If your air conditioning unit runs continuously, it’s possible your ductwork, condenser, or evaporator coil are undersized. It may also be that:
- A relay within the air handler has failed.
- Your air filter is dirty or clogged.
- You have leaks in your air ducts.
- Your home is poorly insulated.
- The temperature outside is much higher than normal.
Any one of these problems could be the sole reason why your A/C won’t stop running. However, it’s also common for two or three of these problems to occur simultaneously. In addition, it may not be a problem that your A/C is running continuously. More on that later.
First, let’s assume your system isn’t designed for longer cycles. How long should the A/C run?
Table of Contents
- How Long Should Your A/C Run?
- How HVAC Technicians Design A/C Systems
- Possible Design Issues That May Keep Your Unit Running Longer
- Other Reasons Why Your A/C Won’t Stop Running
- Related Questions
How Long Should Your A/C Run?
You’ve set the temperature in your home at 75 degrees Fahrenheit and walk away from the thermostat. Now what happens?
If you’re cognizant of the fact that your A/C is running continuously, you recognize that the system should only run in cycles. A “cycle” is simply the period of time in which your A/C is running.
According to Carolina Comfort, an HVAC service company in Columbia, South Carolina, the average A/C cycle should last between 15 and 20 minutes, two to three times per hour. This varies depending on:
- The way the system is designed.
- The temperature difference between the inside of your home and the outdoors.
Take a step back for a second. If you’ve set the temperature for 75°F, and the outdoor temperature is 100°F, that’s a 25° difference your A/C system is trying to manage. Add high humidity levels to the equation, and now your system has to work a lot harder.
What if it’s not 100°F outside? What if it’s 15° cooler? This is where you can eliminate a high outdoor-indoor temperature differential as the primary cause, and look at the design of your A/C system as a whole.
How HVAC Technicians Design A/C Systems
This is where diagnosing gets a little more interesting, and may (unfortunately) lead to a costly fix.
HVAC installers design systems to satisfy the heating and cooling needs of a home. Simple enough at first glance. In practice, there are a lot of factors to consider when deciding how much cold air a home will lose under specific conditions. Two, in particular, stand out:
- Outdoor design temperature is the maximum outdoor temperature of a given area. Using ENERGY STAR’s example, if your home is in Travis County, TX, the maximum outdoor design temperature is 99°F.
- Indoor design temperature is simply the desired temperature of your home – 72°F, 75°F, or whatever suits you.
James E. Brumbaugh, author of the HVAC Fundamentals series, outlines how to develop cooling load estimates in Volume 3. He noted that every cooling load estimate form should take into account sensible heat gain and latent heat gain.
Sensible heat gain is a perceptible increase in temperature due to solar radiation, leakage from running appliances (a gas stove, for example), ventilation, and infiltration. Latent heat gain is an increase in temperature due to the addition of moisture.
When determining the size of your A/C system, Brumbaugh states that HVAC installers must establish a balance between the “hourly heat gain within the conditioned space and the hourly capacity of the air-conditioning unit to remove this heat gain in order to maintain the inside design temperature.”
Possible Design Issues That May Keep Your Unit Running Longer
Installers are like anybody else: They make mistakes. Brumbaugh lists several calculations installers conduct when sizing HVAC ductwork, each of which introduces room for error.
Heat Gain From Solar Radiation
To identify how much heat a room will gain instantly through a window, you need to multiply the window’s area by a shading factor. Then multiply the result by the heat gain by solar radiation. Finally, add the heat gain by convention and radiation.
That’s a lot to cover. The position of the home (30° north, 20° south, etc.), when complete exposure to the sun occurs, and awning sizes can all throw off this calculation.
To account for this factor, you need to determine the amount of heat flowing through structural sections. It’s the amount of British thermal units (Btus) per hour, per degree Fahrenheit, per square foot of exposed surface. This is also known as the U-value, or coefficient of heat transfer.
At this point, an installer needs to gather some information about the home: What type of drywall is installed? Does the home have batt, spray-foam, or loose fill insulation? Is there plywood on the exterior of the home or planks? Each of these materials has its own U-value.
What if the homeowner thinks they have spray-foam insulation when in reality they have batt insulation? This will throw off the A/C installer’s calculation. Spray-foam insulation has a much lower U-value than batt, meaning heat will have a much harder time passing through that material.
This is just the tip of the iceberg. Any miscalculations could lead the installer to recommend an A/C unit that won’t adequately heat your home during a 15-20 minute cycle. In addition, the duct sizing may not carry enough air to the conditioned spaces. Check this out if your air conditioner won’t blow cold air.
Other Reasons Why Your A/C Won’t Stop Running
Let’s assume your system is designed properly. The A/C unit can handle the amount of heat gain your home is experiencing, and your ductwork is sized properly. Now what?
You May Have A Faulty Relay
If your A/C is still running and your thermostat is off, that’s a sign you may have a faulty relay. A relay is a small device that sits inside your A/C unit. It opens and closes electrical circuits to components within the system. For example, they’ll tell the fan to run, condenser to turn on, and so forth.
The relays are usually located just behind the door of your air handler. Shut power off to the unit, open the door, and look at the circuit board. Here’s an example of a relay for a Carrier A/C unit. This can get pretty technical, so call customer support. They can guide you through the troubleshooting process.
Your Thermostat Has Failed
When the thermostat can’t sense the indoor temperature of your room, then the A/C will continue to run. For example, even though the indoor temperature is really 65°F, the thermostat may sense it at 74°F. So it’s going to keep working to try to get it down to whatever temperature you set.
If you have a mechanical thermostat, it’s likely the mercury bulb inside broke. At this point, replace it with a digital alternative.
It’s also possible that the thermostat hasn’t failed per se, but its location is throwing off the temperature reading. If you have a thermostat on a poorly insulated outside wall, it’s going to register a higher temperature than what’s in your home. If you think this is the issue, think back to winter time: Was your furnace running constantly? If so, the location of the thermostat may be at the heart of the issue.
There’s A Hole In Your Ductwork
This is likely if you live in an old home with original ductwork, a steam humidifier, or simply shoddy installation.
Metal doesn’t like moisture. Unfortunately for ductwork, homeowners don’t like dry heat, either. That’s why many homeowners install humidifiers on the supply trunks of their HVAC systems. When the furnace runs, the humidifiers add water vapor to the air. Over time, this moisture will corrode the ductwork.
Take a look at the main trunk and take-offs in the ductwork. Did the installer leave huge gaps between the transitions? Is the supply trunk uninsulated? If so, you can remedy the issue by having (a different) HVAC company come out and patch the ducting.
In fact, if you’re still scratching your head, get in touch with an HVAC service company. They’ll not only be able to identify the problem quickly, but also solve it right then and there.
Could A Dirty Air Filter Cause My A/C To Keep Running?
Yes. If your air filter hasn’t been cleaned in a while, the A/C unit is constantly fighting the ductwork air. That air passes over a coil, which contains refrigerant that cools the air before it travels to your conditioned spaces.
Have any pets? Replace your filters regularly – about every three months. All that fur is getting trapped in the return filter. You can buy them in bulk online or visit an HVAC supply house.
What If My Fan Is Running, But My A/C Isn’t?
This could be as simple as a thermostat setting. A lot of digital thermostats allow you to set your fan to “auto” or “manual.” The “auto” setting means the fan will only run when the A/C unit runs. “Manual” is a little trickier – the previous homeowner may have set the fan to run at a specific time, for example.
However, a refrigerant issue may be the cause as well. If you suspect that’s the case, call a technician certified to handle refrigerant. Look for HVAC service companies that advertise their EPA certifications.