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BACKDEC 19, 2025
Why Wont My AC Turn Off How to Fix a Constantly Running HVAC
BACKDEC 19, 2025
Looking for more guidance? Take a look at our Repairs overview.
That constant, low hum of your air conditioner or furnace running without a break. At first, you might ignore it, but then you notice the air is a little too chilly or stuffy, and a sense of dread creeps in as you think about your next energy bill. If your HVAC system seems to have a mind of its own and won't shut off, you're not alone. This is a common household issue, and the good news is, the culprit is often the small control box on your wall: the thermostat.
A system that runs nonstop, sometimes called "short cycling" if it turns on and off too frequently, isn't just annoying—it's a problem. It leads to significant energy waste, puts excessive wear and tear on your expensive HVAC equipment, and can fail to maintain a comfortable temperature in your home.
But before you panic and call for a costly emergency repair, let's walk through some simple troubleshooting steps together. Most of the time, a few quick checks and adjustments to your thermostat can solve the problem in minutes. Think of me as your friendly expert on the other end of the line, here to help you figure this out.
Your thermostat is the brain of your heating and cooling system. It tells your furnace or air conditioner when to turn on and when to turn off to maintain your desired temperature. When it runs constantly, it's usually because it thinks it hasn't reached that target temperature yet, or because a specific setting is telling it to keep the fan blowing. Let's dig into the most common reasons why this happens.
Grab a step stool if you need one, and let's head over to your thermostat. We'll start with the easiest fixes first and work our way to the slightly more involved ones.
This is the number one cause of a "constantly running" system, and luckily, it's the easiest fix. Your thermostat has a setting that controls the HVAC fan, which is the part that blows the air through your vents.
How to Fix It:
If the system is still running after you've switched the fan to "Auto," it's time to move on to the next step.
Sometimes, the problem is that your thermostat is getting a bad read on the room's temperature. If it thinks the room is warmer or cooler than it actually is, it will keep running to try and hit a target it can never reach. This can happen for a few reasons:
How to Fix It:
If cleaning it and checking for environmental factors solves the discrepancy and your HVAC system shuts off, great! If the reading is still off, you may need to consider recalibrating it (if your model allows) or replacing the thermostat.
For digital, non-hardwired thermostats, low batteries can cause all sorts of strange behavior. The display might work, but the internal components may not have enough power to function correctly, leading to a signal that keeps your HVAC system stuck in the "on" position.
How to Fix It:
Modern programmable and smart thermostats are designed for convenience, but sometimes a setting can be accidentally activated. A "Hold" function will maintain a specific temperature indefinitely, overriding any schedule you have programmed. If someone in your house turned the temperature way down or way up and put it on hold, the system will run continuously trying to maintain that setting.
How to Fix It:
If you've tried everything else, the issue might be with the low-voltage wires connecting the thermostat to the HVAC unit. This is a bit more advanced, but it's a straightforward check if you're comfortable with it.
IMPORTANT: Before you touch any wires, you must turn off the power to your HVAC system. You can do this at the furnace switch (it looks like a light switch on or near the unit) or at your home's main breaker panel.
How to Fix It:
You've gone through all the steps, and that stubborn system is still running. At this point, the problem likely lies beyond the thermostat. It could be an issue with the HVAC unit's control board, a faulty relay, or another internal component.
Hiring a licensed HVAC technician is the right next step. According to Forbes Home, a diagnostic service call typically costs between $75 and $200. While it's an expense, it's far better than letting a minor electrical issue turn into a major system failure.
Dealing with a misbehaving HVAC system can be frustrating, but as you've seen, the fix is often within your reach. By methodically checking your thermostat, you can solve the most common causes of a constantly running system and save yourself from a costly repair bill and a sky-high utility statement.
Want to make home maintenance even easier? Download the Casa app today! Casa provides you with step-by-step guides for hundreds of DIY repairs, sends you timely maintenance reminders, and connects you with trusted, vetted professionals when you need a helping hand. Take the guesswork out of homeownership and keep your home running smoothly with Casa.
