Gas furnace high temperature limit safety switch troubleshooting

Baisc information and trouble shooting on the limit switch. Just because the furnace IFC liht says it is open, does not mean it is bad.
Good pointers to help you solve the problem you have. Due to the dangers of gas heat, electricity, and sheet metal this video is intended for professional use only.

[@danielyoo1718]

Thank you! By your video, I could revive my heater. Otherwise, I had to sleep in 15 degrees like in Siberia. My wife also says thanks to you.

[@ChilesHeatCool]

Your welcome glad I could help.

[@mercurym-7904]

@@ChilesHeatCool I just had a new furnace installed ~ High efficiency “RUUD” I just got a notification from my smart thermostat saying that My main limit switch is open. What does that mean?

[@ChilesHeatCool]

Low air flow from the blower or ductwork. Another possibility is the manifold gas pressure is to high.

[@user-gz9dc2os8q]

Thank you so much for this video! My furnace stopped working at 7pm on Christmas Eve and it's 10° outside. Turned out the high limit switch connections were corroded, cleaned them off and it kicked back on like it should.

[@ChilesHeatCool]

Your welcome. Glad I could help. Feel free to share to help others out.

[@purview4012]

Thank you for this video! I followed your instructions and my heat is going again. I have NO experience in HVAC. In fact this is the very FIRST time I did any troubleshooting!

[@ChilesHeatCool]

Your welcome. Glade my video helped.

[@ShadraqsVids16]

I cant thank you enough. Ive been watching yourube videos for 2 hours trying to figure out why my furnace is blowing cold air and every single video says "just clean the flame sensor" thank you for explaning this 🙏

[@ChilesHeatCool]

Your welcome. Feel free to subscribe and share to help others.

[@jjyemg2397]

For those who read these comments. Do NOT reset a rollout unless you're a professional or know what you're doing. Do this ONLY to troubleshoot one time. CO could fill your house and you'll be dead before the tech comes to replace/repair your system. There are a few reasons why you're rolling out flames so have a Pro assist you.

[@ChilesHeatCool]

Right. That gets covered under my roll out video switch.

[@raykibria5538]

Thank you for that information

[@GranpaTruck]

Reset my limit switch after it was blinking four times and now everything is back to normal. Changed filters and cleaned it and now i got heat in far northern wisconsin and saved $$$.

[@ChilesHeatCool]

Good deal

[@grc5618]

Thank you for posting this. You got my furnace back to running. It tripped after I cleaned the inside of my furnace, and didn't know what happened. Thank you sir!

[@ChilesHeatCool]

Your welcome.

[@mikeleyshon1799]

Appreciate the videos. Have a 1999 Rheem criterion 2, 100,000 BTU/80%. Installed 10/2000 and very reliable. The last 3 seasons I've had short cycling issues. I clean the flame sensor, reinstall and forget it. My usual trick didn't work last week and I was in awe. Tossed down 15 bucks for a new a new flame sensor locally, no change. Today I tapped the top of that roll out sensor with my finger and she's running normally again. Getting near the end of heating season in central Ohio. My outdoor condensing unit is 38 years old. Weather King before Rheem started using the name. Made by Addison products inc. 1984. All said, think it's time to replace the system. Like Rheem/Ruud. See Goodman products on the lower end. Advise if you will

[@ChilesHeatCool]

The quality of the install is more important than brand name. Basically 2 companies make the gas valves and safeties. If the manifold gas pressure and air flow set right during the install, the furnace won't last. If the ductwork isn't correct it won't work right. I put goodman in my house, and I've installed many of them, Lennox is expensive on parts and have part numbers custom to them so you can't just buy a part unless you can match it up. Trane make good furnaces but I hate cleaning the a/c coil due to the spines.

[@pragmatic1111]

Thanks for posting! Very helpful. My three limit switches all tested closed/zero ohms/good. Turned out to be a bad connection of the high-temp limit switch circuit wire on the multi-plug which plugs into the controller module. No voltage was getting onto the high temp limit switch wire. Unplug/re-plug, problem solved.

[@ChilesHeatCool]

Your welcome, glad it helped.

[@260anthony]

Worked like a charm ! God bless

[@ChilesHeatCool]

Your welcome.

[@Mdaneke]

There is a way to reset the high temp limit switch as well. Turn off the emergency switch and disconnect the two wires and take out the 2 screws. Remove the high limit switch and flick it with your finger and that will flatten the disk. Reinstall the high limit switch and connect the wires and turn the emergency switch back on and run the heater until it reaches thermostat temperature. If it goes off you reset it. If not you need a new high limit switch.

