat is not.

How to Check & Change the Fan Setting On Your Thermostat

  1. Go to your thermostat. 
  2. Make sure the fan switch is set to “AUTO” mode, not “ON”. This switch is usually located at the bottom of the panel. See photo for reference: 
Diagram showing the "on" and "auto" fan settings on a thermostat.
When the fan is in AUTO mode, your furnace should only run the fan during a heating cycle and not push unheated air through your vents.


3. Your Furnace Filter needs Replacing.

A dirty air filter makes it difficult for your furnace to circulate air. When the furnace doesn’t have air movement, the internal temperature of the furnace becomes too hot and the limit switch (designed to shut off the furnace when it overheats) automatically shuts off. The best thing you can do when you notice insufficient warm air or cold air is pushing through your vents is to replace your furnace filter. 

Why a clean furnace filter matters: The filter is a vital part of furnace operation and is in charge of trapping airborne particles such as dust, hair, and other debris from entering the blower fan. The filter also ensures that your home’s IAQ (indoor air quality) is safe for breathing.

How often should furnace filters be replaced? You should replace your filter based upon the thickness of the filter and the size of your home. A general guide is if your filter is less than 2 inches to switch it out every 30-90 days. 3-4 inch filters can last 6-9 months, anything higher can be replaced annually. Other things to take into account when considering the frequency of filter replacement are the size of your home, the pets you own, if you burn candles, etc.

A general rule of thumb is if the filter is gray and covered in dust and debris, it’s time for a change. If your filter is not replaced you’ll most likely start to feel cool, weak air blowing out of your vents, an indication that the dirty filter is restricting airflow.


It only takes a few minutes to replace your filter. Follow these steps to ensure you replace your filter correctly and safely! 

  1. Turn off the furnace. Make sure your thermostat is in the “OFF” position. 
  2. Locate the panel and Remove panel. See photo below for panel reference. 
  3. Remove the existing filter.
  4. Insert new filter. If your filter has arrows on it, make sure the arrows point towards the furnace.
  5. Turn the furnace back on. Make sure your thermostat is in the “ON” position. 


Man replacing dirty air filter in furnace.


4. Your Pilot Light isn’t Lit.

Your pilot light might not be staying lit due to a bad thermocouple or “flame sensor,” a dirty pilot orifice, a bad gas regulator, or other reasons.

How to Check & Re-Ignite the Pilot Light

You’ll first need to grab a flashlight and a screwdriver before inspecting your furnace. 

  1. Turn off the power to the furnace by switching the breaker to the “OFF” position and closing the gas valve. 
  2. Locate the front cover panel of your furnace and open the little door. 
  3. You can find the pilot light pretty easily because it is a small flame lit.
  4. If you don’t see the pilot light, check to make sure your furnace has a pilot light. Standing pilot lights are common in old furnaces and newer furnaces have intermittent pilot lights or hot surface igniters. 
  5. Make sure the pilot light is blue. 

The pilot light should always be blue. If it’s not blue and a strange color this indicates the burner assembly is corroded and dirty and has to be cleaned. This is a precarious process and it’s in your best interest to call the experts at Blakeslee and Son.

5. Your Flame Sensor is Dirty.

A flame sensor is an important component located inside the furnace assembly. The flame sensor detects if there is a flame burning inside the furnace. When there’s no flame detected, the furnace flame sensor shuts off to prevent a dangerous gas leak in the house. This also prevents carbon monoxide poisoning. 

How does a flame sensor become dirty? Since many furnaces are located in high-volume dust areas such as basements, laundry rooms, attics, the dust particles stick to the sensor and burn onto it, causing carbon build-up on the sensor. The sensor needs to be cleaned or replaced depending on how badly corroded. 

To clean a flame sensor, it involves removing the small part and using sandpaper or steel wool to remove rust. Since this process is tricky and the part may be costly to replace if you break it, we recommend calling a professional to do the job.

6. There are Leaks in your Ductwork.

Your ductwork is how cooled and heated air circulate through inside your home.

A ductwork leak is when conditioned air from your AC unit or furnace leaks through the holes/cracks/seams of the ductwork before reaching the designated areas and rooms. According to ASHRAE (The American Society of Heating, Refrigerating and Air-Conditioning Engineers), 75% of buildings have duct leakage! 

When there are leaks in your furnace ductwork, it can cause problems for your heating system. You may experience not enough heat blowing to designated areas or too much blowing in one room. 

Blakeslee can help you find leaks and repair your ductwork to ensure you’re not wasting heated or air-conditioned air in your home. 

HVAC ductwork system

Parts, Repairs and Service Made Easy With Blakeslee & Son

Whether your furnace is blowing cold air, your pilot light has gone out, you have ductwork leaks, or you just need to schedule a regular furnace tuneup, Blakeslee & Son is your solution. We’ll even handle all the scheduling for you when you sign up for our Peak Protection Plan!

We’re here to provide you with full-service to your heating and cooling systems to properly ensure safety and great air quality in all areas of your home. The last thing you want is cold air blowing from your furnace on a cold morning! 

Have a heating emergency? We are local in Rockford and ready to help! Give us a call at (616) 866-1921.

Able to get to our house with relatively short notice. The free estimate was crucial in our decision making process. They stayed in scope and finished the job on time. Friendly, knowledgeable staff and plumber. Fixed and updated our 100 year plumbing. Will use again.

Edward O.

Wonderful. Great guys, very professional, explained the new system and demonstrated how to change the filter. Thank you for expediting the air conditioner and installation! They also worked through this heat and the current chaos at our house! Thank you Blakeslee & Son, Inc. You have hired great staff!

Marlene K.