Your furnace has a limit switch on it that serves as a safety control. Understanding how this part works and how to test it is crucial to having a heater that is dependable and safe.
Quick Takeaways
- Your furnace’s limit switch is a safety control that shuts the burner off when it detects overheating or damage.
- A faulty switch can cause the furnace to shut down even when there’s nothing wrong.
- Safety first when testing the limit switch; always shut off the power at the breaker.
- Use a multimeter to complete the testing process with accuracy and ease.
- Schedule regular inspections to spot limit switch problems in their earliest stages.
What a Limit Switch Does
The limit switch on your furnace is a vital component that measures the temperature inside the heat exchanger. Without this part, the heater won’t know to shut down the burner if it starts getting too hot.
Once the system cools down, the switch lets the burner kick back on so it can resume normal heating.
Signs Your Limit Switch Might Be Faulty
Knowing whether your limit switch is working correctly is vital to knowing when to book a repair. You don’t want the furnace to run with a faulty switch because it increases the risk of overheating and system failure.
Here are the main red flags to watch for:
- Furnace shuts off early
- System won’t turn on
- Blower runs constantly
- No warm air output
- Overheating furnace cabinet
- Unusual burning smell
- Visible scorch marks
- Repeated limit switch resets
- Furnace safety lockout
Safety Precautions Before Testing
If you notice any of the signs mentioned above, you can test the limit switch to see whether it’s at fault.
Before you begin, though, it’s essential to take certain safety precautions.
- Turn off furnace power
- Shut off the gas supply
- Wait for furnace to cool
- Use protective gear
- Stand on a dry surface
- Ensure proper lighting
- Only use insulated tools
- Don’t bypass safety controls
- Work in a well-ventilated area
Tools You’ll Need to Test a Limit Switch
Completing the testing process requires these tools:
- Multimeter for electrical testing
- Screwdriver set for panel access
- Needle-nose pliers for wire handling
- Flashlight for visibility
- Wrench or nut driver for mounting screws
- User manual for reference
- Replacement limit switch (if needed)
Step-by-Step: How to Test a Limit Switch with a Multimeter
The whole process for testing a limit switch takes about 15 to 30 minutes. If you come across a step that you’re uncomfortable with, just give us a call.
We’ll send an HVAC technician to complete the testing for you.
- Shut off power and verify safety by checking with a multimeter.
- Locate the limit switch and disconnect the wires (label them if necessary so you know where they reconnect).
- Connect the probes to the multimeter in continuity or ohms (Ω) mode.
- Use the reading to determine if the switch is functioning properly.
- Reassemble the furnace panel after testing is complete.
How to Tell If the Limit Switch Is Bad
When touching the probes to the switch terminals during the testing process, the limit switch should stay closed. The multimeter should beep or produce a 0-ohm reading; this indicates continuity.
If it reveals an open loop or infinite resistance, this means the switch is open even though it shouldn’t be. The best thing to do is to replace the faulty switch so it operates safely and restores proper furnace function.
How to Reset or Replace a Faulty Limit Switch
Simply resetting the limit switch might fix the issue and restore the furnace’s normal operation. To do the reset, just push the manual reset button.
When replacement is the best option, follow these steps:
- Turn off the power and gas supply to the furnace.
- Find the limit switch and remove the panel to access and disconnect the wiring from the limit switch.
- Next, unscrew or unclip the limit switch from the mounting bracket.
- Install the new switch and reconnect the wires.
- Put the access panel back on.
- Turn on the power and gas supply.
- Turn the furnace back on.
- Call an HVAC technician if you encounter any problems
Preventive Maintenance Tips for Longer Lifespan
Booking annual furnace maintenance is the smartest way to ensure the limit switch always functions effectively. We’ll look at the switch to see if it has any corrosion or wear. If it does, we can do a quick replacement.
It also helps if you keep the air filter on the furnace changed regularly. This improves airflow throughout the heating system, which further reduces heat buildup and prevents premature limit switch failure.
FAQ
Why is it so important to have a limit switch?
Your furnace needs a limit switch so it can cool itself down before it gets too hot.
What is the difference between an open limit switch and a closed limit switch?
An open limit switch indicates the circuit is broken and electricity cannot flow. A closed switch means it’s allowing current to pass through the circuit normally.
Is it safe to test a limit switch myself?
Yes, it’s safe to test a limit switch yourself as long as you follow certain safety precautions. We can do the testing for you if you want professional help.
Keep Your System Safe—Test or Replace Your Limit Switch Today
We help homeowners all the time with limit switch testing. Book your appointment today to ensure your heater is ready for cooler weather.
Call Jacksonville One Hour Air Conditioning & Heating today. We’re always on time, or you don’t pay a dime!