Why won't my air conditioner turn off?

“Help!! My air conditioner won't turn off in my house!” This is the exact panicked phrase we’ve had emailed to us from troubled clients time, and time again.

While icy cold air blasting out of your air conditioning unit all day long might sound kind of great, the stress the system experiences - and the sky-high power bills - certainly are not! Plus, it’s frustrating when a system isn’t working properly.

Thankfully, we’ve rectified this problem many times before, and have put together a quick checklist to help you identify just why you're AC system is continuously running.

Fan Limit Switch set to "On"

If a fan limit switch is turned to “on” instead of “auto” the fan will continuously run, regardless of what the temperature of the room is, or what the thermostat says. The fan actually operates independently of what the thermostat says when it is switched to “on”.

Of course, a lot of people tend to switch their aircon settings to the max fan, instead of "auto", and this is commonly the cause of why their air conditioner is constantly running. The fix to this problem is extremely simple, just hit the fan button on your thermostat or remote control until the word "auto" shows.

By engaging the "auto" option for your thermostat fan, you won't need to clean filters as frequently as you would when using the "on" option, either.

Faulty Thermostat

If you’ve ever done some AC troubleshooting on Google, then you’ve probably come across quite a few issues that can be linked to a faulty thermostat. And, your air conditioner not turning off is certainly one of them.

When a thermostat is defective, it may not be able to identify when the correct temperature is reached.

This is usually caused by a thermostat wiring issue and you’ll need to call a service technician to fix your thermostat. Thermostat wiring is complex - especially with an HVAC system - and shouldn't be attempted by someone who isn't sure about what they are doing.

Low-Temperature Setting

Another common thermostat-related problem with an air conditioner that won’t shut off, is the temperature being set too low. This issue is common in a hot and humid environment. When a very low temperature is selected on the thermostat, the air conditioning system will continuously stay on and work to reach this temperature. Even if the thermostat is unable to reach the desired temperature, it will continue to try.

An easy way to pinpoint if this is the problem is to raise the set degree on the thermostat to one slightly below room temperature. If the AC unit successfully switches off once the temp is reached, then you’ve identified the problem. If this is the case, you’ll either need to set the thermostat temperature higher than before or replace your air conditioner with a more powerful unit.

Frozen Evaporator Coil

If you've spotted frozen evaporator coils on your unit then this is a surefire sign of a problem. Air conditioning units should never freeze over as this will usually indicate a refrigerant leak. A refrigerant leak can lead to catastrophic damage to the compressor and needs to be addressed straight away.

The reason why this scenario may cause your AC unit to continuously run is that the cooling cycle can’t be completed. This is due to the lack of refrigerant. To produce cool air, the refrigerant travels through the evaporator coil and absorbs heat from the air. As it absorbs heat, it vaporizes. When the unit is unable to lower the temperature enough to complete its cooling cycle, it will continuously try.

Dirty Condenser Coil

Put simply, the condenser coils release heat from the room into the outside environment and are located on your outdoor unit. Because these coils are located outside, they’re commonly exposed to dirt, debris and other natural elements. For this reason, it’s important that they receive a regular clean.

If grime builds up on the condenser coils, it can cause them to malfunction - and as a result, not allow your AC unit to turn off. This happens because the dirt can obstruct and restrict airflow, which means they won’t be able to cool air as effectively.

As a result, your AC unit will probably push itself and work very hard to reach the desired temperature. Even though the temp can’t be met - due to the dirty coils - the unit will continuously try. A service technician can quickly and easily clean your condensing unit and coils for you in a short amount of time.

Dirty Air Filter

An early sign of a dirty air filter is when an AC system doesn’t seem to be getting as cool as it previously did. A dirty air filter will restrict airflow and the unit will not be able to get cool efficiently. As a result, it will continually run and try to reach a certain temperature.

This causes wear and tear on the unit, and in the most extreme case, can cause the complete breakdown of your AC.

Aside from breaking your expensive unit, air filters can also catch allergens, like pollen, spores and dander as well as bacteria. If the filter isn’t regularly cleaned out these allergens will cause bad air quality and can lead to physical symptoms, like sneezing, red eyes, and congestion.

If you can’t remember the last you had your AC air filter changed, and your system won’t turn off, this could likely be the problem. Air filters should be changed every 90 days. You can change an air filter yourself, or if you’re not sure on how to do it - or what filter to get - you can contact a service technician for a quick change over.


Is it bad if your AC runs constantly?

An effective air conditioner should reach your desired set temperature at some point. If a unit is run constantly there's a good chance that it could be blowing warm air.

While a constantly running air conditioner shouldn't start a fire, a good air conditioning system shouldn't have to run all the time. When an air conditioner is working effectively, it will cool the room down rapidly and not continue blowing, this uses excessive power and could be a sign that something is off with your AC.

The reason for an air conditioning system running constantly could be that the unit is trying to reach the desired temperature, but is unable to. If this is the case there is probably an underlying reason -like the ones mentioned above - that the system can't reach the temp.

Should the fan be on when the Air conditioner is off?

As touched on above, one of the most common mistakes people make with AC is engaging the fan setting. While the fan function is important to distribute air, it doesn't constantly need to be on.

The fan works independently from the thermostat on the air conditioner. So, even if the temperature that you set the thermostat to is reached, the fan will continue to blow. This means that your air conditioner will constantly run until you manually power it down.

If you engage the "auto" function, the fan will only run when the system is on and not run continuously. Choosing the auto function is the most energy-effective option.

By setting your fan to "auto" you'll also experience better dehumidification in your home. When a fan runs constantly, moisture doesn't have a chance to drip outside and will be blown back into your home.

Time to call the experts?

If you are still having issues - it's time to call in the experts. Our Brisbane-based air conditioning repairs team are on hand to give you the assistance you need. Call us on 07 3283 5566 or fill out a quick quote to get your air conditioner back up and running!

Experiencing other air conditioner issues?

Check out our other guides on common problems below:

My air conditioner heat mode is not working

My air conditioner will not turn on

My air conditioner is making a loud buzzing noise

My air conditioner is sweating inside

My air conditioner is icing up

My air conditioner is blowing hot air

My air conditioner is leaking water

My air conditioner smells bad

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