Furnace Keeps Running After Temp Reached

The Furnace Keeps Running After Temp Reached – 9 Causes with Fixes

Good things should never end—who gets tired of being on holidays for instance? But not all good things should run forever and your furnace is one to name. Even if a heated home is something we want in the cold winter, we also don’t want the furnace to run non-stop. Not only does your house get unpleasantly warm, but propane or heating oil will also get wasted. Soon enough, your energy bills will go through the roof and that’s never a good thing.

When the furnace doesn’t stop after it reaches the thermostat, it will wear out long before its time. If left unaddressed, the furnace might break and require repair or replacement. You shouldn’t postpone managing the issues with your furnace. Even if it might not be a severe problem, a furnace that runs continuously is something to address the moment it appears.

Why won’t the furnace stop running?

If your furnace doesn’t shut off with a thermostat, the causes can be various.

A thermostat problem

Two thermostat issues can make the furnace run without stopping:

Incorrect setting of the thermostat

The thermostat monitors the furnace fan. The risk of getting the fan setting to the wrong position by accident is never null. The proper setting is AUTO, so you must check out the thermostat if the furnace runs continuously. Make sure that it’s on AUTO and not on ON.

If the thermostat is set to ON, the furnace fan will run continuously, which is why the furnace sounds like it’s running non-stop. However, if you set the thermostat to AUTO, the furnace fan will operate only when the furnace heats the air inside the house.

Wiring problems with the thermostat

When the thermostat is set to AUTO and all the other components seem to work, too, the thermostat could cause a malfunction. Regular use can make the wiring connecting the thermostat and the furnace wear out. Worn-out wiring may stop the furnace from turning off.

It would help if you didn’t try to fix the wiring on your own but contacted a certified technician to examine the thermostat. Electrical issues can be complex and dangerous to repair. We strongly recommend you contact the professionals to fix the wiring.

The air filter is dirty

If the air filter is dirty, it will stop the air from entering the furnace, which explains its non-stop operation.  As you set the thermostat to heat, the furnace begins to extract cold air from inside the house, to heat it later. The air will go through the filter, which will remove the large particles before the air gets inside the furnace.

However, the filter will collect dirt and debris in time, so the air will struggle to pass through the filter. As a result, little or no air will reach the furnace. Since the furnace pulls less air, it will eliminate less hot air. As the furnace will have to run longer than usual to take the indoor air to the set temperature, it may run all the time.

The problem has an easy fix and you should examine the filter. If it’s dirty, you should replace it with a new filter. Run the furnace as you typically do and wait for several hours to see how the furnace is going. If the furnace keeps running again, you should contact a certified technician.

The blower is faulty

The furnace features a blower that extracts cold air to heat and pushes out warm air inside the house. A motor will power the blower and become faulty after many years. If the limit switch is defective, your furnace won’t shut off even when thermostat is off.

The limit switch is a sensor that informs the system when to turn it on and off. When it’s faulty, the furnace won’t know it’s time to turn it off, so it will continue to perform. The fan limit switch could be stuck on “manual,” so the furnace fan will run even if the thermostat has no power. When the limit switch is blown, the furnace will also perform continuously.

If that’s the problem, you must call the HVAC technician for repair work. Fixing a faulty fan limit switch is better for professionals as it requires specific knowledge and electrical skills.

The air ducts are leaky

If the air ducts are leaky, the heated air possibly leaks into the attic and no longer gets inside the house. When there’s a leak on the supply side of the ductwork, the warm air escapes into the attic. When the heated air is lost, the furnace will generate more heated air to warm the air inside the house. This can lead to a continuous operation.

Here’s how you can tell if the air ducts are leaky:

  • There’s a lot of dust inside the house
  • The heating throughout the house is uneven
  • The utility bills are higher than usual

Should you notice any of these signs, you need to call a duct specialist to examine the ductwork for leaks. If there are leaks, the ducts will need repair (sealing).

The relay requires resetting

Similar to most electrical equipment today, the furnace might need resetting to start performing correctly once again. Once you reset it, the furnace should run as it should. Here’s how you should reset the furnace:

  • Turn the thermostat from “On” to “Auto”
  • Carefully turn the furnace off at the plug or switch
  • Allow the furnace 30 seconds to recover. You want the furnace to come to a complete stop.
  • Turn the furnace back on. It should reset on its own.

The Relay Is Stuck

Let’s say you have reset the furnace, but you don’t get any results. A stuck element could be the cause. You can make some vibration to see if it’s possible to shift the stuck components (a relay on a circuit board, a mechanical switch, or anything similar). Here’s how you can do it:

  • Turn the furnace on
  • Place the palms of your hands on the furnace and shake it with your hands. The blower might stop.

If the shaking fixed the issue, one component was stuck and the chances are that it will get stuck again. Jolting the furnace is only a temporary fix, not a solution per se. You should contact an HVAC technician to look into it and solve the problem.

The gas valve is stuck open

Let’s say you unplug the furnace, turn it off at the switch, and it doesn’t stop running. Even if the problem isn’t common, it’s not impossible either. The gas valve might be stuck open when the furnace doesn’t stop operating after you’ve switched everything off. When the valve is mechanically stuck, it will keep burning natural gas, even if the furnace is off.

Here’s what you need to do:

  • Find the gas stop. Typically, it’s near the furnace.
  • Turn the gas off
  • Call your HVAC technician to see why the gas doesn’t turn off automatically.

The contacts of the relay are pitted

If you have tried all fixes without succeeding, the problem could originate in some contacts in the relay. When a sparkle occurred inside the system, the contacts might have been pitted and the arc weld themselves closed. Its malfunction blocks the connections, so they don’t open anymore. As a result, the furnace will keep on operating.

It’s an electrical problem and you should leave it to the professionals. Call an electrician or an HVAC specialist to assess the situation. An electrician can also verify the system and see if there’s a short circuit that may be at the origin of the problem.

An extra tip instead of a conclusion

Almost 15% of the heated air in the house will escape through drafts commonly found around the exterior doors and windows. It would help if you weatherize your home; there are many ways to do it. Consider adding caulk, weatherstripping, door sweeps, and draft blockers to maintain the heated air inside as much as possible.

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