The furnace limit switch is an important safety component in any residential or commercial HVAC system. Given this, it’s in your best interest to be as informed as possible about this component to avoid any issues or problems down the line. To make things easier for you, here’s everything you need to know about furnace limit switches.
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What is a Furnace Limit Switch?
The furnace limit switch is designed to shut off the furnace if the temperature inside the furnace gets too high. Without a limit switch, your furnace could overheat and cause a fire. In this blog post, we’ll explain what a furnace limit switch is, how it works, and why it’s important to keep it in good working order.
A furnace limit switch is a type of safety switch that is mounted inside the furnace. This switch is triggered when the temperature inside the furnace reaches a certain point. The limit switch is designed to shut off the furnace before it gets too hot, preventing the risk of overheating and fire. The temperature at which the limit switch is triggered is usually between 130 and 160 degrees Fahrenheit.
When the furnace limit switch is triggered, the furnace will shut off until the temperature inside the furnace drops below the limit switch’s set point. The furnace will then restart and continue to operate normally. However, if the furnace limit switch is not working properly, it may not shut off the furnace when the temperature reaches a dangerous level. This can be a major safety hazard and should be addressed immediately.
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How Do You Keep Your Furnace Limit Switch in Good Condition?
It’s important to keep your furnace limit switch in good working order. Regularly inspect the switch for signs of wear or damage. To keep your furnace limit switch in good condition, it’s also important to keep it clean. This can be done by using compressed air to blow out any dust or debris that may have accumulated inside the switch. It’s also important to check the wiring of your furnace limit switch periodically. If any of the wires have become loose or frayed, they should be replaced.
Aside from what we’ve already mentioned, it’s also vital to make sure that the furnace limit switch is the correct size for the furnace. If the switch is too small, it won’t be able to sense the temperature accurately, leading to the furnace shutting down prematurely. If the switch is too large, it could lead to the furnace shutting down too late, allowing the furnace to become dangerously hot.
Conclusion
Overall, the furnace limit switch is an important safety component in any HVAC system. It is designed to shut off the furnace before it gets too hot, preventing the risk of overheating and fire. Make sure to regularly inspect the switch for signs of wear or damage and contact a professional HVAC technician if needed. By keeping your furnace limit switch in good working order, you can ensure the safety of your home or office.
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When I built my mill a couple of years ago I struggled with wiring my limit switches in series. My intent was to use one input on the BoB so I would have a couple for future expansion. That didn't work out so I ended up wiring each switch to and individual input. It's been working just fine until yesterday when I decided I wanted a Z axis zero tool setter. This is where my problems begin.
Here's a picture of my limit switch/tool setter terminal block. The three wires on the upper left are the X, Y and Z plus (+) wires from the limit switches. Each of the four upper right terminals are jumpered with one wire going to an input terminal on the BoB (pin 10 on the Input Signal tab in Ports and Pins). I checked continuity and all is good. You will note that the tool setter plus (+) wire is not connected nor is it connected at the BoB. One step at a time.
The four wires on the lower left are ground wires (-) for the X, Y, and Z axis and the E-Stop circuit. The E-Stop plus (+) is wired to pin 13 on the BoB. The wires on the lower right are also jumpered with one wire going to the ground terminal on the BoB. Checked continuity and it is good across all terminals.
I enabled all limit and home switches on the Input Signal screen and set them to pin 10. As I said above the E-Stop is set to pin 13. The E-Stop works. I checked and am getting 5VDC across pin 10 and the BoB ground terminal but nothing at the terminal block. As a reality check I unwired all the switches and wired in just one and it worked. I can trip the circuit and home the one axis.
All this being said what am I missing or doing wrong? I'd like to get this problem solved so I can continue with wiring in the tool setter.
Tom S.
From your description and the picture it looks like you have them all wired in parallel not Series. Is that what you intended? You would need Normally Open limit switches for a Parallel setup and Normally Closed limit switches for a Series strategy. Best practice is for Limit Switches to be Normal Closed and wired in Series so any wiring fault will trigger the limit circuit. So the question is, are your limit switches Normally Open or Normally Closed? You mentioned that you tried it with one limit swtch connected and it worked. Did you then go straight to all three or did you try it with just two? what was the result? Are you 100% sure your jumper wires are working as expected? It is hard to tell from the photo. You can test this by leaving the jumper wires in place and then using just one limit switch (you said that works) and move it to each of the 3 positions. If it does not work in each position, then your jumpers or wiring block is not working as expected.. If it does work in each position then try adding a second limit switch.
