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Why is My Furnace Blowing Cold Air?

With the breeze getting cooler and cooler, you’re expecting your furnace to provide you warmth for a more comfortable feel inside your home. When you turn on the furnace in your Sacramento home, the last thing you want is to get hit with a blast of cool air. Unfortunately, this inconvenience is actually more common than you think, but it’s also quite simple to fix. 

There are several possible reasons to explain why your furnace is suddenly blowing cold air. More often than not, the problem is something that you can fix without hiring a professional service for furnace repairs. Before you call a heating and air conditioning technician to come out and inspect your Sacramento furnace, check out the following common causes of cool air-blowing furnaces and what you can do to resolve the issue yourself:


  • There’s a problem with your thermostat.

The thermostat is an important component of your furnace. It senses system temperature to ensure that it is maintained at a desired level, serving as the primary control for most heating and cooling systems. Furnace thermostats respond to changes in pressure due to the change in temperature, and a thermostat that’s working properly will only allow air to be released if it passes a particular temperature. Simply put, an operational thermostat makes sure that your furnace is only blowing hot air.

If your furnace is blowing cold air, it’s possible for it to have incorrect thermostat settings. Sometimes, the system control simply gets set into “auto” mode instead of “on”. When this happens, the thermostat will allow the furnace to continuously blow air into the house regardless of its temperature. Auto settings normally trigger while the furnace is still increasing the air temperature to heat your home, so make sure to manually set the thermostat to your desired temperature settings once the furnace is ready to produce hot air. 

If your thermostat is still not working properly after toggling the control settings, check the batteries to see if it needs replacing. If that still doesn’t solve the problem, you probably need to get a new thermostat immediately.

air filter


  • Your air filter is clogged.

If your furnace is blowing cold air, the reason might be as simple as a dirty air filter. The main task of air filters is to make sure that your furnace only releases air of good quality, but it’s easy for the filters to accumulate dirt and dust particles over time. The dirt pileup can clog the filters if left uncleaned, decreasing the air quality coming from the furnace and restricting your total airflow as well.

Most furnaces are designed to stop heating the air once the system feels clogging. When this happens, your heating system will start blowing cold air instead, and it’s probably going to be filled with dirt and dust, too. Regardless if you’re using a gas or oil-powered furnace, you must always check your air filters at least once every three months so that you can clean or replace them as necessary and keep your furnace running optimally.


  • Your ductwork has leaks or connection problems.

If you are still experiencing trouble with your furnace, the culprit might be your air ducts. According to the National Comfort Institute, the average duct system in the U.S. is only 57% effective because of a wide variety of potential problems. The greatest issue is a poorly designed duct system that cannot deliver the proper airflow, usually due to leaks or connection problems. Bad airflow can cause your system to produce cool air even if the furnace equipment is functioning properly. 

Lots of air ducts experience leakage, which can strain your furnace as it overworks itself to make up for the leaked air. By getting your ducts sealed, you will lose less warm air and improve your system’s efficiency.


  • There’s a power issue with your fuse box.

The first thing to do is ensure that your furnace is still getting power. Inspect your fuse box for any tripped circuit breakers. If you do find tripped circuit breakers, it might indicate that you have a problem with your electrical system and you should consult a technician.

If that isn’t a problem, then check your furnace’s limit switch, usually located on the side of the furnace, and ensure that the switch is switched “on”. Some units feature another fuse by the power switch, which can sometimes be blown. If the fuse is blackened, then ask a technician to inspect your system and identify the problem.


  • Your furnace has ventilation problems.

Old furnace models use chimneys for ventilation, while newer versions come with their own fresh air intakes and exhausts that can ventilate along the sides of your house. You need to check these ports of entry and exit from time to time to make sure they don’t become blocked, as clogged ventilation can cause your furnace to malfunction and even damage it in the process. 

Leaves, dirt buildup, and excessive moisture are the usual suspects in blocking ventilation ports, so you need to be on the lookout for those when checking outdoor vents.


  • Your condensate drain line is being blocked.

A furnace working at high efficiency can produce condensation, which isn’t usually a cause for concern because it can just exit your home system through the condensate drain line. However, if there’s any kind of blockage in the drain line, the furnace’s overflow kill switch will automatically turn itself off to prevent water damage.

A good indicator that your home has a condensate line blockage is if you see standing water around the furnace. Once you see this sign, you should clear the condensate drain clogs immediately. Turn off your furnace first to avoid any electrical hazards while working with water. Then, look into the condensate drain pan and remove any water inside using a vacuum cleaner. Remove the dish as well and clean it with soap and water before putting it back on. Continue using the vacuum to suck out clogs along the condensate drain pipe. Once you’re sure that you’ve cleared all blockage, turn the furnace back on to test its performance. If it continues to produce cold air, it’s time to rely on professional services.


  • Your furnace’s pilot light is out.

Your furnace’s pilot light needs to be lit to ensure that combustion is taking place to produce heat. If your pilot light is out, the furnace will start blowing cold air, so it’s essential that you relight the pilot immediately.

To relight an extinguished pilot, you first need to turn off power to your furnace. Next, locate the reset switch on the pilot light assembly and turn it off. It should take ten to fifteen minutes for escaped gases to clear the area, after which you can turn the reset switch to “pilot”. As you do this, use a matchstick or lighter to light outgoing gas at the pilot end.

After you’ve successfully relit the pilot, turn the switch back to “on”. Restore power to the furnace and test it to check if it now produces hot air again. If it still doesn’t, you’ll need to call for furnace repair.

Sometimes, the pilot appears to be lit but it still keeps going out from time to time. This can be caused by a faulty thermocouple. A thermocouple is a sensor that controls the ignition and gas valve functions of your furnace. You can try to clean the gas valve first and see if that improves your furnace’s performance, as it’s easy for dirt to pile up and get the valve stuck in position. If the issue isn’t with the gas valve, it’s likely with the thermocouple, and you’ll need to contact an HVAC professional to replace it with a new one.


  • There is not enough gas supply to the furnace.

If you’re using gas furnace, when your heating system does not get adequate gas supply, it will automatically lock itself down for safety purposes. If your furnace turns off after producing cold air for a while, inspect your gas supply to see if it’s been turned off or if it’s malfunctioning. 

If you don’t find any issues with your gas supply, the ignition problems might be caused by a dirty oil or gas burner. A neglected furnace burner can easily accumulate grime and dirt on the flame sensor, making it hard for the system to heat up due to particle buildup. A compromised flame sensor can also cause your burner to keep shutting off from time to time, resulting in varying air temperatures being released into your home.

To solve this problem, you can try cleaning the burner or the flame sensor yourself. However, it’s better to hire a professional for this task so that he (or she) can look deeper into your system for further damages.


A Final Word on Why Your Furnace is Blowing Cold Air

Understanding the most common reasons as to why your furnace may be blowing cool air is important so that you can try to fix the problem immediately. However, there will be moments when your quick response might not be enough to resolve the concern, in which case it’s probably better to get professional help.


All Year will be able to give you professional inspection services for your heating system, as well as for the identification and execution of the right remedy or repair needed to correct your problem. All Year prides itself in providing top-notch installation, inspection, maintenance, and repair of heating, ventilating, and cooling systems, so you can be assured that your furnace will be in good hands. Contact All Year now and get your HVAC systems ready for the winter season.