If your AC fan is constantly running—well—it shouldn’t be. We’ll help you troubleshoot in this guide.
If the soundtrack of your summer is the constant hum of your outside HVAC unit, you’ve probably got some questions, the first of which is obvious: Is it normal for AC to run constantly?
The short answer: Definitely not. (Unless you have manually engaged your outside fan to run constantly, but it shouldn’t be running for hours on it’s own.)
If your HVAC is functioning correctly, it should keep your house cool by cycling on for 15-20 minutes at a time—with distinct pauses in between each cycle.
In fact, if your outside AC fan is on all the time, that’s a huge red flag that your air conditioning system has an issue (and maybe several) that you need to address.
Which brings us to some other questions: What causes AC to run constantly… and how can you fix it?
Well, you’ve landed in the right corner of the internet.
In this guide, we’ll give you all the info you need to understand the AC fan issue, as well as some troubleshooting tips so you can isolate and solve any problems. You’ll learn:
- 10 different reasons that could be behind why your AC is constantly running
- What is the best, most energy-efficient AC upgrade available (if your current AC is on it’s last leg)
- How to get an AC upgrade (and lengthen its lifespan with insulation and air sealing) at no upfront cost
- FAQs about constantly running AC
Let’s dig in (and hopefully give your outside AC fan a break).
Top 10 reasons your AC unit runs constantly
Here’s a high-level list of the top 10 reasons your indoor air conditioning system or outdoor AC fan is running constantly. Below we’ll cover each of these reasons in detail and help you troubleshoot the problem.
- Your air conditioner’s airflow is being restricted.
- The ductwork is old and leaky.
- The AC unit isn’t big enough for your home’s size.
- Your house is losing all the cold air through air leaks and faulty insulation.
- Your AC’s air filter is clogged.
- The air filter is the wrong type.
- Your refrigerant needs to be replaced or refilled.
- The AC condenser is dirty.
- The evaporator coils are frozen.
- There’s an issue with the AC fan itself.
#1. Airflow is being restricted.
Just what it sounds like! The airflow is restricted somewhere within the system, which means your AC fan has to run constantly in order to compensate for the reduced airflow.
How you can tell:
Start by checking the obvious culprits: AC vents. Walk around your house to make sure all the vents in your house are open and unobstructed. If even a few registers are closed (or clogged by curtains, furniture, your kid’s toys, etcetera), it can severely impede the ability of your AC to do its job.
If all the vents look fine, check to be sure that your air filter isn’t impeding air (more on that later).
How to fix AC airflow:
You might be able to solve this issue yourself by simply opening up the vents or registers that are closed off. But if that doesn’t do the trick, there could be a bigger problem—like a blockage in the ductwork. In that case, you’ll need to call an HVAC professional to come take a look.
Speaking of ductwork…
#2. Your ductwork is old and ailing.
If you’ve got an older home that’s difficult to heat and cool, it might have outdated ductwork that’s seen better days.
Over time, old ductwork can deteriorate and start to fall apart, which can cause all sorts of problems—including restricted airflow and problems with the outdoor fan.
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How you can tell:
Old ductwork is usually pretty obvious. If you take a look at the ducts and they appear to be old and rusted, that’s… not a good thing. Another telltale sign is if you see dust and debris coming out of the vents when the AC is running, because that’s usually an indicator of leaks.
Here’s a useful diagnostic: Hold your hand in front of each of the vents in the house to see if the airflow out of each one of them feels relatively the same. If the airflow is uneven between vents, there’s a good chance that cool air is being lost as it wends its way through your duct system.
How to fix ductwork issues:
If you suspect your ductwork is the culprit, your best bet is to call in a professional contractor to take a look.
They’ll be able to tell you definitively whether the ductwork needs to be replaced (or whether the whole ductwork system needs to be redesigned, which is often the case in older houses).
By the way! If you don’t want to deal with ductwork ever again, we highly recommend a ductless mini-split AC system. They’re magic.
Guess what? Over 51% of your home’s energy use is for heating and cooling (1). When your AC isn’t running properly, that’s a lot of energy waste.
