It’s cold and chilly outside and you suddenly notice that you have no heat coming out of your furnace! All it shows is a flashing light indicator and constant shutting down of your heating system.
That’s mainly because of the primary/secondary limit switch open Lennox.
When you encounter a limit switch error code, it’s mostly because of overheating, plugged filters, or broken capacitors. Other causes can also be that your filters are dirty and it has completely restricted the airflow going into the heater. These problems can be easily solved without proper diagnosis and troubleshooting.
However, to understand the problem, you first have to know what’s the role of the limit switch in your Lennox furnace. So, let’s get going!
How Does Limit Switch Affect Your Lennox Furnace?
A limit switch is a safety device that monitors the temperature inside the furnace and shuts it off if it gets too hot or if there is a problem with the airflow. The role of this switch is to communicate with the blower when the temperature has risen or fallen.
The limit switch is typically located near the heat exchanger and is designed to turn off the furnace if the temperature inside the furnace gets too high.
If the temperature of the heat exchanger rises above the set limit, the limit switch will open the circuit and shut off power to the heating elements. As a result, preventing the furnace from overheating.
Now that you know about the basic furnace limit switch mechanisms, it’ll be easier for you to understand the problem.
When Does the Limit Open in Lennox Furnace?
While there are different Lennox furnace models, the general limit opening temperature is around 135 degrees in the blower compartment. Or, 170 degrees in the burner compartment.
While it can be hard to check the temperature without a pyrometer, usually, the furnace does not reach that level. Mostly, the furnace burner or blower remains on the preset temperature settings but the limit switch opens because of other reasons.
Let’s take a look at what those can be.
Why Does the Primary/Secondary Limit Switch Open Lennox Error Code Occur?
The main reasons behind the primary/secondary limit switch open error are a damaged control board, plugged filters, restricted airflow, and so on. Other causes include A/C coil contamination, undersized duct pipes, etc.
These problems are very similar to that you will find in a Burnham boiler shutting-off scenario. So, if you are familiar with those, it will be very easy for you to understand the following causes.
When you notice that your Lennox furnace is not heating up or turning on, then the most common reason is that you have restricted airflow. Now, you can have airflow restrictions for a multitude of reasons starting with overheating of your furnace and boiler.
Another significant reason for your furnace to face airflow restrictions is that your thermostat and furnace are not syncing up. If that’s the case then you will have improper temperature signals in your furnace.
Dirty or Restrictive Filters:
While dusty and restrictive filters are very different things, they can cause the same issue with your Lennox furnace. If you notice that your furnace or boiler is not producing enough heat then the possibility is high that your filters are dirty.
When your filters are plugged or clogged up, the airflow is significantly restricted. And because of the insufficient airflow, the blower does not get time to cool down and fails to start when you turn it on.
Another common mistake that residents make is using a restrictive filter such as Merv-rated filters. Those tend to severely cut down the airflow going into the boiler to cool down the furnace and give it time to reset.
Bad Limit or Control Board:
Yes, a bad limit control board can cause the primary/secondary limit switch to open on a Lennox furnace. The limit control board is responsible for monitoring the temperature of the furnace and controlling the limit switches to ensure that the furnace does not overheat.
If the limit control board is faulty, it may not be able to accurately read the temperature of the furnace and control the limit switches properly, which can result in the primary/secondary limit switch opening and shutting down the furnace.
Damaged Blower Fan Capacitor:
Behind the blower fan on your Lennox furnace, you will notice a capacitor. Now, why should you be concerned about that tiny component when faced with the open limit switch? Well, that capacitor can be a culprit for your limit switch to malfunction.
If that capacitor renders to be damaged, then the electricity passed through the blower won’t be sufficient. As a result, you will encounter your limit switches malfunctioning.
How to Troubleshoot the Lennox Open Primary/Secondary Limit Switch?
The most common troubleshooting method is to reset your Lennox primary limit switch. Or you can try changing your dirty filters every three months as they can cause this problem in many instances.
This troubleshooting process is as easy as the ones you will find in troubleshooting your Taco Sr504 in your thermostat. So, don’t be afraid of doing it yourself if you understand the process well enough.
