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Is your furnace blowing cold air sometimes? It can be frustrating when you wake up at night or you come back home and discover that your furnace is blowing cold air. Wintertime is already here and if you are like me, you are probably cranking up your furnace more often than before and it is not uncommon to have a furnace not blowing hot air sometimes, and other times, just working properly.
There can be any number of reasons your furnace is blowing cold air ranging from easy-to-fix issues (those that you can fix yourself) to more complex issues that would require you to call in professional service.
Did you know? When you first turn on your furnace, you will have cold air blowing out of the vents when the heat is on. This is because the air that was trapped in your ductwork when you last turned off your furnace must flush out before the newly heated air in the furnace finds its way into your home.
If your furnace keeps blowing cold air beyond 10 minutes, then there are other issues that are causing that, and you should start troubleshooting your furnace as soon as possible.
How Does a Furnace Work?
Before we dive into troubleshooting your furnace, it will be helpful if we get a brief understanding of how a furnace works. This will help us to know and quickly find solutions to issues that can affect how your furnace works. A gas furnace heating cycles goes through the following processes
- When the temperature falls below what is set on the thermostat, the gas furnace kicks into action. Propane or natural gas is ignited to produce heat.
- The burning gas heats up the heat exchanger, a series of metal tubes that transfer radiant heat to the air that passes through them.
- The furnace blower fan then forces the heated air into the ductwork for distribution throughout your home.
- As the hot air fills your home, the cooler and denser air settles at the bottom and is drawn back into the furnace through the return air vents.
- The heating cycle continues until the set temperature on your thermostat is reached.
8 Reasons Your Furnace Is Blowing Cold Air
Different issues can negatively affect how your furnace. But below are the common reasons your furnace is blowing cold air.
1. Furnace blowing cold air because of dirty flame sensors
Flame sensors are part of your furnace’s safety measures. They detect the presence of flames in the furnace. If they can’t detect flames within 10 seconds of starting your furnace, they will close the gas valve to prevent possible leaking of the gas into your home.
However, with years, these flame sensors can accumulate dirt and soot. And when they do, it will be hard to detect flames, in short, they will give false alarms and will keep the gas valves shut. When the gas valve keeps shutting, this is what will happen.
- The burners will not burn any gas (no ignition and combustion of gas).
- If there is no combustion of the gas then, then the furnace won’t produce any heat.
- When there is no heat produced, the furnace can continue blowing cold air only. So, this can cause cold air to blow out of vents when the heat is on.
- A dirty flame sensor will also cause your furnace to make short heating cycles.
What should you do when you have dirty flame sensors?
A flame sensor is easy to find in your furnace. It’s a small metal rod that can be straight and sometimes can have a 90 degrees angle to it.
Make sure you turn off your furnace before starting to unscrew anything. You can use a screwdriver to unscrew it. If it’s dirty and sooty, you can use sandpaper to clean it.
Put it back into its position and then turn on the furnace. This should solve the problem.
2. Furnace Not Blowing Hot Air Because Of a Faulty Circuit Control Board
The circuit control board is the computer of your furnace. It relays instructions to any other part of your furnace.
When the temperature falls below what is set on your thermostat, The thermostat will send a signal to the furnace’s circuit control board about the temperature fall. Then the circuit control board will instruct the furnace burners to ignite and the furnace blower to start blowing the heated air into the air ducts.
If the circuit control board is malfunctioning or is not working properly, it will miss out on carrying out its functions. Sometimes it will instruct the furnace blower to start blowing without communicating to the burners to ignite. Let’s look at what can cause the circuit control board to malfunction.
- Lack of maintenance: Not cleaning the control circuit board can cause dirt to accumulate on the circuit control board. This can negatively affect how it works.
- When you have a furnace leaking water, the water can drip onto the circuit control board and damage its electrical components eventually causing it to fail.
Solution: If you think you have a malfunctioning circuit control board. You can try resetting it.
How to reset a furnace circuit control board.
You can reset it by turning off your furnace with your circuit breaker and waiting for 5 to 10 minutes. Turn on the furnace again. If this still doesn’t help, calling HVAC professionals is in order.
If you ever notice water leaking from your furnace, act, this can bring damage to other components of your furnace. Causing you more costly repairs.
3. Leaky Ductwork Can Cause Cold Air Blowing Out of Vents When Heat Is On
Leaks in your ductwork can cause the heat energy to escape from your ducts before making it into your living rooms. It is through the ductwork that the heated air is distributed around your home. Because these are usually installed outside the heated areas, they may be exposed to extreme temperatures. This can cause damage to them leading to leaks.
Not only that, if rodents find their way into your attic, they can cause significant damage to your ductwork.
Solution: If you can pinpoint where the leaks are, you can seal them off with metallic duct tape or with new pieces. Else, call an HVAC technician to come and take look.
4. Furnace is Blowing Cold Air Because Of the Dead Furnace Limit Switch
Another component of your furnace that is part of the safety measure is the furnace limit switch. The furnace limit switch shuts off the system when it detects overheating within your furnace.
Regular maintenance will ensure that all components of your system remain in good working condition. If air filters remain unchanged for a long time, they can cause your system to overheat.
Repeated overheating of your heat exchanger can
- Cause the furnace to make a short heating cycle
- Eventually, kill the furnace limit switch.
