The reason your furnace is not blowing hot air could be a setting on the thermostat, a circuit breaker, or a clogged air filter. Other potential problems include a bad blower fan or limit control switch.
Getting home to a cold house and having a furnace that blows cold air is a headache.
This post shows some basic troubleshooting ideas to get your furnace blowing hot again.
A/C & Refrigeration offers HVAC Repair to Phoenix Valley homes and businesses.
What To Do When The Furnace Is Not Blowing Hot Air
Do you have cold air blowing out of vents when heat is on? Is your “furnace not blowing out hot air”? If the furnace turns on but no heat comes out, it will be a cold night. Here is a guide to show you how your furnace should be working and tips to fix it and get it running again.
If you have read any of our posts, then you should know that we don’t talk about a topic without explaining how the system should be working in the first place. For this topic, we will give you a basic understanding of how the furnace works so that we can figure out what might be causing your furnace to not blow hot air out. It is vital to point out that we are just talking about the furnace today and not the heat pump. If you have heat pumps, or you aren’t sure about what type of system you have, you need to check out Heat pump versus furnace.
Jump To Section:
Intro: How Furnaces Should Work
- 1. Make Sure Your Thermostat Is Turned To The Correct Temperature
- 2. Check The Circuit Breakers For The Furnace
- 3. Check Your Furnace’s Air Filter
- 1. Check The Blower Fan – Furnace Blower Motor or Fan
- 2. Limit Control Switch – Furnace Limit Control Switch
How Furnaces Should Work
Furnaces work with a simple principle, it uses a type of fuel such as gas, electric, oil, etc. to heat up a series of coils. The type of fuel that your furnace uses for this combustion isn’t important for this topic.
The heated coils called heat exchangers will have a fan called the blower motor that blow the cold air in the house over them and into your ducts where it is then blown all over your house. This process, no matter what type of fuel is used, is how your furnace will be able to blow warm air into your home. Now, this could seem pretty simple, but when it is said that there is nearly a limitless supply of things that could cause issues in this process, its true and you should believe it.
We will be addressing the most common causes for your furnace to not be blowing air, and what you could do to fix it. For information on how the furnace works, check out a great article from Popular Mechanics called How your heating system works. “Maintaining your furnace, especially an older one, will improve its efficiency. Replacing air filters improves air flow and reduces wear and tear on blower motors.” Archiscene.
A. Troubleshooting Furnaces Not Blowing Hot Air.
We will go through this issue like a HVAC repairman would, starting with the most obvious causes to some of the most unlikely of causes. When we start troubleshooting, be sure to check for these things:
1. Make Sure Your Thermostat Is Turned To The Correct Temperature
It may seem like this is a no-brainer, but there are a lot of times where HVAC repairmen will get called to a home only to find that the furnace problem is being caused by the husband is turning the thermostat down and the wife is turning it up. Check with your partner to see and ensure that the temperature is set right. If it isn’t set the thermostat for 5 degrees higher than normal and see if the furnace fan kicks on. If this doesn’t work, then keep checking for other causes of the furnace not blowing air.
Once you have checked the temperature setting ensure that it is on heat mode instead of AC. Again it seems pretty simple, but as the programmable thermostats become more complex, so does the number of ways that they could go wrong.
If you happen to have a programmable thermostat, ensure that the time and date on it is correct. This seems pretty simple, but just remember that the point of having a programmable thermostat is to save energy. It does this in the winter by lowering the temperature of the home when the person is at a work or in bed. If the time is off, the thermostat may think that it is 1pm on Wednesday instead of 9am on Sunday, which causes you to suffer the cold during morning coffee and suffer the furnace not blowing out air because there is a miscommunication between the furnace and you.
Replace the batteries in the thermostat. This is the most likely cause of having an improperly programmed thermostat. If there is a power outage, even if it’s a short one and your thermostat has a dead battery, it could cause the whole system to reset itself. So it is possible for the thermostat to dump settings even if the battery is just low so ensure that you replace the battery to ensure that it is taken care of and move on to more complex issues with the furnace troubleshooting.
If the thermostat is down for the count, you should consider spending a bit more and get a smart thermostat.
2. Check The Circuit Breakers For The Furnace
This can’t be emphasized enough, but we are honestly trying not to treat you like your stupid. We are just going off of experience as the most likely causes and some of these don’t even have to be your fault. If you have recently installed a new water heater, or had electrical work done, it is quite possible that the furnace circuit breaker was turned off and is the cause of your furnace problem, so simply go to your main circuit breaker panel as the next step.