#3. Your AC unit isn’t big enough.
AC units need to be sized correctly: You don’t want one that’s too big (it’ll waste energy, money, and make your house too humid), but you also don’t want one that’s too small (it will run constantly and never really get the job done). So it’s important to get it just right.
Case in point? If your AC system isn’t big enough for the space it’s trying to cool, it will have to run constantly in order to make up for the lack of cooling power.
How you can tell:
This is a particularly common problem when people construct additions on their homes without taking their HVAC system into account. So if your AC issue began after the house was expanded or changed in some way, that’s a big tip-off.
But maybe you’ve never added on to your house? In that case, it’s still worth checking to see if the house has been expanded since the HVAC was installed, especially if the problem is ongoing and you can’t find any other issues.
Here’s how to fix it:
Suspect that your AC unit needs an upgrade? Call in an expert to do an “HVAC load calculation.”
They’ll consider the square footage of the house, the number of windows and doors, the amount of insulation you have, and more to figure out how much cooling power you really need in your situation. If they determine you need a bigger unit, they can help you choose the right one for your home—and install it properly.
#4. Your house is losing all the cold air.
In order for an AC system to work properly, the cold air it produces needs to stay inside the house. And all the hot air needs to stay outside.
But sometimes, that’s not what happens. Most houses are full of tiny holes and gaps that allow too much air exchange with the outdoors.
And sometimes, the AC can’t keep up with all the heat that’s invading your home because of improper or inadequate attic insulation, crawl space insulation, or an uninsulated and unsealed basement.
When one (or all!) of these things happen, your AC just can’t keep up with the influx of outside air. So it runs and runs and runs… trying to catch up.
How you can tell:
Hello, uneven temperatures! If you have a hot upstairs and a cold downstairs, for example, you’ve almost certainly got an air leakage and/or insulation problem.
(Want to know more? Learn how to find air leaks in your home and read on to discover what you need to know about attic insulation. Or watch the quick video below to get a good visual.)
Here’s how to keep your house from losing cold air:
This one is straightforward. Have your house air-sealed and insulated by people who know how.
This step alone can make a massive difference in comfort as well as significantly cut energy waste in your home.
(Making your house super comfortable is what we do at Sealed, by the way. And we can get your house air-sealed and insulated for you at no upfront cost if you’re eligible.)
#5. The AC’s air filter is clogged
The air filter’s job is to keep dust and other particles from getting into the AC unit and clogging it up.
But over time, the air filter can get clogged with all that junk—and when it does, it restricts airflow (remember #1?) and makes the AC unit work harder than it needs to.
How you can tell:
A clogged air filter is usually pretty obvious. Just pull it out of the AC unit and take a look at it. If it’s visibly dirty or gunky, it’s time to switch it out.
Remember to double-check that the new filter is the right size and type for your AC unit. Check the owner’s manual if you aren’t sure. (And remember that Google can usually turn up most owner’s manuals if you, you know, happened to file yours in the wrong place.)
Here’s how to fix a clogged AC filter:
Replacing a clogged air filter is easy: Turn off your AC unit. Pull out the old air filter. Insert the new one (make sure that the “air flow” arrows are pointing toward the AC unit itself). Turn the AC unit back on again. You’re now DIY royalty!
#6. The air filter is the wrong type.
Each type of AC unit requires a specific type of air filter, and if you purchase the wrong type of air filter, it might be too restrictive for the kind of AC system that you have.
The result? Your AC unit struggles to force air through the filter, which slows down the entire system and causes the fan to run constantly.
How you can tell:
If you’re not sure which type of air filter your AC unit requires, the best thing to do is check the owner’s manual to verify the one in your system is actually the correct filter for the job.
Here’s how to solve the problem:
Simply purchase the correct type of air filter for your AC unit! You can usually find these at any hardware store or home improvement store.
An important note, too: If you purchased a more restrictive air filter because of concerns about air quality, know that your concerns about breathing fresh, pure air are absolutely legitimate—and there are other ways to address them.