Lennox Primary Limit Switch Reset:
Resetting your Lennox furnace can easily solve most issues that include overheating, limit switch opening, etc. So, here are the steps to reset your Lennox primary limit switch reset-
Step 1: First, turn off the power and make sure there’s no electricity passing through your furnace. You can just turn it off from your circuit breaker which says furnace or heating system.
Step 2: Go to your control panel which is located on the bottom panel on downflow furnaces or the top panel on upflow models. Lift the panel up.
Step 3: There should be a button named “Lennox limit switch reset”. Press and release the switch and wait for a while to reset your furnace.
Step 4: Once you’re done with the reset, turn the power on to your furnace and the task is complete.
Limit Switch Replacement:
If resetting your furnace does not help, then replacing the limit switches is the most effective in most cases. Before you change the limit switches, it’s best for you to test them.
With the help of a multimeter, you can easily test whether your switches are working or not. Check the voltage of those switches and if they are above the preset limits, then your limit switches are bust.
The tough part about replacing the limit switches yourself is that it can be quite risky. So, it’s best to contact a professional to help you out.
As mentioned before, you can definitely notice the role of your filters when faced with an open limit switch error code. So, it’s very intuitive that changing your furnace filters can eventually solve this issue altogether.
However, don’t purchase Merv filters that are too restrictive as they can be detrimental to your furnace system. Try to use a filter with a Merv rating above 6 as those are adequate for most systems.
Now that you know all about these troubleshooting methods, it will be a breeze for you to solve this issue. However, don’t think all heaters and furnaces will have easygoing fixes that you found out in Beacon Morris heater troubleshooting.
So, don’t panic even if you encounter any minor setbacks!
Primary/Secondary Limit Switch Open in Different Lennox Furnaces
Here is some key information regarding two different Lennox furnace models and their primary/secondary limit switch open issues-
Lennox G26 Primary/Secondary Limit Switch Open:
The common solution for the G26 primary/secondary limit switch open error is to check the switches or capacitors. Most often than not, users found out their switches are damaged or the capacitor has gone past its limit.
To check the switches, you can connect one test lead to the furnace sheet metal and use the other to test for 24 VAC. The reading for the switches for that circuit is 24 VAC, then your switches are alive and kicking. If the voltage is 0, then you know what’s the issue.
Lennox G40uh Primary/Secondary Limit Switch Open:
For a Lennox G40uh model, the primary switch is located just underneath the furnace cover. Check the switch with your multimeter and if it’s fine, move on to checking the secondary limit switch.
Reaching the 2nd switch is difficult as you have to trace back the wires to the blower door. The secondary switch is connected between pins 1 and 7 on the control box. If the plug is disconnected then connect it back and you’re good to go!
When Should You Contact an HVAC Professional?
If you suspect that the open limit switch in your Lennox furnace is causing problems, you should contact an HVAC professional as soon as possible. The open limit switch is an essential safety feature that prevents your furnace from overheating and causing damage or even potentially dangerous situations like fires.
An HVAC professional can diagnose the problem and make the necessary repairs or replacements to ensure your furnace is operating safely and efficiently. It’s best not to try and fix the issue yourself, as working with a gas furnace can be dangerous without the proper training and experience.
Frequently Asked Questions (FAQs):
What are the consequences of a Primary/Secondary Limit Switch being Open?
When the Primary/Secondary Limit Switch is Open, it means that the furnace will not be able to function until the issue causing the high temperature has been resolved. If left unchecked, this can result in damage to the furnace or other components of the heating system, as well as a potential safety hazard if the furnace continues to overheat.
How can I tell if the Primary/Secondary Limit Switch is Open in my Lennox heating system?
If the Primary/Secondary Limit Switch is Open, the furnace will not turn on, and the system may display an error code or fault indicator to indicate the issue. A professional HVAC technician can diagnose and repair the issue.
Can I fix an Open Primary/Secondary Limit Switch myself?
No, attempting to repair or replace a Primary/Secondary Limit Switch yourself is not recommended. Because it involves working with electrical components and can be dangerous. It is recommended to call a licensed HVAC technician to diagnose and repair the issue.
Now you know all about the famous primary/secondary limit switch open Lennox error code and its troubleshooting. All that is left for you to do is to solve this issue by yourself or contact a professional if you don’t feel confident enough!
Always remember that your furnace can excel only when you take care of it. So, be careful about your heating solutions from now on!