Fan limit switch weirdness

a_home_owner

I have an old fan limit switch, model Honeywell L4064B2210. Based on some research I've done, I believe it is faulty and should be replaced. However, I'm seeing a strange behavior that I want to ask about before I replace the part.

I recently replaced the fan blower motor. I am fairly certain I have it wired to spin in the correct direction. During testing after the replacement, I noticed the blower was not shutting off after running for a long time. The first time I tried it, it did stop when I gave it a little poke. Since then it just keeps running until I shut off power at the breaker. This points to a problem with the fan limit switch, as mentioned above.

During the last test I did, I monitored the temperature disk in the limit switch before the unit ran, as it heated up, and after I lowered the thermostat to prevent additional heating. It seemed to work OK except it got "stuck" during the cooling phase and never reached the point to turn off the blower.

Here's the weird part: after I tuned off the power, the fan limit switch reset and returned to a normal position within a few short minutes. So clearly the limit switch itself isn't physically stuck, but I can't explain the way it's acting. The best ideas I can come up with is the movement of air from the blower is enough to keep the helical temp sensor from returning to the normal position, or I messed something up when wiring in the new motor.

My money is still on a bad limit switch that needs to be replaced, but I wanted to ask here to see if anyone can confirm that or has other ideas.

Thanks!

EdTheHeaterMan

That is how they work. This is not a precision testing instrument used in Rocker Surgery! The tension needed to snap that switch to the off position is part of the design calibration so it does not keep cycling the fan on then off then back on several times after the heat exchange (HX) cools off.

You need to move the OFF temperature tab about 10 degrees higher in order to get the fan to stop without blowing cold air into the home after the call for heat is done. See if that does the trick.

I remember trying to get this just right the first time I replaced a L4064 fan limit control so many years ago.  Of course you don't want cold air blowing all the time during the winter. So I selected 90° as the fan off dial setting.  I figured 90° was plenty warm enough to stop the fan from running and blowing cold air.  But I was wrong and the fan just kept running like your system does.  So I then adjusted it to 120° off and the fan continued to run until the dial stopped at the 120° mark and stopped turning.  After about 45 more seconds the switch snapped off and the blower stopped.  

Was I finished?… Not by a long shot.  The HX still had some residual heat left in there and the fan limit dial started to climb back up and after about 65 seconds… CLICK the fan back on blowing more heat off the HX for another 45 seconds until it dropped to the point where the dial stopped turning and then CLICK at about 90° and the fan was off.  It did that 3 more times.   So 120° was not the magic setting so I tried 110° and got the fan dial to only click back on once and stop after that.   That extra tension is so you don't have the HX cause the switch to go on and off several times after every cycle is over.  

Hope this info helps

Mr. Ed

mattmia2

probably just cooled more and eventually shut off. where is the off stop set?

a_home_owner

The lower/OFF limit is set around 100. I did nudge it up a little before the last test to see if it would help, but it didn't make a difference. I let it run for probably about an hour+ without any more heating cycles, so I don't think the extra 5 minutes with the power off should have allowed the switch to cool much more than it already had.
Note: this switch probably dates back to the early 80s (if not earlier!) so being faulty seems most likely. Perhaps it got pushed "over the edge" or was damaged when the motor went out?

a_home_owner

One other quick note: it was working last year, but the motor failed the first time I ran the system this year, so I don't know if it was working properly before I replaced the motor.

mattmia2

maybe the vibration from flipping the switch nudged it over?

a_home_owner

https://forum.heatinghelp.com/discussion/comment/1871072#Comment_1871072

Thanks for that info Ed! To be clear, I did NOT replace the switch yet. I'm trying to figure out if it's actually bad before I spend the money on a new one. Your comment will be really helpful if (when) I do replace the switch.

a_home_owner

https://forum.heatinghelp.com/discussion/comment/1871077#Comment_1871077

I did try gently banging on it from various angles, that didn't do anything to shut it off. There shouldn't have been any vibration from flipping the switch since the switch was in a breaker box on the other side of the basement 😉

mattmia2

did the dial turn past the off stop? does it have a relay for ac?

HVACNUT

An electrical meter will confirm your diagnosis in 2 minutes.

EBEBRATT-Ed

As @EdTheHeaterMan said fan/limit controls are not exactly the most precise controls made. They usually have to be tweaked, and they are just a bimetal control so they can get tired.

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1871078#Comment_1871078

The bi-metal spiral spring may be getting old and you may need a replacement, OR, Just try to set the low number to 120° and see it that helps. You may be able to tweak the adjustment and get some more seasons out of that old part. If your heater is using that technology, it is pretty old. It may be time to think about something newer before replacing any old parts. You just replaced a motor, now a limit switch, what will go next? Just something to think about

mattmia2

https://forum.heatinghelp.com/discussion/comment/1871152#Comment_1871152

the hx and the gas valve are about the only parts left unless it is oil.