Quickly Testing Gas Furnace Safety/Limit Switches!

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[@rayanbraydon53]

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[@yehudabrennan9420]

@Rayan Braydon instablaster ;)

[@rayanbraydon53]

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[@rayanbraydon53]

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[@yehudabrennan9420]

@Rayan Braydon happy to help xD

[@johncarcamo6772]

Seriously you did better than my gas furnace teacher.

[@acservicetechchannel]

Glad it was helpful for you, thanks!

[@abeltaura4894]

I paid 12k lol

[@veteranjeeper1120]

These videos you make ,have helped me become a better tech and great for touch on on knowledge!

[@acservicetechchannel]

Great to hear!

[@naturestrail2296]

I'm sitting here trying to figure out why my thermal switches have 120v? It's a 1979 Ruud heat pump I believe. Also been trying to figure out why there's a grey wire no label, blue wire labeled B, and no orange wire? No telling who's worked on it in the past when the thermostat was changed by the previous owners. Also, when it's set to heat pump it has electric arches jumping between a couple contacts you can hear inside the air handler. When you set it to EM heat this goes away. Any help will be greatly appreciated! Thanks

[@TrollinTheInterwebs]

I like watching your videos, keeps me from getting rusty.

[@acservicetechchannel]

Glad you like them!

[@vetdadcubby4722]

Simple, effective and professional!! Great job sir!

[@acservicetechchannel]

Thank you kindly!

[@aaronr164]

I wish there was someone like you to explain commercial boilers

[@izzymoney85]

I like to say Thanks for a very well done video this is something the new guy can benefit from bye learning a simple way of doing things keep up the good work 😎

[@acservicetechchannel]

Thank you very much Esmar!

[@tytig94]

I freaking love your videos, they make my life easier. Thanks!

[@acservicetechchannel]

Glad you like them!

[@Dalewdone]

Great video. Answered my question a while back about using the dollar bill to clean the flame sensor.

[@acservicetechchannel]

Glad to help

[@RayRay-nb7tn]

Great video on gas furnace safety switches. Haven't been commenting lately. Been busy with friends Sprint car racing. Season ended so back to the grind again. Thank you for the videos. Be safe.

[@acservicetechchannel]

Thanks, you too!

[@RayRay-nb7tn]

You are very welcome. Thank you.

[@ShelterG]

I would like to add on more method. Measure voltage between limit connectors. If O volt means no voltage drop and limit is good. If 28 volt means open circuit - bad limit.

[@acservicetechchannel]

Great tip Eugene!

[@farmthug0085]

Thanks for the tip. I'm new to HVAC but it's all starting to come together

[@jimmysegovia2688]

thank craig for sharing! all your lessons are really awesome! i have been watching and learning from the start with your videos thank you so much for sharing.

[@acservicetechchannel]

You're very welcome!

[@johncarcamo6772]

Thanks for the clear explanation

[@acservicetechchannel]

Happy to help

[@petebuehn7789]

Thanks for the great video, helped my solve my problem!!

[@acservicetechchannel]

Glad it helped!

[@joemikhaiel3710]

Thanks for sharing the knowledge

[@acservicetechchannel]

So nice of you

[@acrepairnearme]

Great topic. Sometimes I do this to check automatic reset limits. Just to make sure it is in working condition. I’ve put the flame from the lighter(blue part) and warming up limit switch metal shield till it will click, and make it OL, and then waiting for it to cool down and click back. Another technic: if I see that the igniter won't go ON, I jump the pressure switch just a few sec later after the inducer starts, and then if the igniter starts to light up, I know the problem is most likely related to the pressure. The switch itself or clogged tube.

[@acservicetechchannel]

The only thing is those switches are built so small and precise that I really would not put a flame to it to test it because it may overheat especially the alum shell on the outside and it may warp causing pressure on the bimetal disc. I have done it in the classroom to fault a furnace for a student for the flame rollout switches but I wouldn't put that heat on those in the field. Hope you don't mind me saying and thank you for your support!

