Troubleshooting the limit switch on the 80% AFUE gas furnace

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A guide to determining if the limit has failed and if so why. This video is part of the heating and cooling series of training videos made to accompany my websites: www.graycoolingman.com and www.grayfurnaceman.com to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome. If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

Comments

grayfurnaceman says:

That is definitely not code any where I know of! Its a propane hose I use for temp hookups. Code here is black pipe to outside the appliance. Thanks for the close look.
GFM

rogersaf1 says:

What type of supply gas line is that? I’v never seen automotive hoses and clamp gas line before, code in va is bonded steel or wardflex…

grayfurnaceman says:

Thanks
GFM

sparkyUonline says:

Good video,great practical approach to showing real life troubleshooting.

grayfurnaceman says:

I encourage you to do so. When I was teaching, I had to pull someone off a unit most years because he was afraid to lose the refrigerant. By the way, tight fitting gloves are a bad idea, as they get saturated with refrigerant and continue the burn until they can be removed. Thanks for the comment.
GFM

ROBERT FLEMING says:

Ya i could,it happened on a lennox h.p. back seating the high side service valve it did’nt shut off correctly while taking the hose loose,R-22,the customer talking to me,i thought at 1st it was just what was traped in line,a few turns i knew something was wrong so i 1st tryed to tighten line but so much liquid froze nut on hose,instead of letting it go like i should i kept trying to get it to stop,i did’nt want the customer or me or my boss to be out the money to replace,dumb,lesson learned

grayfurnaceman says:

Wow that must have been some burn. Here’s hoping for a return soon to the biz. I bet you could do some good vids on refrigerant safety! Take care.
GFM

ROBERT FLEMING says:

very helpful video,i love these gas furn videos,this is something i dont get to work on very much,i have had to change some inducer motors/blower wheels and a few gas valves,ive worked on several after thinking about it,but not as often as i would like,i am out of work do to frostbite to my hands from R-22,Dec.29 makes 1yr out now,and still not getting any good news yet about my rht hand,got it bit bad,but the vids on ytube and reading a lot help me keep up,i hoping to get back asap,may be awile

grayfurnaceman says:

Whoa, sounds like design problems big time. If they are 4x10s with a max of 125 CFM that’s only 750 plus the baths. Sounds like no heat ,no cool no nothing. I smell early failure. Merry Christmas.
GFM

OcRefrigeration says:

Again, a Very good Vieo! Just had a call like this on friday. too funny. credit card limit open. 140-30. guardian furnace. 4 years old. dirty filter and customer closed off 2 registers downstairs. drywalled over them. i told him to open them back up. not enough air flow for a 4 ton furnace. 100,000 btu. it only has 4 mains and 2 bath. he closed off 2 mains. gotta love those engineer homeowners from boeing. and he’s a real engineer and should know better.

grayfurnaceman says:

The limit is probably stuck off or there is a manual reset limit off. It is usually located behind the gas valve. On the vid you can see the left edge of it behind the gas valve. Turn off the power and remove the limit then tap it on the switch. It will probably come on. It still needs to be replaced. Do not bypass!! Hope this helps.
GFM

edward arroyo says:

I am working on an gas furnace 1993 arcoaire model no GNE075B12A1 the problem is the same fans running blowing cold air. Found all three filters to be clogged possibly 2-4 years old. I cant seem to locate the limit switch on this model any suggestions?

gusyve1 says:

I did clean the rod once before and it worked fine for a while, I’ll try bypassing the limit switch tonight, about the test with the flame rod I couldnt test it because the flame doesnt stay on for me to touch it to the chassis. I ordered a flame rod since it went bad before, i figure I’ll replace it for a new one. do you have a vid about a cracked heat exchanger?

grayfurnaceman says:

It should not. However, if the flame shuts off within the trial for ignition period, it is probably not sensing flame. Have you cleaned the flame rod?
The only other solutions I can think of are a bad control board or the possibility of a heat exchanger crack near the limit switch. Try temporarily bypassing the limit. Do not leave it bypassed!
GFM

gusyve1 says:

I will check on that tonight after work, if that is the problem, why would the board give the 4 blinks for a bad open high limit switch? or could the code mean other problems also?.

grayfurnaceman says:

We are looking at the flame sensor. Checkout “How does the gas furnace flame rod work.” If it is not shorted to ground and the wires are tight, it is probably a board problem.
GFM

gusyve1 says:

i do not have the meter to check for microamps, the flame will lit and shut off whitin seconds, which sensor you want me to check ? the flame sensor? and how to check for a faulty flame sensor?

grayfurnaceman says:

OK, lets start over. If the burners come on then shut off within 7 seconds, it has a flame proving problem. If you have a meter with microamps, you can check the microamps. Most will prove at .5 microamps. If you dont have that meter, check the sensor, after isolating it, with an ohmmeter to chassis. Should be open.
If burners run for 2-3 min, with room temp set higher than room temp, there is not enough air or the heat ex is cracked near the temp probe.
GFM

gusyve1 says:

i checked the rollout limit and it didnt need to be reset, as where I stand i checked the the limit and rollout switch, the flame sensor i have sanded it down before so i think it could be that it needs replacement, but am not too sure, the flames will start just fine but will shut off and the fan will just keep working. at this point what else would you recommend to do? thank you GFM.

grayfurnaceman says:

Update: I thought you were looking at an older vid. The one you commented on is the latest vid. Be sure to look for both the fusible link and the manual reset limit.
GFM

grayfurnaceman says:

If you have a manual reset rollout limit, you can reset it by pushing the button. I just did a vid on rollout switches. You might check it out. I am not sure where you would find the rollout switch on that unit. It should be located above and close to the burners. You need to find out why it has shut off. Could be cracked heat exchanger, plugged ht exchanger. If the limit shows continuity, it has not failed. Good luck
GFM

gusyve1 says:

Hello, i have a comfortmaker RPJ2 with the same 4 blinks that on the cover shows open High limit switch, i did the continuity test and its good i hear the beep sound on the meter, i have the same problem fan works but no flame. what else should i check for? or should i switch the high limit switch? thanks.

tkarmakid says:

Good info. for the high temp limit, in one of your other videos you describe a method of testing these by removing the neutral wire on the fan, letting the furnace run for about 3 mins and seeing if it kicks off, thus proving the switch to be functional. My question is: is there a way to know if the limit is an automatic reset or a one-shot-then-replace type? Is there one shot limits or are all high limits auto reset?

brennanac1 says:

Good to know thanks again!!!

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