How To Test Furnace Limit Switch? A Comprehensive Guide

Introduction to Furnace Limit Switch

To dive into the topic of furnace limit switches, let me start by introducing you to what a furnace limit switch is and why it is important to test this crucial component. A furnace limit switch is responsible for controlling the blower motor to maintain proper airflow and prevent overheating. In this section, we will cover what a furnace limit switch is and the importance of testing it regularly. So, let’s get started!

how to test furnace limit switch

What is a Furnace Limit Switch?

A furnace limit switch is a safety measure that stops the furnace from getting too hot and causing fires or other harm. It’s a tiny electrical part situated near the heat exchanger in the furnace. It keeps tabs on the temperature inside the furnace. When the temperature reaches a specific level, the limit switch will turn off the burner or shut down the entire system to stop it from heating more.

Moreover, limit switches can protect your furnace from unnecessary wear and tear. By turning off the system if it gets too hot, limit switches can help extend its lifespan and reduce repair costs over time. Not all furnaces have limit switches installed by default, so check yours if you want an extra layer of protection.

Schedule an appointment with a professional HVAC technician if you haven’t had your furnace examined recently. They can make sure your furnace is working properly and safely, including searching for the presence of a limit switch. Don’t wait – look after yourself and your home by taking action now.

Testing the limit switch is like checking your smoke detector – you don’t want it to go off, but it’s great you have it if it does.

Importance of Testing the Limit Switch

Assessing the limit switch of your furnace is critical for safety and correct performance. Here’s how to do it:

  1. Turn off electricity. Make sure there is no current running through the furnace before handling it.
  2. Pinpoint the limit switch. It looks like a small box with wires at both ends, usually close to the heat exchanger.
  3. Examine with a multimeter. Utilize a multimeter to find the ‘NC’ or normally closed terminal. Put one probe of the meter on this terminal and the other probe on the common terminal. Turn on power; if you get a reading of zero, then you need a new switch.
  4. Reset if tripped. Locate and press the reset button if you have a tripped limit switch. Test again using step 3 to make sure it is functioning properly.
  5. Keep up with inspections. Regularly examine your limit switch to prevent dangers and keep your furnace performing optimally.

It’s essential to be aware that not all furnaces have the same limit switches, so it’s best to read the manual or contact an HVAC expert for help. Moreover, testing should be done by professionals if you’re uncertain or can’t safely complete the task.

Get to know your furnace limit switch before it has the chance to get to know you!

Understanding Furnace Limit Switch

To understand your furnace limit switch and solve related issues, you need to check different aspects like the type, operation, and mechanism of the system. In this section, ‘Understanding Furnace Limit Switch’ with ‘Types of Limit Switches, Parts and Operation of a Limit Switch, and Signs of a Faulty Limit Switch’ as sub-sections, you will learn how to detect faults and identify defective parts of the limit switch.

Types of Limit Switches

Limit switches are important for furnaces. Let’s check out the different types available. Here’s a table with their features:

Type of Limit SwitchFeatures
Snap DiscLow Cost, Fast Response
Bi-MetalCan Handle High Currents
Mercury TubeMore Accurate Temperature Control, Longer Lasting

It’s essential to pick the right limit switch for your furnace. Don’t take risks with safety.
Be educated and know the power of a limit switch. Get the right one for your furnace’s optimal performance.

Parts and Operation of a Limit Switch

A furnace limit switch is essential for controlling the temperature and keeping you safe. Let’s take a closer look!

The 3 main parts of a limit switch are

  1. Bimetallic element – senses temperature changes
  2. Contact arm – makes or breaks electrical connections
  3. Spring – controls contact arm movement

Plus, there are other components like terminals, mounting brackets, and a reset button.

It’s important to get the right limit switch for your furnace. Different furnaces need different kinds. Make sure it can handle high temperatures.

To extend its life span, here are some tips:

  • Clean with compressed air
  • Ensure proper insulation around the furnace
  • Check all electrical connections

Follow these and you’ll keep your furnace drama-free!

