High limit swtich keeps tripping

[pablo94sc]

Had some issues with my ancient gas furnace (Carrier 58ssb095-120cc) and traced it back to the high temp limit switch tripping causing it to shut down the burners right before the blower kicked on. Replaced the inducer due to age, the limit switch, filter, and cleaned the blower fan. Now it's tripping shortly after the blower kicks on - maybe 20s. Flames are still pretty blue with no sign of rollout. Return suction and air coming out of the vents is strong.

Haven't had a chance to check the temp near the limit switch with a probe yet, but what is the chance that my new limit switch is weak?

 

[jhelrey]

Vent blockage?

 

[monkeyspanners]

Any baffles in the air flow that could have rusted out etc? Any chance the switch for the blower could be switching on too late?

 

[Rockhead261]

May I ask if you are sure you're working with the automatic reset limit switch? I know it may seem dumb, but lots of folks don't know the difference between a flue spill switch, rollout switch, or a limit. If you're pressing a button to reset it, it's the limit switch.

Is it natural gas or propane? Have you checked your gas pressure? That furnace should be firing at 3.5" w.c. on NG but tolerates derating to 3" quite well. Be sure the correct orifices are in place.

If all is kosher, jump the limit switch temporarily and monitor operation. It could reveal some other issue.

 

[pablo94sc]

Sure you can ask. It's NG, haven't checked the pressure of the gas flow and not sure I have the ability to do so. The high temp switch is closing as I'm getting voltage across it the burners shut off. It seems to reset, open, after a couple minutes of the blower running starting the cycle over until the thermostat calls for off. Flow through the vents seems normal, and return is strong as well.

Before replacing the switch, it'd trip before the blower was called for (50 sec on delay), and after cooling down would restart the cycle and typically run normally until called temperature was reached.

Tested a new control board (emergency spare) and no difference. Runs the same as the old board with either switch.

Thought about a dirty a coil, but would that cause an overheat before the blower kicks on?

 

[n8n]

Had a similar issue, it was regulator letting too much gas through. It'd apparently drifted out of adjustment over the years. I didn't do that myself but had a pro check it - I didn't have a thermocouple and didn't want to mess with things I couldn't instrument.

Sent from my XT897 using Tapatalk

 

[pablo94sc]

That's definitely something good to know and check. Thanks for the tip and help so far!

 

[Rockhead261]

I'd start with a manifold pressure adjustment and take it from there. I used to adjust those on the low side to aid longevity. It's a great machine but the exchangers didn't like being overheated, hence the relatively low limit temperature.

Keep us posted.

 

[BillK]

Pablo,
If it does it regularly enough, and you have an infrared type heat sensing gun, I would try to see what the temp is at the limit switch. Unless you got a factory switch I would still call it suspect. Seems like too many parts are being made cheaply now days.

 

[theoldwizard1]

I had a similar problem on a much newer furnace.

Turns our when it is extremely cold outside (well below zero) the burner would run so long that it tripped the high limit. Furnace guy installed a new regulator and adjusted it as low as possible. No luck. His only suggestion was replace the pulleys and belt for ones that run at a higher speed.

I did NOT change the pulleys. The furnace "short cycles" when extremely cold, and sometime will not reach the thermostat setting. This has not happened in the past 5 - 7 years.

 

[dw1]

Had some issues with my ancient gas furnace (Carrier 58ssb095-120cc) and traced it back to the high temp limit switch tripping causing it to shut down the burners right before the blower kicked on. Replaced the inducer due to age, the limit switch, filter, and cleaned the blower fan. Now it's tripping shortly after the blower kicks on - maybe 20s. Flames are still pretty blue with no sign of rollout. Return suction and air coming out of the vents is strong.

Haven't had a chance to check the temp near the limit switch with a probe yet, but what is the chance that my new limit switch is weak?

Do you think possibly the blower motor is slow to start up or run? reason I am asking, I just went through this on my dads furnace, (he lives 70 miles from me) anyway, he bought a blower relay and HTL, I replaced both, several days later it was doing it again, I made the trip back to check it out, trying to cycle, I reached in and pushed started blower squirrel cage and it took off, pulled blower out and the "Plastic" run capacitor had a crack down the side of it, $8.00 later, all was well. It was trying to run, sometimes it would, sometimes it would stop on its own, no air across the HTL and it will open up likes its doing.

 

[pablo94sc]

I'd start with a manifold pressure adjustment and take it from there. I used to adjust those on the low side to aid longevity. It's a great machine but the exchangers didn't like being overheated, hence the relatively low limit temperature.

Keep us posted.

Is there a way to get a tool to test the manifold pressure that doesn't cost $1000? This is something my friend doesn't have. I'm willing to step it down a hair manually by eye-balling it, but I'm sure that's the safest option for testing.

Pablo,
If it does it regularly enough, and you have an infrared type heat sensing gun, I would try to see what the temp is at the limit switch. Unless you got a factory switch I would still call it suspect. Seems like too many parts are being made cheaply now days.

