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Furnace Fan Not Working? 2 Reasons Why Your Furnace Keeps Running

May 02, 2023

It’s not normal to hear your furnace running constantly, even if you have the heat on during colder months. Is your furnace running, but you don’t feel any heat? With either issue, you could be using the wrong setting or looking at a shorted wire:

Wrong Setting:

  • Your thermostat is on the wrong fan setting.
  • Your fan limit switch is on “manual override.”

A Shorted Wired:

  • A thermostat wire shorted.
  • A wire at the fan limit switch shorted.

Troubleshooting these possible problems can be frustrating. Our technicians at Patrick Riley Cooling, Heating, & Plumbing service the greater Phoenix area, and they’ll walk you through each diagnostic step so you can pinpoint why your furnace keeps running. We’ll also let you know when a professional is needed to solve the problem.

Live in the Phoenix area and need a tech now? Just contact us and we’ll send a professional over immediately.

Check your thermostat settings

Does your furnace fan keep running with the thermostat off? First, confirm that your thermostat is on the correct setting. Follow these troubleshooting steps:

  1. Set your thermostat to a lower temperature than the current room temperature. Setting your thermostat at a lower temperature shuts the furnace down because the space is already at the desired temperature. If the furnace was in the middle of a heating cycle, wait 3–4 minutes for it to stop running.

    Did your furnace stop running? If so, you may be habitually setting the temperature too high. Setting the thermostat to a temperature that your furnace struggles to reach can cause it to heat continuously as it tries to reach that temperature.

    Frustrated because you simply want to be comfortably warm? Your thermostat could be struggling to reach your ideal temperature because the house needs to be weatherized properly, preventing cold air from entering the home as hot air escapes.

    If your furnace keeps running when the thermostat is off or after the temperature is reached, continue to step 2.
  2. Set your thermostat to AUTO and not ON. If your thermostat is set to ON, it will run continuously, regardless of whether your furnace is heating or not. Avoid setting your thermostat to ON—it wastes money because it forces the fan motor to run constantly, despite the furnace burners being cool. With your thermostat set to AUTO, you should only hear your furnace running when it’s in the middle of a heating cycle.

Learn more about furnace repairs.

If your furnace fan is set to AUTO and is still running, then move on to the next section.

Check your fan limit switch setting

What in the world is a fan limit switch? Your furnace has a fan limit switch located directly under its hood. The fan limit switch controls when the furnace fan turns on and off. A probe inside the switch monitors the temperature of the burner assembly. The burner assembly generates the heat, and the furnace fan distributes the heat. When the thermostat indicates heat is needed, the fan limit switch turns on as the burners heat, thus warming your space. When the thermostat indicates no heat is needed, the fan limit switch turns off as the burners cool.


If your HVAC fan keeps running but there’s no heat, your fan limit switch may be set to manual override. Follow these troubleshooting steps:

  1. Locate your fan limit switch and check to see if the white knob (see pic below) is pressed in. If so, your switch is set to manual override, or “always running” mode.
  2. Pull the white button out to reset your fan limit switch to AUTO. The auto setting causes the furnace fan to automatically turn on when the furnace is heating and shut off when the preferred temperature is reached.

Settings are correct, but my furnace keeps running

If your thermostat and fan limit switch are set to AUTO but your furnace keeps running, you most likely have a shorted wire in your thermostat or fan limit switch. Unfortunately, you’ll need a professional to fix shorted wires.

When do I need help from a Patrick Riley technician?

If your furnace keeps running with the thermostat off or it doesn’t reach your desired temperature, a professional can inspect your system and home to provide possible solutions.

If you check all your settings but your furnace keeps running, a professional can assess if your thermostat or fan limit switch needs to be rewired or replaced.

Contact us for furnace repairs in the greater Phoenix metropolitan area, and we’ll send over an experienced Patrick Riley technician who can stop your furnace from running continuously!

Related reading:

Why choose Patrick Riley Heating Technicians?

Patrick Riley Heating service technicians pride themselves on being accurate and professional. We take the time to assess the issue and teach you how to troubleshoot before recommending replacement. Pricing will be discussed up front, and recommendations will be appropriate for your home. We make heating services transparent for you, and financing options are available, so there’s no need to run around price shopping. If your furnace fan shouldn’t be running continuously, then neither should you.

AZ HVAC & Plumbing Service Area

Originally published: March 1, 2022

Written by: Patrick Riley Cooling, Heating, & Plumbing

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By Ty Lindsay

Ty Lindsay is the Director of Field Operations at Patrick Riley | Isley’s and a 15-year veteran of the plumbing and HVAC trades. In 2010, Ty earned his Journeyman’s plumbing license. He became a Master Plumber five years later and earned his Journeyman HVAC technician’s license that same year. Ty’s breadth of knowledge in plumbing and HVAC includes both residential and commercial work. He’s been a loyal member of the Patrick Riley | Isley’s team since 2016.