[@alfredoalvarez1510]

Very good explanation, thanks 😊

[@luiselizalde6757]

As always excellent explanation., Thanks.

[@acservicetechchannel]

Glad you liked it!

[@tinysand3517]

another great one Greg

[@acservicetechchannel]

Glad you enjoyed it

[@mikimiki195]

Great video Sonic you reading voltage on switches. That means switches are good

[@alexismcmackin]

Yes, more of these!

[@deanmartin1966]

Great video, thanks for sharing

[@acservicetechchannel]

Thanks for watching!

[@jonathanraul4086]

Thanks great video once again

[@acservicetechchannel]

Glad you enjoyed it

[@cri8tor]

Great tutorial. Cheers

[@acservicetechchannel]

Glad you liked it!

[@cri8tor]

I have a Goodman GMP075-3 furnace, installed in 1997. My AC and Heater are off at the thermostat. Fan is on Auto. The blower fan in the furnace turns on for a few minutes randomly almost everyday. Could that be a Fan Limit Switch issue? Will go into the attic and check some of the switches when it cools down. Cheers

[@thecreativehuman75]

You are by far the BEST HVAC instructor on KZbin! Do you have a book ( aside from your AC book) designated to service trouble shooting? Many thanks for all you do.

[@ParabellumX]

Modern Refrigeration and Air Conditioning, 21st Edition, by Althouse/Turnquist/Bracciano. This is the HVACR Encyclopedia.

[@melo8668]

Awesome information awesome channel !

[@acservicetechchannel]

Thank you!

[@AceMoney-Sportsbook-Robbery]

I have a Luxaire older model - I have decent knolwedge from troubleshooting this furnace over the years . I was having trouble remembering what i did a few years ago to get it running correctly. I had to change and fix a few different sensors , limit switch & a Capacitor for the blower motor. basically all the stuff you just went over.. I came across your video.. quick easy and right to the point. quick refresher coarse . I got a limit switch issue I think ( 4 Blinks on my modle) Ill go test them now.. thank you.... Quick question if you see this "OL" that means its bad the ones with the button in the middle/ they should read 0.00 right ? ohms

[@bradmironik6137]

Glad to know another means of testing switches. I thought you can test across the limit and find out if the switch is opened/closed via voltage readings (24v/0v). Correct me if that is not correct.

[@johnnynguyen3838]

Is it on specific furnace that you have power across these safeties while the furnace is idled? (Power is ON but no call for Heat)

[@bauhnguefyische667]

Always a simple explanation here on how ‘hopscotch’ works with a electric meter.

[@Millennmify]

Showed me how to test the switches. Found that the thermal limit switch was open showing no ohms. Replaced switch and Goodman furnace works again.

[@MrMasoud911]

Hi Craig and thanks again for your great videos, just a novice question, if we disconnect the power to the furnace, how come we have voltage on those switches? Thanks again

[@acservicetechchannel]

There is power to the switches even when there is no call for heat, thanks!

[@MrMasoud911]

@@acservicetechchannel alright, thanks

[@RC-ud9hh]

Wow you blew through that lesson! Just throw this novice right into the furnace fire and repeat. 😆

[@Karnol428]

Are all gas/ propane low voltage systems common connected to the ground?

[@abnergarcia6252]

What would happen if you touch the flame sensor with power on? I always clean them but remove the wire first. Sometimes the wire spade connected is bare metal and is not insulated. I wondering if there is a risk of getting shock. The furnace is power on but there’s no call for heat at the time I cleaning it.

[@TheMaster5150]

If there is voltage going to the flame sensor when touching the spade with one hand, and you touch something grounded with your other hand⚡....just like the test he did with the meter to get a reading. I've seen retards pinch wires with meters and bump into something grounded and get shocked🤯

[@abnergarcia6252]

@@TheMaster5150 is voltage only going through flame sensor when there is a call for heat? Or after burners are lit?