[@ChilesHeatCool]

That is not fixing or resetting. It's like hammering your starter to make it work. It's only going to last so long till it's done. What it there is a legitimate problem and the "reset" causes a fire. Trust me, it's so much cheaper and easier in the long run to fix it right. Very important on things that can kill you. But, this is America and you have the freedom to do that if you want.

[@debosway]

So if you suspect switch is bad just replace it? Don’t flick it to try to reset it. Thank you for letting me us know about potential fire

[@Gus11445]

Very rarely do switches go bad. Usually there is a problem setting it off. Did not show the actual troubleshooting. 1. Is the filter fresh and merv 7-8. 2. Is there any obstruction anywhere, secondary heat exchanger plugged, evalorator plugged, a lot of registers closed off, returns blocked, are dampers open? If it is still overheating, check gas pressures and combustion. Check blower speed/heat rise on system and adjust blower speed accordingly.

[@ChilesHeatCool]

Lol high temp limit is different than pressure switch lol.

[@ChilesHeatCool]

That was mentioned dude

[@dmitripogosian5084]

For troubleshooting. My furnace short cycles - it reaches temperature set on thermostat, switches off, starts again in like 30 seconds, switches off, starts again etc .... How to check it is not an overheating ? (though reaching thermostat temperature seems to point at that it is not)

[@carvajalcarlos645]

Thank you for your video.

[@ChilesHeatCool]

Your welcome.

[@dkshrute1700]

Great video. Don’t know if you’ll see this but. I’ve got a diagnosis question. I’ve got a Lennox furnace and recently got the trouble code of primary / secondary limit switch open. Filters aren’t clogged. I cut power and gas to the furnace. Jumped the primary limit switch, powered up furnace and now I have flame and heat as it should. Cut power / gas again and reconnected limit switch, rechecked and system works fine. Problem happened again two days later. Do I most likely have a bad limit switch or is there anything else you recommend me checking.

[@ChilesHeatCool]

Air flow and gas pressure will cause the limit to trip. Do you have a dirty blower wheel, dirty secondary heat exchanger (high efficiency furnace), dirty air conditioner coil, blocked return, to many supply dampers close or registers close, blower speed to slow. That's all the air flow stuff I can think of or a zone system we're the zone dampers are closed or dump some damper isn't opening. The only thing I will say you will need a tech for is the gas pressure. Your gas valve has a regulator on it to step the pressure down. You need a tool called a manometer to make sure it's adjusted to manufacturer specifications. I say call a pro because you're messing with the gas, and any accidents can cause insurance to say we won't pay as that will hurt their profits.

[@ChilesHeatCool]

kzbin.info/www/bejne/r5PcqJaYoNGjgtE This is a long-winded video, but see what the temperature rise is can tell if the switch is bad. Make sure the supply air temperature reading is out of sight of the heat exchanger. The switch will have some number l140-35 for an example as they vary. The l140 is the limit of 140 degrees, and that's when it opens to cut off the gas. The 35 means it must cool off 35 degrees to close and allow the gas to flow again. So 140 minus 35 equals what 105. So the supply air temperature should be 105 degrees f. If it doesn't perform close to the specifications, than it's bad. If its rated at l145, opens at 135 its opening to soon its bad.

[@waaazupd1948]

I only figured out what the problem was because on the blow a motor I noticed the little red switch when I looked at it just right. I messed with it and noticed that the little button pushed back in and the furnace turned back on right away. I live in Wyoming where in the middle of Winter and it was freezing cold. The gas pilot lit but the furnace itself didn't turn on until I pushed that button in it turned on immediately. Had I known the switch was there I would have figured out the problem a long time ago. And then I ran into your channel and it explained what that switch did. I guess a question I have is should I replace it since it engaged?

[@ChilesHeatCool]

Maybe. You said pilot so it's old but if the furnace is overheating that would trip it. If its cheap enough like $10 I'd replace it cause it's $10. Make sure the blower wheel is clean and filter are not to dirty. Air flow and gas pressure effect furnace temperature that can cause it to over heat.

[@waaazupd1948]

@@ChilesHeatCoolthanks, appreciate it. Me and my lil dog were freezing. I stayed up till 3 am looking on the net for a resolution and since I'm a truck driver I'm not home very often. I was afraid I was gonna have to call a service tag and if I did they probably would have laughed at me since it was a simple solution. I did blow out all the dust before I noticed the red button then changed the air filter after it turned on. It dies sound better now after the dust was removed. I usually changed out the filters each winter but since they were pretty dirty I will be changing them out each month if needed.