Signs of a Faulty Limit Switch

If your furnace isn’t working well, check for any issues. It may be a faulty limit switch! Here are signs of a faulty switch:

  1. Furnace turns on & off often
  2. The fan runs all the time
  3. House isn’t heated effectively
  4. High-pitched noise from furnace
  5. Safety shut-off function activates often
  6. Blinking light on the control board

Fixing a faulty limit switch can be tricky. It can involve delicate electrical work, so it’s best to call an HVAC technician. Remember, limit switches are important for safety. Without them, your furnace may overheat or even cause a fire hazard.

In 2011, Carrier released an air handler system with a defective limit switch, which caused furnace issues. This happened due to mistakes in the product’s assembly. It shows how even tiny errors can lead to big problems for homeowners.

So, get your tool belt and start checking the furnace limit switch yourself!

Tools Required for Testing

To test your furnace limit switch with accuracy and precision, you need the right tools. With the right tools, you can ensure that your furnace operates safely and efficiently. For this section on “Tools Required for Testing” with “Multimeter, Continuity Tester, Electrical Tape, and Screwdrivers” as sub-section solutions, we will discuss in detail how each tool fits into the testing process and why it’s important to have them on hand.


Table displays quantity, its symbol, and unit of measurement.

QuantitySymbolUnit of measurement

Multimeters come in analog and digital. Analog to measure changes fast. Digital to read accurately. Some models measure capacitance/temperature. The accuracy of multimeters varies due to different models. 

So, consider which tests you will be performing. Check the circuit’s continuity. Put it to the tester! Rejoice in the assurance of success.

Continuity Tester

Continuity testers are key for troubleshooting circuits. Follow these steps to use it correctly:

  1. Before starting, turn off and disconnect the circuit from any power source.
  2. Connect one end of the tester to a point in the circuit, and the other end to another point. If there is a continuous path between these points, the LED light will light up – this indicates continuity.
  3. If no continuity, check individual components to find the break.

Remember, all continuity testers don’t work the same way. You must calibrate some before use, and they might only detect certain electrical connection ranges. Get to know your device. Don’t rely on multimeters to detect breaks in wires – continuity testers are made for this purpose and give accurate results.

Using continuity testers can save time, ensure safety and prevent long troubleshooting sessions. Add one to your toolkit now. Electrical tape is also great for patch jobs – so professional.

Electrical Tape

Electrical tape is a must-have when it comes to testing electrical components. It’s made of plastic, with adhesive on one side – allowing it to conduct electricity. This makes it perfect for insulation, securing wires, and marking areas. Plus, it’s durable, versatile, and affordable!

What really sets it apart is that it can withstand harsh environments like extreme temperatures, UV rays, moisture, abrasion, and chemicals. Electricians and technicians use it in many industries, like construction, automotive, aerospace, and marine engineering.

The first electrical tape was invented by 3M in the 1940s. It was called Scotch Tape 33 because of its originator Scotchlite. Nowadays, many brands have created their own versions. 

With a screwdriver, testing can be a DIY project – just watch out for any software mishaps!


The flathead screwdriver is great for single-slotted screws in electronics. The Phillips’s head has a cross-shaped head for laptops and cameras. Torx screwdrivers fit star-shaped screws in computers and phones.

Choose the handle shape and grip for comfort. Make sure the driver size is right to avoid damage. Organize by size and purpose. Label storage spots and keep them dry. Corrosion can reduce lifespan.

If at first, you don’t succeed, skydiving may not be for you. Be careful before taking the leap!

Precautions Before Testing

To ensure a safe furnace limit switch testing process with the right solution, you need to take certain precautions. When dealing with any switch-related issues, first switch off the power supply followed by waiting for the furnace to cool down. To make sure you’re safe from any damage, such as electric shocks, always wear appropriate protective gear. In this section, we’ll introduce you to these sub-sections – Switch off Power Supply, Wait for the furnace to cool down, Wear Protective Gear, and discuss their importance.

Switch Off Power Supply

Power supply is a must-switch-off component before any electrical testing. This is for you and the safety of your surroundings. Here’s a 4-step guide to switching off the power supply before testing:

  1. Locate the main switchboard panel supplying power to the area being tested.
  2. Trip or turn off all breakers or switches in the panel.
  3. Use lock-out tags. This will stop others from accidentally turning the power on during tests.
  4. Finally, use a digital multimeter to check if there is no voltage before beginning electrical work.