[@TheMaster5150]

@@abnergarcia6252 When the door switch is in the closed position, there should be power to the flame sensor, safety switches, transformer, board...etc. He has a magnet, or tape on the switch to perform power test without any calls. With the door off the switch, you shouldn't be getting power. Fwi...I was taught with 3 phase, and 240v check each leg to ground, even if your not getting a reading from leg to leg because you maybe reading the same potential "no difference" between the probes as the polarity could be the same. My point is...if you're not 100% sure, aways check to ground, and a beeping volt tester first, because Doctor's have found getting shocked causes scarring to the heart.

[@acservicetechchannel]

Yes power is on any time the door switch is shut and power to the furnace. Thanks for answering TheMaster5150!

[@johnharrisonschulz]

You forgot to show where the high limit switches are in the blower compartment. Some furnaces have manual reset limit switches in the blower compartment too.

[@cck0728]

As usual excellent video. So, it means that switches can be checked via resistance and voltage? Voltage should be same across and resistance should be "zero". Thanks.

[@martineucza486]

How do you test switches and flame sensor rod with the furnace on?

[@wxfield]

Why are you using the red lead for your ground?

[@robertcherry4971]

What is the best way to test a high limit switch while in furnace,to prove what temps it is opening and closing at?

[@Krystle161]

Hello! I ran into your video because my inducer motor isn’t starting. There is no flashing light on my furnace though. Just a solid light. I tested each of my 3 limit switches and for one of them, one plug is giving 28v and the other is giving 0.03v. I also tested the pressure switch when calling for heat and both are giving low voltage. Any thoughts and advice? Thank you!!!

[@l.a.m7645]

Flame rollout switch and spill switch the same?

[@EclecticLensYT]

So.. how to do you test the ignitor?

[@Kkayoodle]

I have an older Carrier/Payne 3 wire auto pilot ignitor type furnace. I wanted to see if you could tell me how I can tell the diff between my limit switch & flame roll out switch? Most videos I watch have either a button pop type limit switch or a 2 longer pronged limit switch. Problem is I have both. So I'm thinking one of them may be a flame rollout switch but I can't tell. The parts # on it is smudged so couldn't look it up to verify what it is. Ty

[@DylanTonoguva]

How do I check air combustion temperature sensor in my rinnai hd200e hot water heater?

[@domas4463]

Is this multimeter good for checking resistance of a/c thermistors? (kilo ohms) Thanks

[@alfredsosa24]

Hi I having problem to check a furnace with limit switch open that I think is not the limit switch I want to know what to check in order to fix the issue

[@johnnynguyen3838]

It makes sense that the safety sensors should have power going through them when there's power to the furnace. However, I have come across furnace that is powered and no power to the safety sensors. Which is why I asked my question above/below.

[@Seabass1206]

how will the censors possibly trip if there’s no current running through? im no expert but it sounds like someone jumped the safteys

[@zeeshawn15]

I was always told to not call 24volt's common a ground... ground should be reserved for 120 volts, while 24 volt should be called common.

[@parneypots]

👍👍

[@ozarusa192]

I have HEIL 96% furnace that is shutting down after about 6 - 7 minutes completely , including thermostat. Code call for limit switch i replaced switch but it still does the same thing, it does it only when required longer operations to bring heat in home for like 3 to 4 degrees up. Acts like its overheating. Fan blows after shutdown for about 3 minuts and restars heating cycles and works again for same amount of time 5:37 6-7 minutes and shuts off. Air flow is good and clear clean filter. I checked A coil no dust so i am verry confused and concerned what is it and why is doing that

[@MG-sg2ci]

What ended up being your issue? I'm having the same issue with my payne furnace.