[@richardfowler3254]

Thanks, I will need to test ours with my meter... At Grandma's house what is happening is that the heater runs and will cycle off normally when the t-stat is satisficed but when it attempts to reignite the burner the gas valve opens (and lights) but then the gas flows cuts out right away till it cools way down. I am going to need to locate the limit switches and test them. The heater has been working fine for about 12 years now but in the last couple of months this gas cut out has started.... I did air balance for 38 years not service so your tips will give me the starting point to see if I can figure this out...

[@ChilesHeatCool]

If it lights and goes out, might be a flame sensor. Glad my videos can help.

[@misodinamosa]

Wow that’s awesome, thank you!

[@ChilesHeatCool]

Your welcome.

[@comm2531]

Great video.

[@ChilesHeatCool]

Thank you. Try to make HVAC videos to help and inform.

[@zorst99]

This is my problem do you think this would be the switch .......Problem: When desired house temperature is reached the heating portion of the cycle terminates, but the furnace blower fan fails to turn off after removing excess heat from the furnace. The furnace must be manually unplugged to accomplish.

[@ChilesHeatCool]

Yes if its an old fan limit electromechanical control. Newer style furnace it may be set to long or the contacts are sticking on the relay on the board, and it would need a new board.

[@jadyas87]

I have a Rheem 3 years old unit. With an issue related. Here's what happens: Temperature is set for 70 furnace starts heating up and turns on. Furnace light says “H” 30min later Flame in the furnace goes out and blower continues. This happens well before temperature is reached Furnace light blinks “22” Furnace kicks back on after almost 3min on the second try, flame is lit, changes to “H” with a yellow light next to it. This cycle keeps going but 2nd time flames go out is after 10min then 7min. After that my set temp is reached! Do you know what that could be?

[@ChilesHeatCool]

Dirty fan, dirty filter, to many supply dampers closed, manifold gas pressure to high, dirt coil, dirty secondary heat exchanger.

[@InspiraAccionDiaria]

I did replaced the heat sensor as well as the mentioned high limit sensor...but it keeps reading Primary/secondary limit switch open.....how can I reset the roll up sensor?

[@ChilesHeatCool]

kzbin.info/www/bejne/h4nTg6KphZead9Esi=mC8Pvfs0ELjvWxlg Button on top.

[@Shanders86]

So my furnace, exhaust and intake are clear with no obstructions. If the door is on the burner box, it tries to light but will only stay lit 5 seconds then off. Then it will try 2 more times and put itself out. We have changed out the exhaust fan, the flame sensor and the pressure switch….. we have no idea where else to go….. we are currently living with the burner box open. The flames going into the exchanger are coned and don’t show any signs of the heat exchanger being cracked…. HELP.

[@ChilesHeatCool]

It might be related to the flame sensor. It could be a dirty burner or bad board or bad wire or the furnace lost ground. I know you you replaced the flame sensor but what was the micro Amp reading? This link is how to test the flame sensor. kzbin.info/www/bejne/m6GYeWSAarl1Z9E

[@doughairrell4048]

I ordered a new limit switch that will be here in a couple of days. Can I bypass the faulty switch until the new one arrives. The filter needed replaced, which was causing the switch to limit. I put in a new air-cleaner filter.

[@ChilesHeatCool]

Its not recommended to by pass safties of a gas furnace.

[@Boxerdad27]

Can a bad igniter cause your furnace to throw an open limit switch code?

[@ChilesHeatCool]

Nope. Only an open limit, bad board, or bad wires or connectors.

[@dynamite2179]

So my blower thing never kicked on when I unhooked like in video

[@ChilesHeatCool]

Possibly a bad board or a bad motor. kzbin.info/www/bejne/pJytZaRjYrx6ja8

[@ramblinman7153]

So my limit switch keeps tripping. I used a multimeter to verify that it reads 24v when it trips. I also tested continuity and it was good. I removed the limit switch and used a jumper to keep the furnace running while using a meat thermometer to check temperature in the port. Temperature never came close to the 200F specified on the limit switch. So I replaced the limit switch and it's still tripping. Air filter brand new and still trips if removed. Blower is 1 year old, furnace 6 years old. All registers are open and should be clean. Unfinished basement so only registers on upper levels. No kids or pets that might stuff something in the registers. There are no dampers either. AC unit was replaced last summer, so the coil should be relatively clean. What else could it be? Evey time a tech comes out, they suggest something different with a huge expense. Kind of like going to a jiffy lube and being up sold a cabin air filter replacement for 75 bucks. Moreover, it never locks out on me. And I only seem to have the issue when bringing the house to temperature in the morning. Say from 65 to 70. It runs fine during the day and night to keep things at temperature.