See if your home indoor air quality is healthy and clean here (and learn some effective ways to ensure your home air is safe to breathe).
#7. The refrigerant needs to be replaced or refilled.
The refrigerant is the liquid that cools the air the AC unit blows into your home (it’s often called freon).
If the refrigerant level is low, or if the refrigerant itself is past its expiration date, the AC unit will struggle to cool your home to the desired temperature and your outdoor fan will be in constant overdrive.
How you can tell:
The AC unit’s air isn’t as cool as it used to be. Maybe you’ve noticed a slow decline in the level of coolness in the air that blows from the vents? That’s a giveaway that there’s something up with your refrigerant.
Here’s how to fix your AC if it’s low in refrigerant:
This issue can be due to a leak in the AC coil. Or it might simply be because the system is old and needs to be refilled. (This is one of the ways to know when it’s getting close to time for an HVAC replacement.)
In either case, this is not a DIY fix—refrigerant must be handled by a professional contractor.
So if you think your AC unit might need more refrigerant, it’s time to call in the cavalry. And quickly. Refrigerant problems are a health concern as well as a comfort issue (you really don’t want freon poisoning).
#8. The AC condenser is dirty.
The condenser unit is the part of your AC system that sits outside your home, and it can collect a lot of dirt over time. When the condenser coils are covered in dirt and grime, the unit has to work harder to do its job, which means that the fan will run constantly in an attempt to keep your home cool.
How you can tell:
Do a quick visual inspection of the outside unit. If it’s covered in dirt and debris—or those pesky cottonwood tree seeds if you live in the Midwest or Eastern U.S.—the condenser is likely too dirty to do its job efficiently.
Even if the outside of the unit doesn’t look particularly alarming, it’s worth using a flashlight (or a cell phone light) to look at the coils themselves to look for accumulated gunk.
How to fix a dirty AC condenser:
Cleaning condenser coils is a fairly easy DIY job, but you’ll need to gather some materials and equipment, including a specialized cleaner, a screwdriver, and wire brushes.
Here’s how to clean your AC coils—in plain English. We recommend you read the whole thing before embarking on the task, though, especially if it’s your first time.
Tired of repairs? Get super-efficient, hassle-free heat pump HVAC upgrades at no upfront cost if your house qualifies.
#9. The evaporator coils are frozen.
If the coils in your AC unit get too cold (usually because of an airflow issue), they can actually start to frost over—which completely blocks the flow of air and kicks the fan into high gear.
How you can tell:
The most obvious sign that the evaporator coils are frozen is that there’s ice on them. (We’re guessing you saw that coming.)
But you might also notice that the air coming from the vents is significantly warmer than it used to be.
What to do if your AC evaporator coils are frozen:
The first thing you’ll need to do is turn off the AC unit to give the coils time to thaw.
Once they’re thawed, you can clean them and then take a look at the air filter to see if it needs to be replaced. If the coils freeze again, it might actually be an issue with the refrigerant level, so you’ll want to have a professional take a look.
#10. There’s an issue with the AC fan itself.
Finally, there might just be something wrong with the fan! A broken fan blade, for example, will cause the fan to work less efficiently, which means your outdoor fan will run 24/7 in an attempt to make up for the lost airflow.
Another possibility? The motor that actually turns the fan (it’s called a blower motor) might need a repair.
How you can tell:
Hear strange noises coming from the condenser unit? Those might be an indicator of a fan or blower motor issue.
Otherwise, diagnosing a fan issue is definitely a job for the pros. If you’ve eliminated all the other potential causes, it’s time to schedule a maintenance check.
How to fix an AC fan:
This is one for the experts. Not just because of the specialized knowledge required to diagnose and solve issues regarding the “innards” of your AC system, but because these kinds of repairs can be pricey and complex—anywhere from $300 to $800, depending on who you ask (2, 3).
And you’ll want a professional opinion about whether it’s even worth the cost to repair your system.
Truth is: When this kind of thing starts going wrong, it very well might be time to invest in a new AC system. Read When to replace your air conditioner to learn more.
And while we’re on that subject, we’ve got a recommendation for you.