[@ozarusa192]

@@MG-sg2ci temperature switch overheating due to too small ac coil. 17.5 inch furnace 14 inch coil. guy who installed coil did bad job cheating telling me that that size coil is fine. well after replacing coin and getting more airflow on proper size coil 17.5 runs fine and no overheating . if you have proper size ac coil check if is clean, alot dirt can collect in coil fins an restrict air flow, check presure or rollout switch.they are not expensive . but most likely is airflow restriction this high efficiency furnaces are sensitive on overheating due to airflow

[@huanab]

What if there is no voltage at all? bad control board not sending power ?

[@sideskraft]

My furnace fan stays on all the time. I tested the air pressure sensor and I'm only getting 25V on one terminal when the furnace is running (there's no continuity). I removed the inducer hose from the furnace tube and gently applied a vacuum with my mouth and I could hear the contacts closing, and I verified continuity with my meter. I did this several times to establish repeatability. My understanding is that the airflow passing over the tube in the furnace creates a vacuum which moves the switch diaphram and closes the contacts. I assume this closed circuit tells the furnace controller that it's OK to shut off the furnace fan. I've checked the condensate drain hoses and everything is clear. I also looked inside the metal furnace inducer tube that the switch hose fits over and saw nothing, but I did gently run a screwdriver in and out and moved it around a bit, but nothing came out, so it must be clear. Is it possible or probable that the pressure switch diaphram has aged such that it now requires excessive vacuum to sufficiently move and close the contacts? If this switch never closes will the fan run all the time?

[@lifeovdeath]

Did you ever get a solution? I’m in the same situation.

[@jordanblair5410]

@@lifeovdeath I had the service guy come to repair it, but unfortunately I don't recall what was replaced.

[@AustinTXRealEstatebyFelix]

What if you read 10.2? My unit is no longer kicking the heating on and the AC works.

[@turbot_]

So 0L means my safety switch needs to be replaced? I have just a wall heater eith thermopile and cant figure out whats wrong with it. I had to take the whole cover off to find the stupid safety switch. Since I have no voltage at my thermostat I'm really hoping it's just the switch and not the wiring.

[@supertreat1]

When I test my flame sensor the fan turns on for a bit and it registers 3 V - also my primary switch doesn't register any voltage- does that mean it's bad?

[@chesstime356]

Great video thanks.

[@taephillips3276]

Heard steel wool was too harsh and it’s sand screen but not sure

[@pablogomez7519]

I'm having issues with my furnace. the blower starts, the inducer spins (I think the pressure switch is good) the ignitor does not turn on but the gas valve does click and release the gas. I did replace the ignitor. getting code 34.

[@frankjames1955]

i did something dumb, .. i changed my thermostat and nothing worked except the main large fan always runs no matter under what setting it is.. i put back the old thermostat but it still does the same thing... the green light on the gas valve gives me an code of- "limit circut fault"

[@quymai3860]

I have a guy came in to swap out the controller board on my unit, warranty works, but then he induced an issue to my heat furnace. now its not turning on and throw an E04(trane system), Open thermal limit, rollout switch or RAF switch, they are safety switches that tied in a loop. All checked good but still not comming on. any idea?

[@tagetes1442]

My fairly old Bryant Plus 80 heater started cycling on and off every 3 minutes (that is three minutes of flame....three minutes of cold air...three minutes of flame...three minutes of cold air, so my house wasn't heating very quickly to say the least. I jumped each stop limit switch with a piece of copper wire until, lo and behold, I came to the main furnace box stop limit switch and while jumped the furnace DID NOT cycle off prematurely. I replaced the wires on that stop limit switch and the heater went back into its troublesome cycling. So this tells me that the error code was right in that the problem was a stop limit switch and my testing proved which one it was. NOW...to find out why that main furnace box stop limit switch is acting up??? (I actually replaced it with new one and the new one creates an open after three minutes as well. So Do I have this all right?