[@ChilesHeatCool]

Hmm with out seeing it i wonder if the blower speed is right or if the gas pressure is to high on the manifold. Manifold gas pressure should be checked by a profesional or sometimes the utility company might do it free. I think I have a video that's part of a furnace service series talking about the temperature rise and sequence if operation. Only 10% maximum on registers should be closed in a house. You could try opening one in the basement and see if it helps. Also make sure the blower is clean. Do a temperature rise check and compare it to the data plate on your furnace. League reasons I say mess with the blower motor speed if you can. Now if its an older furnace the fan limit switch has to be set right.

[@ramblinman7153]

@@ChilesHeatCool no registers in basement as it's not finished. I did a temp rise test and it is well within limits. Temp at return was roughly 70F and at supply 120F.

[@5822huron]

@@ramblinman7153 Take high limit out and just let it hang. Don't let it tough metal cabinet. If furnace never trips limit you have your answer. Are the techs checking HEAT EXCHANGER?

[@5822huron]

Also try using cheap blue filter. Cap could be low on blower. Wheel could be dirty. Blower speed too low. Adjust manifold gas pressure on both stages.

[@ramblinman7153]

@@5822huron it was an overfired furnace. Funny as it hasnt been a problem until now. Also don't understand why the installer didn't set it properly when it installed in the home 6 years ago

[@hetfieldofjames5261]

My furnace runs for only 2 minutes then the limit switch opens I ran it without a filter (the filter is new) and there is little to no dust on anything it shouldn't overheat and open the switch .

[@ChilesHeatCool]

If you have a high efficiency furnace the secondary heat exchanger may be dirty. If you have air conditioning you could have a dirty evaporator. You might have to call some one to have a manometer hooked up to test gas pressure.

[@palerider7924]

Is there any other sensor that detects low air flow? I was getting a code 33. My filter got plugged up with white minerals powder from my running a cool mist humidifier all the time. I changed the filter and stopped running the cool mist now furnace runs normal. The mystery though is I never had to push the reset button on the limit switch all I had to do was cycle power to the furnace to get it to run for a while until it stopped with code 33 again.

[@ChilesHeatCool]

Limit switches don't have a reset. They just auto reset till it breaks. Dirty coil for the air conditioner, dirty secondary heat exchanger on a 90% furnace. Living in high elevation causes the low airflow code with furnaces due to low air density. Blocked returns or to many supply registers closed.

[@palerider7924]

@@ChilesHeatCool I'm at 5300 feet. My limit switch has a button but I didn't have to push so does repeated cycles ultimately cause the button to pop up and then the switch is broken?

[@ChilesHeatCool]

That altitude may be causing part of it. No the button is a manual reset. The only safety that has a reset is a roll out switch. The limits switches are all auto reset unless this is something brand new. If its heating I'd call it good. I know the altitude of Colorado springs and monument Colorado always gave the higher end furnaces issues due to altitude. They worked it's just the low air flow code since the motor can't tell air density just airflow per rpm of the motor.

[@palerider7924]

@@ChilesHeatCool 30 year old Bryant. All the problems started at season start up. First bypass humidifier cage motor not running so no humidity cooling bypass air. Then I installed a better filter nano kind. Then started running cool mist humidifier around the clock. Code 33 starts. Filter covered with white minerals. Installed $1 barely a filter and block humidifier bypass. Now runs fine no codes. Waiting now 5 weeks for cage motor from skuttle. No stock. Paid for it but don't know if it will ship this season. House 25 % humidity very dry.

[@ChilesHeatCool]

I would have someone check it. Just do an annual service. Becareful with a to cheap filter that doesn't really filter. They let the equipment get to dirty.

[@M0h0m3d]

Does the dirty air duct make high limit switch trip

[@ChilesHeatCool]

Only if the return is dirty enough to restrict airflow.

[@FerrellMcCollough]

Thank you for your video! I'm at a loss with my Carrier Infinite Furnace. Limit fault switch Error 33 is triggered. I replaced the limit switch 180-20F two times. The wall thermostat is capable of running a system check. I removed the filter to test the system and still got Error 33. The system check "Furnace LOW, MED and HIGH" on HIGH heat I get: Airflow 1288 CFM, Inducer RPM 3315, Blower RPM 776 and Static Pressure 0.30 without filter. I vacuumed the blower fins, they was only a mild film of dust on them. All vents in the house are open. System Limit fault is triggered only during HIGH heat. System was new 2012. Any ideas, thank you!