Low-voltage circuits can cause fatal electric shocks. So switch off & lock out power sources with caution. Take safety measures when testing electrical systems & equipment – accidents occur without warning. Follow these steps to stay safe and prevent unwanted injuries and damage.

Be careful with electricity. Make sure to follow these steps and stay safe! Don’t be the one feeling the burn when testing the furnace.

Wait for the Furnace to Cool Down

When dealing with furnace testing, it is essential to wait for the furnace to cool down. This may seem obvious, but caution and proper procedure are paramount to avoiding harm or destruction.

To safely cool a furnace before testing:

  1. Trip the power and gas valves.
  2. Clear any nearby combustible items, tools, or debris.
  3. Allow the furnace to cool for at least one hour.

Failing to wait can cause injury, fire, and other disasters. Always double-check that the equipment is cool enough to handle. Waiting for furnaces to cool down prior to testing is a long-held custom in the industry. A steelworker was grievously burned when he attempted to test molten metal too soon because he thought it had cooled enough. This serves as a reminder of why this step is so important.

Wear Protective Gear

Protecting oneself is a must before testing. Wear the right gear – safety glasses, gloves, lab coats, and closed-toe shoes – to avoid injury. Without the proper protection, one could face hazardous material exposure and even lab-specific health complications.

Sadly, the lack of safety protocols has caused terrible accidents in labs, leading to loss of life and severe injuries that could have been prevented if protective gear had been used. This shows why it’s vital to take precautions before every test: for yourself and other members associated. So before you test the furnace limit switch, make sure it’s not a limit to your patience!

How to Test Furnace Limit Switch

To test your furnace limit switch with its multiple types like a high limit switch, fan limit switch, and so on, you need to follow an easy five-step process. First, locate the limit switch hidden in your furnace unit which can be tricky. Next, test the limit switch for continuity using a continuity tester. Then, test for voltage drop by checking the reading of the meter. After that, check the high-limit switch settings for accurate control of temperature. Lastly, replace the limit switch if found damaged.

Locating the Limit Switch

  1. Firstly, make sure the power is off before touching any wires or components.
  2. Then, have a look inside the furnace.

               Note the fan and heat exchanger – this will tell you where the limit switch should be!

  1. Look for a small rectangular component with two or three wires.

                If you can’t find it, refer to the manual or get help.

  1. Once you’ve located it, check for any damage or wear.
  2. Then, use a multimeter to make sure the switch is working properly.
  3. Faulty limit switches can lead to overheating.

                So, make sure you check and test all systems before using your furnace.

                 Locate those switches now!

To find the limit switch on your furnace, follow these steps. Accordingly, make sure the power is off before touching any wires or components. Then, look inside the furnace and note the location of the fan and heat exchanger, which will help you locate the limit switch. You should see a small rectangular component with two or three wires. If you can’t locate it, refer to the manual or seek help. Once you’ve located the limit switch, remember to check it for any damage or wear and test it with a multimeter to ensure it’s working properly. It’s important to do this because faulty limit switches can lead to overheating. Therefore, check and test all systems before using your furnace and locate those switches now!

Testing the Limit Switch for Continuity

Testing the furnace limit switch for continuity is key for proper functioning. This test can help diagnose issues and save money! Follow these 6 easy steps:

  1. Turn off the power – Shut off the power source and unplug it.
  2. Locate the switch – Find it near the furnace’s burners.
  3. Disconnect wires – Detach them with a screwdriver.
  4. Test meter – Set your multimeter to “Ohms” or “Continuity”.
  5. Check resistance – Place one probe on each terminal. Observe your multimeter’s display for a 0-50 ohms range.
  6. Reconnect wires – Reattach them and turn on the furnace to see if it works.

Remember, each heating system varies. Refer to the user manual for specific instructions. Voltage drop testing is like playing hide and seek with a ghost!

Testing for Voltage Drop

Testing for voltage drop is a must to guarantee your furnace limit switch is working correctly. Measure the amount of voltage lost while power flows through the switch. It’s important to test regularly, especially when the furnace starts to act up.

Here’s what you’ll need: a multimeter. Now, here’s what you do: turn off the furnace, locate and remove the limit switch, attach the multimeter to the leads, and set it to measure resistance. For added caution, unplug all equipment, put on insulating gloves, and avoid touching any live wires. 

