What is a Furnace Limit Switch?

What Is a Furnace Limit Switch?

Your furnace is packed with safety components designed to keep your home comfortable and your family safe. One of the most critical — yet often overlooked — parts is the furnace limit switch.
If your furnace suddenly stops blowing warm air, cycles on and off frequently, or the blower won’t shut off, the limit switch could be to blame. But what exactly does this little part do — and why is it so important?
Let’s break down everything homeowners in Tempe, AZ need to know about the furnace limit switch, how it works, what can go wrong, and when to call HVAC Tempe Pros for help.

What Is a Furnace Limit Switch?The furnace limit switch, also known as the high limit switch or fan limit switch, is a safety control device inside your furnace.
Its main jobs are to:

1. Turn the blower fan on and off at the right times, and
   
2. Shut down the burners if the furnace overheats.
   

In simple terms, the limit switch acts as a temperature monitor. It measures the heat inside your furnace’s heat exchanger and ensures the system doesn’t run too hot, preventing damage and potential fire hazards.

Where Is the Furnace Limit Switch Located?You can typically find the limit switch mounted on or near the furnace’s heat exchanger — the part that transfers heat from the burners into the air that circulates through your home.
Most switches look like a small metal plate or probe with electrical terminals and a small white or black plastic body. Some may have a small reset button in the center.
If you open your furnace’s access panel (after turning the power off), you may see the limit switch connected to the wiring harness near the burner assembly.

How the Furnace Limit Switch WorksThe limit switch plays two key roles in your heating system’s operation:
1. Fan Control Function

• When the burners ignite, the furnace heats up.
  
• Once the air inside the heat exchanger reaches a safe temperature (typically around 110–140°F), the limit switch tells the blower fan to turn on.
  
• The fan then circulates warm air through your ductwork and into your home.
  

When the furnace shuts off, the limit switch senses the temperature drop and turns the blower off once the heat exchanger cools down (usually below 90°F).
2. High-Temperature Safety ShutoffIf the furnace temperature rises too high (often 200°F or more), the limit switch will automatically shut off the gas burners to prevent overheating.
This protects your heat exchanger from cracking — a costly and dangerous problem — and prevents potential fire risks.

Common Signs of a Bad Furnace Limit SwitchA faulty limit switch can cause several noticeable issues with your furnace. Here are the most common symptoms homeowners in Tempe should watch for:

1. Furnace blower runs constantly
   – If your blower never shuts off, even when the thermostat isn’t calling for heat, the limit switch may be stuck in the “closed” position.
   
2. Furnace short cycles (turns on and off frequently)
   – If the limit switch trips repeatedly due to overheating, your system may shut off prematurely.
   
3. No heat or intermittent heat
   – If the switch fails completely, it may prevent your furnace from turning on at all.
   
4. System goes into “lockout” mode
   – Many modern furnaces automatically lock out after repeated overheating events to prevent damage.
   
5. Burning smell or unusually hot furnace cabinet
   – This could indicate overheating — a dangerous situation that demands immediate attention.
   

What Causes a Furnace Limit Switch to Trip or Fail?The limit switch itself is designed to last many years, but it can fail prematurely due to a variety of underlying problems:

• Dirty air filter: The #1 cause of overheating. A clogged filter restricts airflow and traps heat.
  
• Blocked or closed vents: Air can’t circulate properly, leading to high internal temperatures.
  
• Dirty blower motor or coils: Dust buildup causes poor airflow and heat retention.
  
• Malfunctioning blower fan: If the fan doesn’t turn on when it should, the heat exchanger overheats quickly.
  
• Shorted wiring or electrical issues: These can cause false readings or switch malfunctions.
  

In many cases, the limit switch isn’t the real problem — it’s a symptom of poor maintenance or another airflow restriction issue.

Can You Reset a Furnace Limit Switch?Some furnace limit switches have a manual reset button. If your furnace shuts off due to overheating, you can try pressing this button (after turning off power and allowing the furnace to cool).
However, if the switch keeps tripping, don’t just reset it repeatedly. The overheating problem will continue — and could cause major damage or safety risks. It’s always best to have an HVAC professional diagnose the cause.

How HVAC Tempe Pros Can HelpAt HVAC Tempe Pros, our licensed technicians specialize in furnace diagnostics, repairs, and safety inspections.
If your furnace isn’t running properly — or if you suspect the limit switch is malfunctioning — we can:

• Inspect the switch, blower, and heat exchanger
  
• Check for airflow restrictions or clogged filters
  
• Test electrical connections and temperature sensors
  
• Replace or recalibrate your limit switch if needed
  

Our goal is to restore your furnace safely, efficiently, and affordably — before a small issue turns into a costly breakdown.
📞 Call us today at 602-566-9684
🌐 Visit www.hvactempepros.com
We proudly serve homeowners throughout Tempe, AZ, and nearby communities.

Preventing Furnace Limit Switch ProblemsRegular maintenance is the best way to prevent overheating and switch failure. Here’s how:

• Change your air filter every 1–3 months
  
• Schedule annual HVAC maintenance before the heating season
  
• Keep vents and registers open to ensure proper airflow
  
• Vacuum around your furnace to remove dust and debris buildup
  

These simple steps can extend your furnace’s life and keep your heating system running safely all winter long.

Frequently Asked Questions About Furnace Limit SwitchesQ1: How much does it cost to replace a furnace limit switch?
In Tempe, AZ, the average cost for a furnace limit switch replacement ranges from $150 to $350, including parts and labor. The cost depends on the furnace type and accessibility.
Q2: Can I replace a furnace limit switch myself?
While it’s possible, it’s not recommended. The switch is connected to your furnace’s electrical and gas systems — incorrect installation could damage your furnace or cause safety hazards. Always hire a licensed HVAC technician.
Q3: How do I know if my limit switch is bad or just tripped?
If your blower runs continuously or your furnace won’t start, the limit switch may have tripped or failed. A technician can test it with a multimeter to confirm.
Q4: What causes the limit switch to trip repeatedly?
The most common cause is restricted airflow from a dirty filter, blocked vents, or a failing blower motor. Fixing the underlying issue will prevent repeated tripping.

Final ThoughtsYour furnace limit switch may be small, but it plays a huge role in your home’s safety and comfort. It keeps your furnace from overheating, protects your equipment, and ensures your blower fan runs at the right time.
If your furnace is cycling irregularly, blowing cold air, or running constantly, don’t ignore it — it could be a sign your limit switch or airflow system needs attention.
📞 Call HVAC Tempe Pros today at 602-566-9684
🌐 Visit www.hvactempepros.com
Your trusted HVAC experts in Tempe, AZ are here to help you stay safe, warm, and energy-efficient all year long.