[@ChilesHeatCool]

I'm thinking gas pressure is to high or dirty coil if you have air conditioning. If this is a condensing furnace the secondary heat exchanger could be dirty.

[@FerrellMcCollough]

@@ChilesHeatCool I checked out the coil and it had a thin dust film on it. I cleaned it but that didn't fix the error. Then I changed the wall thermostat setting from COMFORT to EFFICIENCY. It results in a higher CFM during all 3 modes: LOW, MED, HIGH. During the HIGH test the COMFORT CFM=1288 and the EFFICIENCY=1520 (below 1600 spec) and I didn't get a limit fault switch error. I'm probably fine keeping it on EFFICIENCY for a short time but I worry a bit if it's masking a problem.

[@ChilesHeatCool]

I bet the manifold gas pressure is high. It might just be off by enough to cause an error like that.

[@chadwolfeschledgelsteinhau9697]

Dont change the swich that was a wast of time the error says its open. Means ur temp was up faulty compaseter on blower motor

[@johncovello3375]

Is it normal for the heater to consistently hit the temperature to activate that limit switch? Our limit switch operates every few minutes and I wonder if it's normal for the heater to be that hot to constantly activate that switch?

[@ChilesHeatCool]

No. It is a problem. The high temperature limit is to essentially prevent the furnace from catching on fire. Check your filter. If its clean then the blower wheel or a/c coil could be dirty or to much gas pressure or wrong fan speed. Air flow and gas pressure are the two things that effect that.

[@whywoulditellyou00]

Is it possible they go bad and trip at a wrong temp reading?

[@ChilesHeatCool]

It could. If they trip a lot they eventually just break.

[@loluv1069]

I tested the ohms on the high limit switch and the numbers were jumping all over the place does this indicate the switch is bad?

[@ChilesHeatCool]

It could be that, your meter batteries, or a bad connection with your meter leads.

[@shunjohn3567]

Hello will the ignitor heat up and the furnace start if I have a bad limit sensor?

[@ChilesHeatCool]

No. It will fault out and stay off when it is on limit switch. If the furnace is igniting the gas and shuts off, its probably a dirty flame sensor. I have a video on how to test and clean that.

[@shunjohn3567]

@@ChilesHeatCool okay thanks for your help

[@ChilesHeatCool]

Your welcome.

[@chasecash901]

I troubleshooted a high limit switch and after 5 min of bypassing the furnace caught on fire from the heat exchanger

[@tacktiician4978]

Soo i just replaced my pressure sensor switch and still having the same issue. It fires up runs fine with heat and flames for sometimes day, sometimes 10 minutes, then kicks off and throw a pressure limit code.. im lost now..

[@ChilesHeatCool]

Is the port to the pressure switch clear?

[@ChilesHeatCool]

kzbin.info/www/bejne/lXS2ZGCKpKhgnKc

[@tacktiician4978]

@@ChilesHeatCool the port on the actual switch or the port the tubes go into? Both seem to be clear

[@ChilesHeatCool]

Port on the furnace. They some times get clogged on the furnace if its a damp location. If its a new switched that is the same rating, did you use the new hose or old hose? If they still include a long hose with the new one did you cut it down to size? Some times the new hose is to long and kink just right. If that's all good then you will need a manometer to test the vacuum on the inducer. The squirrel cage in side could be rusted out or crystallized.

[@tacktiician4978]

@@ChilesHeatCool thank you. Same hoses, just checked both all clear. I guess the inducer is the next check. 👍

[@M0h0m3d]

I have 170f-30 switch can i use 170-40 instead?

[@ChilesHeatCool]

Maybe. It will open at 170 degrees if there is a problem. It will just take longer to close. I'd say try it and see if it will work. Make sure the length is the same.

[@M0h0m3d]

I had a code for high limit switch , but ultimately found it is the molex connector with yellow wire which is loose. any suggestions how to fix it?

[@ChilesHeatCool]

Pin puller with a new pin but those are hard to find. New wire harness will work. You could try to see if a small screw driver can expand one side of the plug to make a better connection but if done wrong could ruin it. One side fits in to another plug so expand the male side a tad bit more carefully. To much and it could get ruined.

[@The_Silver_Pharaoh]

P8

[@halbestcanada2305]

Not useful

[@ChilesHeatCool]

Ok