Make sure your high-limit switch is as high as the temperature in your home – warm and cozy!

Checking the High Limit Switch Settings

Testing a furnace limit switch? Essential step: check the high-limit switch settings. Follow these three steps:

  1. Power supply off. Wait 30 minutes for the components to cool.
  2. Locate the switch. Clean with a soft-bristled brush or air.
  3. Test with multi-meter set to “ohms”. Read “open” when inactive, and “closed” when activated.

Also, ensure wires are connected and electrical panels are fastened back. According to The Spruce website, the high-limit switch is a safety device. It shuts down the heating system when the furnace’s interior gets too hot. Time to replace the broken heart – the old limit switch.

Replacing the Limit Switch if Damaged

If your furnace limit switch is damaged, replacing it is the best way to keep your heating system running properly. Here is how to do it:

  1. Turn off Power: Turn off the furnace power and tidy up the area.
  2. Remove the Old Limit Switch: Take out the screws and carefully take it out.
  3. Attach New Limit Switch: Put in the new limit switch and get everything back together. Then, turn on the power and check if your heating is okay.

Remember, safety always comes first. Make sure to get a limit switch that fits your furnace. My friend experienced problems with her heating for months before realizing it was the limit switch. She followed a guide like this one and has had no problems since then.

To be honest, a broken limit switch is a great excuse to stay home and enjoy some hot cocoa!

Common Issues with Furnace Limit Switches

To address common issues with furnace limit switches, I recommend delving into overheating, tripping of the limit switch, rust and corrosion, wiring and connection problems, and dirty air filters. In this section, you will understand the various problems you may encounter while operating your furnace and the potential solutions that can be used.


When your furnace heats up too much, the limit switch is the first thing that comes to mind. It acts as a safety valve, protecting your furnace from dangerous temperatures. If it doesn’t work right, serious damage and risk to your home could occur.

To avoid this, have routine maintenance checks done on your system. Also, if you notice scorch marks, unusual noises, or smells coming from your furnace, contact a professional right away. Another issue is when the limit switch gets stuck in the open position. This means your furnace won’t work properly and it may use more energy.

It’s important not to ignore any warning signs of an overheated furnace or faulty limit switch. Get help from a professional HVAC technician straight away. Otherwise, you could end up spending more money.

Tripping of the Limit Switch

The limit switch is an important part of any furnace. But sometimes it can trip, causing problems.

  • Dirty air filters can block airflow, leading to overheating and limit switch tripping.
  • A faulty blower motor can also cause inadequate warm air circulation, tripping the limit switch.
  • Too much obstruction around external vents can result in poor airflow and limit switch tripping.

It’s important to remember that furnace issues are not only annoying but potentially dangerous too. If the temperature rises too quickly, this can lead to a fire risk. To avoid this, make sure your furnace has a good supply of filtered air. Replace dirty filters regularly. Clear any obstructions from external vents. And if the problems continue, get a professional to take a look.

Good maintenance and regular inspections can help reduce the chance of a limit switch issue happening again. Rust and corrosion can also cause it to trip.

Rust and Corrosion

Rust and corrosion can cause chaos for furnace limit switches. This arises when there’s moisture, humidity, or condensation nearby. These problems can lead to malfunctions or total failure. Rust and corrosion could mean more issues within the furnace. Maybe there’s a ventilation issue, gas leak, or maintenance problem. An experienced professional should handle it quickly.

To dodge rust and corrosion woes on furnace limit switches, keep all parts clean and dry. Have a qualified heating specialist come in for regular maintenance checks.

One freezing day, we received an emergency call from a homeowner about their furnace. We inspected and found rust and corrosion on the limit switches, causing them to malfunction. 

After replacing the parts and performing maintenance, the furnace was back on – just before more cold weather!

Wiring and Connection Problems

Have trouble with your furnace limit switch? Wiring and connection issues are common problems. Loose wires and insecure connections can cause signal interruptions and lead to faulty readings.

To keep the limit switch functioning properly, make sure all wiring is tight and connections are secure. Check for any signs of wear and damage on wires. Cover exposed wiring with electrical tape, or replace it if necessary.

Check that everything has been installed correctly. Consult a professional to confirm. This will help you avoid unnecessary repairs or replacements. Be diligent with maintenance checks and have a pro inspect things annually. 