When the furnace limit fails or is killed by the overheating, the system will think that there is overheating in the system, and it will turn on the blower fan to try and cool everything down. That is why the furnace will blow out cold air.
Solution: Now, a furnace limit switch is something that you can’t easily find. And you don’t want to spend your day trying to figure out where it is located. That’s why on this once, we advise that you call in a licensed HVAC technician to come and do an assessment.
5. Gas Furnace Blowing Warm Air Not Hot Because of Dirty Air Filters
Furnace filters screen out dirt, hair, and other objects you don’t want to go through your furnace. If you leave air filters for a long time without replacing them, they can get dirty and clogged. When this happens, air that is coming into the gas exchanger is restricted. Because of that, less air is taking away the heat so overheating takes place.
This triggers the furnace limit switch to kick into action and shut off the system. Your furnace will make short heating cycles which could be the reason for your gas furnace blowing warm air not hot.
To change furnace air filters, you need to:
- Turn off the furnace on the switch or thermostat
- In Some heating systems, air filters can be found at the bottom of the blower unit, in others they can be found at the return ducts near the center of your house. Check if they are dirty. You can hold them up to the light and if you can’t see the light, then surely, they need a replacement.
- Remember to check the size of your filter before getting a new one.
- Make sure you fit it right, check the direction of the arrow, it has to be away from the entrance and towards the furnace.
Solution: You need to change air filters once every 3 months depending on the condition of your air. If you keep pets or if there are some people with alleges, you can change them even more often.
6. Blocked Registers or Vents Could Be the Reason Your Furnace Is Not Blowing Hot Air.
Are you not getting the hot air only in one or two rooms? This could be something to do with blocked registers or vents.
Solution: Move away objects such as beds, furnaces, rags from the vents so they can circulate the air in your rooms without being blocked.
7. Furnace Blows Cold Air Because Freon Is Leaking
Are you using the heat pump to heat your home? There is one more possible reason your furnace blows cold air. You could be running low on refrigerant. Remember, heat pumps do not produce heat but reply on the refrigerant to transfer heat between the outside air and your inside home. So, if there is no refrigerant in the system, there won’t be any heat transfer because the fans in the indoor unit (condenser) will be blowing on the cold coils, give you only cool air not hot
Your refrigerant could only be running low if there is a leak somewhere within your system.
Solution. If you think your heating system is running low on refrigerant. Make a service call to HVAC professionals so that they can come and verify the refrigerant charge.
What To Check First When Your Furnace Is Not Blowing Hot Air
Thermostat settings
When you notice that your furnace is not blowing hot air, the first thing you need to check is your thermostat. Many homeowners mistakenly put their fan to ‘on’ instead of ‘auto’ or ‘heat’.
When the fan is set to on, the furnace blower will blow cold air when the furnace is breaking. Because of that, you will have cold air blowing out of vents when the heat is on.
Sometimes, a low battery could cause your thermostat to misbehave, sending wrong temperature signals to the circuit control board of your furnace. If the battery is low, make sure you replace it with a new one.
FAQ
Why is my furnace running but no heat is coming out?
This could be several things your furnace could be running without heat coming out. These include
- Incorrect thermostat setting
- Dead/drying blower capacitor
- Dirty or clogged air filters
- Blocked registers or vents
- Leaky ductwork
Why is my furnace blowing cold air and won’t shut off?
If your furnace is blowing cold air, it could mean that you probably have dirty air filters. And if your furnace won’t shut off, your thermostat could have issues too. A low battery is a common cause of this. Make sure you do not have a dying battery. A battery that is low can affect how a thermostat works, sometimes failing to turn on and off your furnace.
If that doesn’t help, try resetting your circuit control panel. You can do this by switching off your furnace using the circuit breaker for 5 minutes, then turning it on again.
Why is my furnace blowing air when off?
If your fan setting is ‘on’ instead on ‘heat’ or ‘auto’, your furnace blower will continue blowing air even when the furnace is on break. To resolve this issue, you need to change fan setting from ‘on’ to auto so that it will automatically coordinate with the furnace.
Why does my furnace come on every 5 minutes?
Dirty or clogged air filters will cause your furnace to short cycle.
When your air filters remain unchanged for quite some time usually more than 3 months, they will cause issues with your furnace. The common problem they cause is, they will restrict airflow into the heat exchanger.
When this happens, overheating is going to take place and when it does, the furnace limit switch will kick into action and shut off the system. The thermostat will send signals about the dropped temperature, your furnace will be attempting to heat again, and the short cycles continue.
Why does my heat feel like cold air?
Are you using a heat pump to heat your home? When your heat feels like cold air been blown into your home, it could be a sign that your heat pump system is in defrosting mode.
If your outdoor unit has ice surrounding it, it will try to blow some warm air on it to melt the ice away. Whatever the cause this should not take more than 10 minutes before your systems starts sending heat into your home.
Final Thought on Furnace Blowing Cold Air
Having seen how some issues could negatively affect how your furnace works. It must also be noted that regular and proper maintenance could actually prevent your furnace from not working properly and breaking down.
You should have furnace maintenance tune-up at least twice a year. During maintenance tune-up, an HVAC technician can examine all the components of your furnace and do a thorough clean up.
So, the next time your furnace is blowing cold air, try out the DIY we have provided in this guide before calling and HVAC.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.