The best AC on the market
If you decide that you’re due for an overall upgrade, the best AC system on the market right now (2022) is—by far—the air conditioning system known as a heat pump.
If you’ve never heard of a heat pump, here’s a quick overview: Heat pumps are basically super-efficient air conditioners that can work in reverse.
In the summer, a heat pump AC system will provide the best air conditioning you’ve ever experienced. It does this by pulling heat out of your environment and circulating a constant flow of cool, fresh, purified air.
In the winter, they reverse the process and act as heaters by taking heat energy from the outside air (yes, there’s still heat out there in winter) and circulating it throughout your home for comfortable, even warmth.
One of the best things about heat pumps is that they are 3 times more energy-efficient than traditional HVAC (4). And, if your house is eligible, you may be able to get a heat pump AC system for zero upfront cost.
And because heat pumps are an all-in-one system—heating, cooling, dehumidification, air purification—you’ll only need to maintain one system.
Which, in our experience, means fewer headaches (and less Googling around to find articles like this one).
A well-maintained heat pump can last 20–25 years.Home Inspection Insider
In short, heat pumps are incredible. And there’s a lot more to know, so if you’re interested in learning more about heat pump AC systems, we recommend starting with The Ultimate Guide to Heat Pumps.
And, finally, if your interest is really piqued by all this, we’ve got a proposal for you.
Solve your AC and home comfort issues for good. (For zero upfront cost, too.)
If you’re sick of malfunctioning equipment, not feeling comfortable in your own home, and not knowing quite what to do about any of it… let us swoop in to save the day.
At Sealed, we’re air conditioning and climate control nerds—and we know how to make your home as comfortable and energy-efficient as possible (even if it’s a really old house or has really stuffy, stale air).
If your house qualifies (find out in 2 minutes here), we’ll come alongside you to make your house feel amazing.
Air sealing? Insulation? New AC?
We’ll figure out what your house needs—combining our knowledge with your personal preferences—and then create a 100%-custom home upgrade plan. No cookie cutter solutions here.
We’ll also hire expert local contractors, oversee the whole project, and stand behind our work, making sure it’s done right.
If I don’t have to spend any extra money to get a huge improvement to my home, it’s just a no brainer at that point.Scott R., Sealed customer
Basically, we’ll do all the stressful stuff—and for no upfront cost (if your house qualifies).
All you’ll need to do is enjoy an incredibly comfortable home.
It’s a great plan that Sealed customers love. See if you qualify—right now—in two minutes.
FAQs about constantly running AC
Scan below to view some of the most commonly asked questions about an AC that won’t shut off, or use the links below to skip ahead.
- How long should an air conditioner run?
- How long should AC stay off between cycles?
- Why does the fan keep running on my air conditioner?
Don’t see your question listed here? Call us. We’re here to help!
How long should an air conditioner run?
A general rule of thumb is that an air conditioner should run about 15 to 20 minutes at a time.
If your AC is consistently running longer than that—or if it’s running constantly—there’s a good chance you’ve got an issue to address!
(Tap here to get detailed information about how to solve the problem.)
If your AC turns on for shorter amounts of time (say, it stays on less than 10 minutes for each cycle), it’s called short-cycling and means that the air conditioning system is too large for the amount of space in your home.
While short-cycling might seem harmless, it’s actually a big problem: An AC system that short-cycles wastes energy and won’t be able to properly dehumidify your home.
How long should AC stay off between cycles?
There should be a delay of at least 5-15 minutes between AC cycles (depending on the AC system and the size of your house). If there’s no delay, or if the delay is much shorter than that, it’s time to do some troubleshooting.
Why does the fan keep running on my air conditioner?
Some of the most common reasons that the fan on your AC system keeps running are:
- Too much hot air is getting into your house because of air leaks and poor insulation—your AC can’t keep up
- Something is impeding the airflow in your AC system and ductwork, causing the fan to work harder
- Your AC system isn’t the right size for your house
- There’s an air filter issue
There are many more reasons!
If your outside AC fan is constantly running, be sure to read our full set of diagnostics.