It's a factory switch, but who knows how long it had been sitting on a shelf. My friend has a Fluke with the temp probe, so we'll be checking the temps this weekend.

I had a similar problem on a much newer furnace.

Turns our when it is extremely cold outside (well below zero) the burner would run so long that it tripped the high limit. Furnace guy installed a new regulator and adjusted it as low as possible. No luck. His only suggestion was replace the pulleys and belt for ones that run at a higher speed.

I did NOT change the pulleys. The furnace "short cycles" when extremely cold, and sometime will not reach the thermostat setting. This has not happened in the past 5 - 7 years.

It does it when it's 60 outside or 20. Unit is in a closet in conditioned space, and does it when the door is open or closed (re: all the time).

Do you think possibly the blower motor is slow to start up or run? reason I am asking, I just went through this on my dads furnace, (he lives 70 miles from me) anyway, he bought a blower relay and HTL, I replaced both, several days later it was doing it again, I made the trip back to check it out, trying to cycle, I reached in and pushed started blower squirrel cage and it took off, pulled blower out and the "Plastic" run capacitor had a crack down the side of it, $8.00 later, all was well. It was trying to run, sometimes it would, sometimes it would stop on its own, no air across the HTL and it will open up likes its doing.

Fan kicks on instantly when I turn the setting to ON at the thermostat. It may be running slower than normal, so I plan to move the wire from Med-Low to Med speed and see if that at least lets it run a little longer before the thermal limit trips.

I'll be pulling the access panel off the evaporator this weekend to check for fallen insulation around the exchanger and dirt build-up on the a-coil, as well as additional testing. It's getting warm out so it's not an immediate need, but I definitely would rather have it fixed now instead of waiting until it gets cold again.

Thanks for all the suggestions and help guy!

 

[zmaxmotorsports]

May I ask if you are sure you're working with the automatic reset limit switch? I know it may seem dumb, but lots of folks don't know the difference between a flue spill switch, rollout switch, or a limit. If you're pressing a button to reset it, it's the limit switch.

Is it natural gas or propane? Have you checked your gas pressure? That furnace should be firing at 3.5" w.c. on NG but tolerates derating to 3" quite well. Be sure the correct orifices are in place.

If all is kosher, jump the limit switch temporarily and monitor operation. It could reveal some other issue.

Not all limit switches have reset buttons on them.

 

[zmaxmotorsports]

pull the A coil out and check to see if its blocked.Do you have a picture of the limit switch?

 

[zmaxmotorsports]

Do you think possibly the blower motor is slow to start up or run? reason I am asking, I just went through this on my dads furnace, (he lives 70 miles from me) anyway, he bought a blower relay and HTL, I replaced both, several days later it was doing it again, I made the trip back to check it out, trying to cycle, I reached in and pushed started blower squirrel cage and it took off, pulled blower out and the "Plastic" run capacitor had a crack down the side of it, $8.00 later, all was well. It was trying to run, sometimes it would, sometimes it would stop on its own, no air across the HTL and it will open up likes its doing.

I don't recommend push starting a blower motor.

 

[zmaxmotorsports]

Is there a way to get a tool to test the manifold pressure that doesn't cost $1000? This is something my friend doesn't have. I'm willing to step it down a hair manually by eye-balling it, but I'm sure that's the safest option for testing.



It's a factory switch, but who knows how long it had been sitting on a shelf. My friend has a Fluke with the temp probe, so we'll be checking the temps this weekend.




It does it when it's 60 outside or 20. Unit is in a closet in conditioned space, and does it when the door is open or closed (re: all the time).



Fan kicks on instantly when I turn the setting to ON at the thermostat. It may be running slower than normal, so I plan to move the wire from Med-Low to Med speed and see if that at least lets it run a little longer before the thermal limit trips.

I'll be pulling the access panel off the evaporator this weekend to check for fallen insulation around the exchanger and dirt build-up on the a-coil, as well as additional testing. It's getting warm out so it's not an immediate need, but I definitely would rather have it fixed now instead of waiting until it gets cold again.

Thanks for all the suggestions and help guy!

A plugged up A coil will affect the cooling side of system also.I just saw your picture,yes that's your limit switch.

 

[ctfjr]

Is there a way to get a tool to test the manifold pressure that doesn't cost $1000? This is something my friend doesn't have. I'm willing to step it down a hair manually by eye-balling it, but I'm sure that's the safest option for testing.

It's pretty easy to make a reasonably accurate manometer. A U tube manometer can be made with some 1/4 OD plastic tubing, a piece of wood to attach it to & an adapter to the test plug size on the gas valve (1/8" ??) or if there is one on the gas manifold.
http://www.britannica.com/EBchecked/topic/612191/U-tube-manometer

use water colored with food coloring to make it easier to measure the difference between the fluid heights. LP is about 11", nat gas normally around 4-5"

Should cost all of a few bucks

 

[SALIV8]

have you cleaned your unit and heat exchanger?

probably just need a good cleaning. easy peasy.

 

[pablo94sc]

I'll be cleaning the a-coil this weekend and also take the front panel off the furnace to clean inside the exchanger plenum.