From your description and the picture it looks like you have them all wired in parallel not Series. Is that what you intended? You would need Normally Open limit switches for a Parallel setup and Normally Closed limit switches for a Series strategy. Best practice is for Limit Switches to be Normal Closed and wired in Series so any wiring fault will trigger the limit circuit. So the question is, are your limit switches Normally Open or Normally Closed? You mentioned that you tried it with one limit swtch connected and it worked. Did you then go straight to all three or did you try it with just two? what was the result? Are you 100% sure your jumper wires are working as expected? It is hard to tell from the photo. You can test this by leaving the jumper wires in place and then using just one limit switch (you said that works) and move it to each of the 3 positions. If it does not work in each position, then your jumpers or wiring block is not working as expected.. If it does work in each position then try adding a second limit switch.
Thanks for your response and questions. My switches are NC so that being said I need to wire them in series. I connected one switch only. Didn't try two. Jumped right to three and got no where.
I hear what your are saying about parallel vs. series wiring. Bear with me. I'm electrically challenged but I think I grasp the concept. I'm out of town for the next five days but will give it a try when I get back. I'll post how it works out.
Tom S.
I am also out of town for a few days but with NC switches you must wire them series. so the ground goes into one switch, then out of that switch into the next, our of that switch into the third, and then finally into the BOB. there will be NO common GRD connection as each switch is daisy chained from one to the next.
Thanks. Can't wait to get back in the shop and get my mill running again.
Tom S.
Back at it today. Wired the switches in series, or so I thought. Obviously I'm not getting it because I'm not getting lights on the Mach 3 Diagnostics screen when I actuate a switch. Here's a picture of my terminal block and an explanation on how I wired it.
Starting at the upper right is the wire from pin 10 on my BoB. The COM wire from the X axis switch which is connected to the upper left terminal which is jumpered to the second terminal from the upper left. The X axis NC wire is connected to the terminal second from the top on the right side. The Y axis switch is wired in series, or so I thought, from the X axis switch to the Z axis switch with the last terminal on the lower left going to the ground terminal on the BoB. Pins and Ports is setup with pin 10 designated for all switches.
I'm a reasonably intelligent person but for some reason I have a mental block when it comes to electricity and wiring. Any input is appreciated.
Thanks,
Tom S.
I'm sorry Tom, I don't understand your explanation, but try this:
First make sure all of your limit switches are wired Normally Closed. Check this with a Multimeter ohms setting.
Then with the power off, again use the the Multimeter to follow the path. Put one lead on the wire going to pin 10, then check at the X switch with the other lead, then make sure that you have the signal at your terminal block, then make sure you have the signal at the Y com terminal at the switch, then check to make sure that toggling the switches causes a state change and so on. Trace through the circuit one connection point at a time until you get back to the Bob Com terminal. Then if all is well, it should work as desired.
It also helps sometimes to draw the circuit out on a piece of paper, just to help troubleshoot.
I'm sorry Tom, I don't understand your explanation, but try this:
First make sure all of your limit switches are wired Normally Closed. Check this with a Multimeter ohms setting.
Then with the power off, again use the the Multimeter to follow the path. Put one lead on the wire going to pin 10, then check at the X switch with the other lead, then make sure that you have the signal at your terminal block, then make sure you have the signal at the Y com terminal at the switch, then check to make sure that toggling the switches causes a state change and so on. Trace through the circuit one connection point at a time until you get back to the Bob Com terminal. Then if all is well, it should work as desired.
It also helps sometimes to draw the circuit out on a piece of paper, just to help troubleshoot.
Thanks Jim. Below is the diagram I used (limit switch circuit only). My BoB is powered through a USB hub so the DC power supply shown on the diagram is not applicable. I'll do the test procedure you laid out and see if I can find the problem.
Tom S.
Tom,
The C from the Limit Z should probably go to the +VDC not -VDC. The typical circut is that the BOB is looking for +5v or +Vdc to indicate that all is well and nothing to indicate that there is a problem.
So if you connect C (common) from the Limit Z and then through all of the limit switches as indicated to the BOB then with all of the switches in their Normal postion it will allow the +Vdc to get to the BOB. If any switch actuates then it will break the circuit and prevent the +Vdc from getting to the BOB.
BTW, I would expect the same for the E-STOP as most E-STOP switches are Normally Closed