The sound of an AC fan running constantly is not something any homeowner wants to hear. But why won't your AC fan shut off?

If your AC fan is running constantly and will not shut off, it's likely as simple as an incorrect setting on your thermostat. Check to make sure it's set to "auto" mode instead of "on." If that doesn't solve the problem, check your system for any signs of airflow blockage or electrical failure, either of which may require the help of a professional to fix.

In this article I'll walk you through different problems that could be causing your AC to run constantly. I'll also offer a few DIY solutions that can help you troubleshoot the issue before calling a professional. 

What Is the AC Fan and How Does It Work?

The air conditioner fan is an essential part of the cooling system. The fan pulls warm air into the air conditioning unit through one vent, pushes it across the evaporator coil where the air is cooled down by the refrigerant in the coils, and then pushes the cool air back into the the house.

The fan blades rotate clockwise when the unit is operating and counterclockwise when the unit is off. The speed of the fan determines how fast the air moves. A higher-speed fan produces less airflow. The size of the fan blade affects the amount of air produced by the fan. Larger fans produce more air than smaller fans.

The thermostat is what usually controls the fan turns. It will turn the fan on when it detects a higher room temperature than the desired setting, and off when that desired temperature has been reached.

But some things can cause a disruption to the system, and when that happens, the fan sometimes does not shut off.

Why Won't the Fan on My Air Conditioner Turn Off?

There are several possible reasons the fan could be running constantly. It could be a problem with one of the components inside the unit, or it could indicate a bigger issue.

Whatever the issue, it's best to get it solved sooner rather than later because a constantly running fan can increase your electricity bill as well as cause unnecessary wear and tear on your air conditioner. 

Let's take a look at some of the common culprits and what you can do to avoid these situations. 

The fan is set to the "on" setting

You have the option to either set your AC fan to "auto" or "on" on your thermostat. The "auto" function turns your AC fan on to begin the cooling process every time your thermostat detects that the temperature goes above the set amount. Then, when the thermostat detects that the room temperature has gotten back down to the desired temperature, it shuts the fan off again. 

Having your thermostat set to auto will not only be more convenient and comfortable temperature wise, it can also save you money on your energy bill.

The "on" setting, on the other hand, will keep the fan running on your AC even when the desired room temperature has been reached. The thermostat will signal to turn the compressor off so that the AC is no longer cooling, but the AC fan will stay on and keep pushing air around. 

So if you are wanting to keep air circulating around the room for a short period of time, it's okay to set your fan to "on," but I wouldn't recommend leaving it in that mode since it will ultimately just cause more wear and tear on your fan and cost you more on your energy bill.

Faulty thermostat

When you think about it, the thermostat is one of the most important parts of your home heating and cooling system. It controls how much heat or cold gets into your house, and it does so based on the current conditions inside your home. If something goes wrong with the thermostat, it could lead to problems with the rest of your HVAC system.

A faulty thermostat can cause incorrect readings of room temperatures and can also cause the fan to run continuously. If the wires that connect the thermostat to the fan are faulty it may send mixed signals and signal it to keep running. 

The best way to avoid fan problems due to a faulty thermostat is to regularly check that your thermostat is working properly. Here are a few things to keep in mind to keep your thermostat working at its finest:

1. Check the batteries or wires regularly - depending on if your thermostat runs on batteries or is hard wired.
2. Make sure there isn't a lot of dust around the sensor. This could block the signal.
3. Keep the unit away from heat sources like fireplaces and direct sunlight.
4. Make sure the thermostat box is level - if it isn't level, it may not get the proper readings.
5. Clean the thermostat at least once a month.

AC unit is too small

Your air conditioning unit needs to be properly sized to match the size of your space. This ensures that it provides enough cooling power to keep your entire house comfortable during extreme heat.

An oversized AC will cause unnecessary waste and damage to your equipment. You could end up paying hundreds of dollars every month just to cool down your home or office.

On the flip side, an undersized air conditioning system won’t cool down your home properly. This could lead to higher energy bills and even damage to your HVAC equipment. Your AC unit might look fine on the outside, but if it doesn’t have enough cooling capacity, it could overheat.

If your AC unit can't keep up with the demand, your fan will keep running constantly, which will increase the amount of work your AC unit must do. As the heat builds up, the compressor will run longer and harder, and eventually, the compressor will burn out too.

To avoid this scenario, make sure your AC unit is sized correctly. I recommend checking with a local HVAC technician before committing to a new AC unit so that you can make sure you are getting the right size for your house.

Electrical issues

A short circuit or broken relay switch will prevent fans from operating correctly. You'll know you have a problem because the fan won't turn on. Check the wiring connections between the fan and the thermostat. Also make sure the fan is plugged in to an outlet that's powered.

Reduced airflow

Blocked airflow not only causes an increase in heating costs, it can also wreak havoc on your air conditioner and its parts. 

As I mentioned before, the fan pulls in the air and pushes it through the unit, so if there is any block in airflow, your fan will have to work overtime in order to get the air moving from the room, over the coils and back out. 

The blockage could be coming from several different places throughout your AC system but some of the common places you will want to check are the air filters, coils, and the ductwork. 

You should regularly clean your vents and air filters to avoid any decreases in airflow. You can use a vacuum attachment to suck up the dust and debris that builds up over time. Make sure to check your filters every month to ensure that they are working properly.

I also recommend having a professional clean out your ducts every couple of years to keep your home comfortable and healthy. 

Old ductwork

Blocked ductwork can restrict air movement, but old ductwork can cause problems to the system as well. If the ductwork is old and failing, the AC will have a harder time properly cooling your house. That will, in turn, overwork your AC and its parts, like the fan. 

If you don't maintain your ductwork properly, you could end up paying thousands of dollars to replace it. Routine HVAC maintenance checkups usually include duct inspection where they will check for any broken or loose parts. But if you don't typically get your unit inspected, do a duct inspection yourself every once in a while to make sure nothing need fixed. 

Dirty condenser coil or evaporator

Dirty coils will restrict airflow as I mentioned above, and that's not good for any part of your AC system. If you notice any excessive noise, leaking water, ice buildup on any part of the unit, or poor performance, it could be a sign your coils are dirty or even you could be overcharging your AC.

Cleaning your condenser coil is an easy DIY project that can usually be done with a garden hose since it's located outside. Check for an visible signs of dirt and debris - grass trimmings typically get caught up over the condenser coil - and simply hose them away.

Evaporator coils may be a bit more tricky to clean. You'll need to remove the lid to your AC unit and usually the A shaped evaporator coil is right there in front. If it is, you can clean it with an over the counter coil cleaner, or a homemade water-vinegar solution. In some cases, the evaporator coil is a bit harder to get to, in which case I would give an HVAC technician a call. You don't want to risk further damaging your unit trying to get in there to clean the coil.

When to Call a Professional

Finally, don't forget to schedule routine maintenance. A professional technician can look at the system and see what needs to be tuned up before you get yourself into one of the situations above that requires fixing. 

Use this list of potential fixes to troubleshoot your issue and keep enjoying the refreshing cool air from your AC without the annoyance of a constantly running fan. If you need professional help, don’t hesitate to call us.