That manometer is a genius idea! Thanks!

 

[zmaxmotorsports]

I'll be cleaning the a-coil this weekend and also take the front panel off the furnace to clean inside the exchanger plenum.

That manometer is a genius idea! Thanks!

You clean the heat exchanger through the area the burners sit in.

 

[pablo94sc]

You clean the heat exchanger through the area the burners sit in.

I was thinking of coming in from the panel for the draft inducer. Not good?

 

[pablo94sc]

Well, decided to turn the gas pressure down about a quarter turn and that seems to have helped, so once I can get a manometer on it I'll be able to dial it in and verify that was the cause. It still trips, but it's taking longer to do so. I can up the fan speed to compensate and that seems to allow a normal cycle, but since it's warmer out now, I'll suffer with it being a little cool in the house.

I'll update once I get it all tested. Thanks!

 

[Rockhead261]

Well, decided to turn the gas pressure down about a quarter turn and that seems to have helped, so once I can get a manometer on it I'll be able to dial it in and verify that was the cause. It still trips, but it's taking longer to do so. I can up the fan speed to compensate and that seems to allow a normal cycle, but since it's warmer out now, I'll suffer with it being a little cool in the house.

I'll update once I get it all tested. Thanks!

[emoji481]

Sounds like you're on the right track. Definitely get a manometer on there - having the pressure too low creates problems as well (condensation). I'd shoot for somewhere between 3.00" and 3.25" W.C. That always seemed to be the sweet spot for me.

As far as cleaning goes, those heat exchanger cells are not easily cleaned. You cannot pull the front cover without removing the entire heat-ex assembly (which really isn't that difficult though it requires complete dissaembly/removal of the gas manifold, inducer, and wiring harness). You might be better off just replacing the cells if you see alot of corrosion. In addition to tearing all the insulation, the real problem with removing the heat-ex cells is the 8 sheet metal screws that hold each one. The cells themselves are high carbon steel and don't tolerate being fiddled with after they've been heat-cycled for a couple decades. They can shatter like glass if you drop them on a concrete floor. I've even had new ones crack from being over-tightened and/or incorrectly torqued. Ask me how I know this.

One more thing to check - pull out the limit switch you just installed and look around with a telescoping mirror and a flashlight. See any cracks or holes in the heat-ex? Pay close attention to any darkened areas you see. Carrier offered lifetime warrantees on some of their heat exchangers, so be sure to check if yours applies. Replacing the cells is a 4-5 hours job for a properly equipped tech so it'll be worth a rebuild IMO if you end up finding a breach. I installed hundreds of those furnaces back in the 90's and they were always among my favorite.

 

[pablo94sc]

Started doing it the original way again... Overheat once and then work properly. Timed the unit and it took a minute-twenty before it shut down the burners and kicked on the fan. It's supposed to have a 50 second on delay before the fan starts, so beginning to think it may be something with the blower motor. Maybe the capacitor is on its last leg and without it there isn't enough juice available to start the motor during a call for heat? Thoughts?

 

[Elginz]

Neighbors was the blower motor not spinning up to speed. Everything came on at the right time, then quit. The blower sounded a little like it was laboring, but I couldn't say for sure because it was the neighbors and I don't regularly listen to there blower motors. Changed the filter and it went a little longer before stopping. Service man came out and replaced the blower motor, all better.

 

[pablo94sc]

Could be the motor. I need an anometer (?) to test air speed and cfm. Hoping a $2 capacitor solves it, otherwise I may call out a tech. Getting tired of working on it. Lol

 

[nehog]

... It's supposed to have a 50 second on delay before the fan starts, so beginning to think it may be something with the blower motor. Maybe the capacitor is on its last leg and without it there isn't enough juice available to start the motor during a call for heat? Thoughts?

If it was the capacitor, you'd hear the motor humming and trying to start. I'd monitor the voltage to the motor (that should be easy) and that will tell you if the problem is at the motor or the controller.

 

[pablo94sc]

If it was the capacitor, you'd hear the motor humming and trying to start. I'd monitor the voltage to the motor (that should be easy) and that will tell you if the problem is at the motor or the controller.

I had installed a new board (emergency backup) previously with no change, so I put the old one back in. I'll swap again when I get my manometer and test the gas pressure. In the mean time, I'm keeping the heat down.

There is a plus side to this - I'm learning a lot on how to troubleshoot HVAC systems, get to buy new tools, and spend more time in my heated garage!

PS: It's amazing how cheap parts are for a 30+yr old furnace compared to a newer model. Found a gas regulator for $15, control board for $45, draft inducer assy for $120, high temp limit for $15, and capacitor for $2 - all new or NOS.

 

[pablo94sc]

Got a Fluke and tested the run cap... good. Motor is old and doesn't spin as freely as I'd expect, but no bearing whine, so suspect. The step down transformer looks a little toasty. It tested 16ohm on the 120v side and 1ohm on the 24v side. Can't believe I didn't see the charring before. Need to figure out why it heated up to the point of discoloring the wires and replace those components.