[@jimbarron8688]

Actually, with the customer switch contacts closed simply use a jumper between R and W. If the ventor motor starts you know all the safeties are closed and normal. If the main blower starts you know the high limit is open. Simple.

[@shellyblanchard5788]

I had to recalibrate my limit switch because it was shutting down the furnace before it warmed up real good to heat. But the fan won't turn on in automatic, you have to the fan on swtich. The wire broke last winter and had to splice it. I thought maybe the had burned out , But wasn't the case. I think the switch on mechanism is not working. I took out the splice, hooked it to the wire , it still didn't come on.

[@lifeovdeath]

Did you end up finding a solution? I’m about to get rider a new limit switch but I just tested it and it had voltage. My furnace turns on fine but thermostat is reading wrong temperatures, fan doesn’t turn off after lowering tempering although flame goes out.

[@shellyblanchard5788]

@@lifeovdeath I think my thermostat is a little off to. It could be yours may be in draft cause it to stay on longer than usual. Put the fan on automatic. Wait about 15 minutes to see if it shuts off. If is blows cold air, turn on the heat and see what tempture it comes on at. Let the the heat stay on till room warms up , check the temperature side and see it what says. If it's a digital, click downward till the burner clicks off. If it is a lever one click left or down. See how many degree it off.

[@shellyblanchard5788]

@@lifeovdeath one thing to if you have a wire at the side of the thermo coupling you may have a loose wire. Twist it just a bit just enough to tighten , it should stay lit when you light it. One thing to , if you have central air, check your out side compressor to see if it is running for a.c.. It may be running. You may have to pull the breaker on it if has a lever.

[@lifeovdeath]

@@shellyblanchard5788 thanks. finally got it figured out 4 days later. So turns out the brand new thermostat I got was shot which got me really confused and made me run loops as to what’s wrong. Just got a different thermostat and it worked. :)

[@shellyblanchard5788]

@@lifeovdeath right on. Glad to help.

[@dsd-downshiftdave8056]

Correct me if I'm wrong, checking each side of safety to ground determines the safety is open or closed, however, it does not prove that the safety will open if there is an issue, if the safety is stuck in the closed position and will not open Then your test didn't prove the safety Is functional

[@jkbrown5496]

At least some boards test the pressure switch to ensure it is open before turning on the inducer fan. An auto diag youtuber, Schrodinger's Box, has a recent video using a failed pressure switch off his furnace to give a lesson multimeter scale selection for resistance. On continuity, the switch shows open, but on the 20M ohm scale it has a high resistance. Again, his board reported this fault (due to a humidifier leaking onto the furnace electronics).

[@acservicetechchannel]

thanks for sharing!

[@naturestrail2296]

I'm sitting here trying to figure out why my thermal switches have 120v? It's a 1979 Ruud electric only heat pump setup. Also been trying to figure out why there's a grey wire no label, blue wire labeled B, and no orange wire? No telling who's worked on it in the past when the thermostat was changed by the previous owners. Also, when it's set to heat pump it has electric arches jumping between a couple contacts you can hear inside the air handler. When you set it to EM heat this goes away. Anyone have a clue?

[@xephael3485]

why test the voltage? Just put it into continuity mode and check of they're passing current.

[@pablogomez7519]

I'm getting reading of 24volts from all the safety devices but my ignitor (brand new) still will not turn on. I am getting gas I can hear it click and hiss. but no glow from the ignitor. please help anyone 🙏🏽

[@christophbimmermann238]

It looks like you are only checking whether the limit and safety switch(es) "tripped". So if a furnace is not working, this is a good method to figure out which safety switch caused it, therefore allowing more detailed searching for the source of the problem. BUT I would not consider this testing of safety switches. For that, you would need to trip them and check whether they trip at the right temperature/condition. In my opinion, this is more a troubleshooting step, than actually testing of safety-related features. So don't believe that your safety switches work if you never actually checked if they can open ...

[@jamesoconnor5349]

Ms