Dirty air filters need special attention too – they can’t be thrown in the wash!

Dirty Air Filters

What was the last time you changed your air filters? Dirt in filters can cause trouble with a furnace’s limit switch. As dust and debris gather, it blocks the airflow. This makes your heating system work too hard. This can damage parts, including the limit switch.

The dirty filter has a serious effect on the furnace limit switch. Even though they’re not connected, the filter causes too much strain on the limit switch. The heat can build up beyond what the furnace exchanger can manage. This can trip off the limit switch.

One family had lots of heating problems. Despite cleaning parts, it didn’t help. Eventually, they found out why. They hadn’t been cleaning or replacing their filters! Replacing filters saved them thousands of dollars over four years!

Furnace maintenance is like a flu shot – a bit of a pain, but it’ll stop bigger issues.

Tips for Maintenance and Care

To keep your furnace working efficiently and avoid potential issues, you need to regularly maintain and care for it. With this in mind, I recommend regular cleaning and inspection of the various components. In addition, the furnace limit switch can be affected by dirty air filters, so changing them regularly is important. However, some issues require a professional touch, so it’s a good idea to have your furnace serviced by a professional at least once a year.

Regular Cleaning and Inspection

To keep your equipment running smoothly and efficiently, make sure to follow these six tips:

  1. Unplug the equipment before cleaning to avoid electrical shocks.
  2. Wipe with a clean, dry cloth to remove dirt, dust, or debris.
  3. Check for loose parts and tighten them if needed.
  4. Don’t use water when cleaning electronic components.
  5. Lubricate moving parts with oil if needed.
  6. Clean and replace filters.

Be sure to look for unique details specific to your equipment. For example, some machines need special tools or varied maintenance depending on their use. To ensure successful maintenance:

  • Create and stick to a maintenance schedule.
  • Train employees on proper techniques and safety measures.
  • Get a professional service every few months.

Regular maintenance will help extend the lifespan of your equipment and save time and money. So, take care of your machinery now, for a trouble-free tomorrow. 

Don’t forget to clean your air filters or else you’ll be breathing in more than just clean air.

Changing Air Filters

Taking care of household appliances is key for their longevity and functionality. An important part of HVAC maintenance is switching out the air filters. This simple task can improve air quality, prevent pricey repairs, and save you money in the long run! Here are 6 easy steps to change air filters:

  1. Locate the filter – usually behind a vent or panel.
  2. Turn off the HVAC system.
  3. Carefully remove the filter and remember its size and type.
  4. Clean the area around the filter.
  5. Install the new filter according to its instructions, facing the proper direction.
  6. Turn the system back on and enjoy the fresh air!

Air filters should be changed every 1-3 months depending on the season and your usage. Not doing so can lead to higher energy bills, dirtier ductwork, and system breakdowns. Investing in higher-quality filters can extend their lifespan and provide better filtration.
One person reported a noticeable difference after replacing their dirty filter – better airflow and less dust! Don’t forget this easy step – keep up with cleaning and replacing your HVAC’s air filters for optimal performance. 

Like going to the dentist, getting your HVAC serviced once a year is necessary for a healthy future.

Professional Service Once a Year

Regular maintenance and care to ensure your appliances are efficient and long-lasting. Get a professional service every year!

A trained technician will inspect, clean and repair any parts that need attention. They will also check for potential issues. This yearly visit not only ensures proper functioning but also helps you save money. Plus, it extends the lifespan of your appliance. Schedule services at a convenient time for you. Stick to it every year to keep your appliances in top condition. 

Maintaining your sanity while keeping your house in order? That’s a win-win!

Conclusion and Summary.

The furnace limit switch is an important part of any heating system – it comes in two types, high and fan limit. Testing it can keep your equipment safe and prevent major breakdowns. Testing can be done with mechanical dial indicators, safety limits, preset values, or electronic control boards. After testing, inspect air filters and set thermostats to a max of 90°F+ for optimal temperature and maximum power. Faulty limit switches can cause problems with HVAC systems and safety issues like overheating. Don’t neglect this essential component! Monitor readings and replace them when needed. Service all affected zones regularly to avoid blower motor shutdowns and pressure problems.