I can’t tell you how many times I’ve heard someone complain that their air conditioner runs all the time. Now it’s certainly possible that an air conditioner running continuously has a problem. But is the simple fact of continuous operation the problem?

A better question

If I were interviewing homeowners about their house, I might well ask how much their cooling system runs on the hot days. But it wouldn’t be to assess whether it’s up to the job or not. It would be to get an idea of how close it might be to the right size.

But if I wanted to find out how well it’s operating, I’d first ask if they’re comfortable on those cold and hot days. If the answer were yes, and the air conditioner runs a lot—even if it doesn’t run nonstop—that would be a good thing. It wouldn’t mean there weren’t any problems. But there’s no reason to be concerned about runtime if the air conditioner is keeping the house cool on the hot days.

Runtimes tell you about air conditioner sizing

I’ve written many articles about air conditioner sizing over the years. Most recently, I wrote about using runtime to figure out what size system you need when it’s time to replace the current AC. Last year, I looked at what it means for an air conditioner to be properly sized. Briefly, it means that the air conditioner was sized according to the Manual J and Manual S protocols from the Air Conditioning Contractors of America. You can read the full article there for the details. Here I’ll focus on the runtime aspect.

The quote above is from the overview of Manual S, the equipment selection protocol. From that, we can deduce that your air conditioner should run continuously. But under what conditions?

Well, those would be the design conditions, which are:

• Indoors: 75°F (24°C) and 50% relative humidity
• Outdoors: your local design temperature, which is the temperature your area goes above only 1% of the hours of the year

But even if the installer went through the proper design procedure, your AC may not run continuously. The Manual J cooling load calculation is usually a bit inflated, so sizing to an inflated load can reduce runtime. Also, air conditioners almost never come in exactly at the right capacity to meet even the actual cooling load. That adds even more buffer.

Other reasons your AC may run a lot

Going back to the long runtime issue, what you want is for the AC to run close to continuously at the design conditions AND keep everyone comfortable indoors. But here are three reasons you might have long runtimes. The first two can result in people being uncomfortable:

1. The air conditioner is undersized. This isn’t likely, because the great majority of ACs are oversized, usually by a factor of 2 or 3.
2. The air conditioner isn’t working properly. This happened to me in 2009. Our AC was 25 years old, running continuously, and not keeping the house cool, so we got a new one. But you may not have to replace yours. It might just need a repair. (Have you checked the filter?)
3. The air conditioner has variable capacity. Inverter-driven equipment, like minisplits, ramps up and down to try to match the load. Since most of the hours are at outdoor temperatures below your design temperature, the system doesn’t shut off. It just goes to a lower speed.

For the first two, you need to call your HVAC company. You should be OK with number 3 as long as you’re comfortable.

Long runtimes are good!

One reason people think long runtimes are bad—even if they’re comfortable—is that they assume this means higher energy bills. But wait—if the reason for longer runtimes is that the system is properly sized, that means it’s smaller than the typical air conditioner.

And that results in lower power use while it’s running. Those two things—runtime and power use—end up canceling out. When one goes up, the other goes down. When you multiply them together, you end up with pretty close to the same energy use. (See my article about power and energy if this doesn’t make sense.)

Another reason long runtimes are good is comfort. Getting blasted with cold air for 10 minutes and then feeling the house warm up for 10 minutes isn’t all that comfortable. Keeping the cool air blowing at a lower rate over a longer time is more comfortable.

And then there’s humidity. Here in my humid part of the world (the southeastern U.S.), removing water vapor from the air is just as important as dropping the temperature. Having long runtimes in properly sized air conditioners is the best way to do that. Depending on your situation, you still may need a dehumidifier, but you always want the AC to do the bulk of your dehumidification.

There you have it. Don’t fret about long runtimes if everything’s working right and you’re comfortable. In fact, you should celebrate an air conditioner that doesn’t shut off during hot weather.

---

Allison A. Bailes III, PhD, is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.