Check that the breaker to the furnace is on, but understand that the blower motor may have a separate circuit breaker, so check that as well.
Next, go the furnace and check to see if there is an on/off switch or another circuit breaker that may be shut off, is on.
If any of these are turned off, then turn them on once and allow the system to run. If they turn off again, that this is a sign that there is an electrical problem in the wiring of the system and you will have to call an HVAC contractor. Don’t keep resetting the faulty breaker to your furnace as this can cause an electrical fire.
3. Check Your Furnace’s Air Filter
Your next step should be to check the furnaces air filter as this could cause the furnace to stop blowing air out for a few reasons. Firstly, the filter could be so saturated with debris and dirt that the air isn’t making it into the ducts which causes you to believe that your furnace isn’t blowing air out, when in fact it is, but the clogged filter is blocking it. This is less likely, but some people don’t actually change their filters like they should. You need to change the filter every 3 months.
The second issue with a clogged furnace air filter and more likely is that the restricted air flow out of the furnace is causing the furnace to overheat. Each modern furnace has a safety shut off switch that gets tripped when the furnace gets too hot. Having a clogged filter can cause the furnace to overheat, shut itself off and make you think that the furnace isn’t blowing air when it is the clogged air filter that is causing the furnace to shut off as a safety reason.
Unless it is brand new, the next stop for troubleshooting your furnace should be replacing the air filter.
B. Advanced Furnace Troubleshooting
Don’t let “advanced” and “simple” throw you off, we aren’t talking about experience level. You will have to perform the furnace troubleshooting steps above before going into this section, otherwise it may not actually work. Working your way from easy to hard ensures that you have covered the bases needed for advanced troubleshooting, then continue by using these steps in order:
1. Check The Blower Fan – Furnace Blower Motor or Fan
After you have check the simple issues above, the most likely cause could be a problem with the blower fan, if you remember is what circulates the air over heated coils in your home. In order to check the fan to make sure it is working right, start by turning the furnace to the off position. Then turn the thermostat to the fan only mode. There are a few ways to do this depending on your thermostat, but the most common are:
Turning the heat to off then changing the fan from auto to on.
Or, there are some thermostats will have a fan only position.
Wait for at least 3 minutes for the thermostat to have a decent delay between the input and execution.
If the fan turns on, then you know that the blower isn’t the issue because it is running, and you can move on to the next step.
Although, if the fan doesn’t turn on then go back to the circuit breaker and check to see if the breaker was tripped. If it was tripped, then there is most likely an issue with the wiring and you will need to get a HVAC technician.
If the circuit hasn’t been tripped, then the issue is likely to be the thermostat or the fan has completely failed and consideration should be made to call out a HVAC technician or replace the thermostat with a new one. Start with a new thermostat because it is a cheaper option and don’t let the HVAC technician talk you into getting a new blower right away. Always go from cheap to expensive if you have no way of ruling out the issue.
Side note: If you are spending the money, check into the newest programmable thermostats or smart thermostats which happen to be thermostats that will learn your behavior through the week based on location and then adjust the temperature for energy efficiency and comfort.
2. Limit Control Switch – Furnace Limit Control Switch
Remember that you shouldn’t be jumping straight to this section. This is a later step in a multistep troubleshooting process. Skipping right to this step is like a doctor stating that you have a rare disease when you only have the common cold. If you have followed the above steps, and have the blower fan working, but it still isn’t running with the furnace in heat mode, then the cause may be the limit control switch on the furnace.
What is the limit control switch and how it can cause your furnace to not blow air out?
The limit control switch happens to be one of those parts that have different names like furnace fan limit control switch, fan limit switch, etc. which can be frustrating. But they are all the same thing.
Basically, the limit control switch is a thermometer that is attached to a switch and the switch tells the blower fan to turn on/off. When the furnace is off, this is what senses that there isn’t heat being blown out by the heat exchanger and tells the fan to shut off. If it gets warm from your furnace turning on, then it will turn the fan on to circulate the warm air. This is how the limit control works normally.
Although, it does have a secondary safety roll that is designed to sense whenever the furnace gets too hot and then shuts off as a safety precaution, including the fan. So it depends on if the limit control switch is shutting down the furnace because it believes it is overheating or because the switch is damaged and needs to be replaced. Then replacing the switch is your best step in this troubleshooting process.
Here is what you are getting in calling a local HVAC contractor. We would say that it is quite possible that replacing this part will save around $150, but only if you are handy. If you want to do it yourself, then take a look at this short video for more information on how to replace the limit control switch.