Why Won't My Furnace Turn Off?

August 8, 2023

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why won't my furnace turn off

It's the middle of summer, but your furnace keeps running nonstop. The sweltering heat is making your home unbearable. Every time you turn the thermostat down, the furnace continues blasting hot air. What's going on here?

So, why won't my furnace turn off? In short, a furnace that continues running despite the thermostat being turned down could be caused by a faulty thermostat, short cycling, a bad limit switch, or a gas leak near the furnace. Diagnosing the specific problem and either performing repairs or calling an HVAC technician can get the furnace back to shutting off properly when it reaches the set temperature.

A furnace that won't shut off can be extremely frustrating. But don't sweat it (even though your house already feels like a sauna)—the problem is likely an easy fix. Here are some possible causes and solutions to get your furnace back to cooling it on the heat.

Possible Causes and Solutions

1. The Thermostat is Malfunctioning

A thermostat is essentially the brain directing the entire furnace system. It senses ambient temperatures and tells the furnace when to kick on and shut off. If it's damaged or improperly calibrated, it can cause continuous heating.

Thermostats have internal components like switches, fuses, relays, and gears that can wear out and malfunction over time. Power surges or spikes can also disrupt their programming and electronics. Improper wiring during installation is another common thermostat issue.

It's smart to change the thermostat batteries first in case low power is the culprit. If tapping the unit doesn't fix the issue, it likely needs professional replacement. Smart thermostats may also require software updates or reinstallation/reprogramming.

Thermostat problems account for the majority of furnace units that fail to shut off properly. Thankfully, these control units are inexpensive to replace and easy for an HVAC tech to swap out, restoring normal furnace operation.

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2. The Furnace Has a Short Cycling Problem

Short cycling is when the furnace turns off before reaching the set temperature, then quickly turns back on. This makes it seem like it's running nonstop. Short cycling can be caused by a dirty filter, closed vents, or an oversized furnace.

Check your furnace filter to see if it's clogged with dust and needs replacement. Next, walk around and open all the vents—maybe the kids were playing a prank on you by closing them. If the filter and vents look fine, your furnace might simply be too large for your home. Call a technician to inspect the sizing.

And if all else fails, sacrifice a goat to appease the Trickster God, keeping your furnace stuck on full blast. Hey, desperate times call for desperate measures.


3. The Limit Switch is Faulty

The limit switch is a safety device that prevents the furnace from overheating. It's normally closed and sends power to the furnace components. If it senses the temperature rising too high, it will open to cut off power and shut down the furnace.

A faulty limit switch that's stuck open can prevent the furnace from igniting at all. One stuck closed will never trip to stop the furnace from running. This causes it to keep heating past the set point with no off cycle.

Limit switches are inexpensive parts that are fairly easy for an HVAC technician to test and replace. Old or oxidized limit switches prone to sticking are common culprits behind furnaces that won't stop running. Replacing the faulty switch restores the furnace's ability to maintain the proper temperature range and turn off when needed.

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4. Why Won't My Furnace Turn Off? It Could Be You Have a Gas Leak

A gas leak can be very hazardous and prevent the furnace from shutting off in a couple of ways. First, leaking gas can disrupt the electrical current required for the gas valve to close properly. Second, it can cause high carbon monoxide levels that prevent the flame sensor from doing its job.

If the furnace ignites despite a gas leak, it can lead to an explosion. That's why it's critical to act fast at the first whiff of a gas odor by leaving the area and calling the gas company immediately.

Technicians have sensitive equipment to detect gas leaks and pinpoint their location. They may need to tighten loose fittings, seal cracks in the gas line, or completely replace damaged sections. The furnace can't operate safely until any and all leaks have been fully repaired.

Ignoring a possible gas leak risks serious explosion, poisoning, or even death. Always err on the side of caution by evacuating and getting professional help if you suspect a leak around the furnace. Safety first!

why won't my furnace turn off

When to Call for Repair Help

Most of the time, a furnace that keeps running can be fixed with some easy DIY troubleshooting. But not always. Call in the pros if:

  • Tapping or replacing the thermostat doesn't stop the furnace from continuously running
  • The furnace is short cycling despite having a clean filter and open vents
  • You smell natural gas around the furnace
  • The furnace is making strange noises, smoking, or showing other signs of technical difficulty

A puzzling furnace can often benefit from a professional diagnosis. HVAC technicians have specialized tools and expertise to get your furnace back in working order ASAP.

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Enjoy the Relief of a Repaired Furnace

Is your sweltering home making you sweat just thinking about it? Take action to get that furnace fixed. With some simple repairs, you'll once again feel the sweet relief of a properly functioning furnace keeping your home at a comfortable temperature. No more sauna—just comfy, calming coolness. Ahhh, much better!

About 24/7 AC Repair Conroe

Are you looking for a reliable heating and cooling company that is prepared to meet your needs affordably? 24/7 AC Repair Conroe is fully licensed, bonded, and insured and has a wealth of experience. Give the team a call at (281) 595-1808 to schedule an appointment.


Is it OK for my furnace to run constantly?

It is generally not normal or recommended for a furnace to run nonstop without turning off. Continuously running furnaces typically indicate a problem like a faulty thermostat, dirty filter, closed vents, gas leak, or other issue that should be inspected and repaired for proper and safe function.

Why won't my furnace shut off and is blowing cold air?

If a furnace is running continuously while blowing cold instead of hot air, the issue is likely a stuck relay or faulty control board not signaling the system to heat. A technician can determine if a faulty relay or control board is causing the furnace to get stuck, blowing cold air, and recommend fixing or replacing the malfunctioning part.

Why won't my furnace shut off even when the thermostat is off?

If your furnace continues running even after turning the thermostat completely off, there are a few possible causes. A stuck relay that fails to receive the "off" signal from the thermostat could be forcing the furnace to remain on. Issues with the flame sensor can also bypass the thermostat's instructions and keep heating activated. Additionally, a faulty control board might not be communicating the thermostat's commands to the furnace components. In any of these cases, a technician can pinpoint the malfunctioning part and get the furnace responding properly to the thermostat again.

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