Are you sure the temperature limit switch is tripping and not the pressure switch?

When i put the old limit switch back in the unit runs fine

You said the said the problem occurs intermittently. I was not sure if putting back the old switch worked once or has it worked over several days.

You measured a temperature of 132 F degrees. The switch is rated for 180 F degrees. Given your measurement, it would seem the switch is defective or incorrectly labeled.

Yes, the issue seemed to happen randomly. I have my thermostat on a scheudule. So at night the setpoint for heat is low. 63. Then starting early around 3am it gradually increases about a degree an hour until 6am. So at 6 the setpoint it now 66. During these 1 hour temperature increases the furnace ran and made temp with no issues. Then at 7 the schedule pushed the temperature to 69. The furnace has to run longer to satisfy this 3 degree jump. This is usually when the furnace fails on limit. I’ve measure the heat both on the 1 degree rise in temp setting and the 3 degrees rise in degree setting. The temperature never got above 135. i did happen to be down there when the limit switch opened and I did a continuity test and verified the limit switch was in fact open. But yes, I beleive it is a faulty limit switch. I did a test and with the old limit switc. i slid 3 filters in the filter slot instead of one. I also closed several supply registers and wired the fan up for low speed. The old limit switch did end up opening and the furnce flame went out. i immeadiatly took a temperature reading at the a coil and it read 185. I removed the extra filters, opened the returns and several minutes i heard the inducer draft motor kick on. I took another temps reading and it was at 148 which tells me the old limit switch seemed to be working properly. Once it fired and ran for several minutes i turned the unit off and wired the fan back up for its previous speeed.

Yes, I am an HVAC technician but very new. Mainly do preventative maintenance but don’t get the chance to dig further into things at the moment until I start school at my union. I’ve asked a few people I work with and they seem to think the new limit swtich is just defective. Wondering what the online community thinks.

There's a saying that goes like this: If you're not testing, you're guessing.

If it's not that it must be this: What is known as a 'parts changer'. From a preventative stand point you need to 'know' what fails, why it fails, outside of the realm of changing it 'just because it's old'.

This is a skilled profession, there is no skill in guessing... there's this board we're posting on that proves it. (anyone can play, but that doesn't mean they should.)

Now these words I've provided here may sound harsh to you, but truth often does that. If you are to go far in this profession it will require you to 'know why' --- if all you do is guess? who is going to pay you for that?

Nothing is better than protecting your income potential and having people call and request 'only you'. No union provides that, you do that on your own.

If you apply yourself to learn, you can go anywhere you want.

Austin Air. i guess from day 1 you knew anything and everything? I do know how the parts work and how to test them. I know that limit switches open and i know the reason for it. If you actually read my post you will see that i went through a series of troubleshooting. And yes, i know how to use a meter and test. Since you are so experienced, why not share that knowledge and give me your professional opinion what the issue was? Mine is that is was a faulty limit switch since putting the old one back solved the probem.

Austin Air, and just so i don’t get critizied for being a parts repacer, i replaced the following parts for the following reason. The main reason being the age of the unit and i know parts can fail due to wear.

Ignitor-had some surface areas that were not glowing cherry red during the ignition sequence.

Pressure switch-found hairline cracks in the stubs where the tubes connect.

Flame sensor-unit 17 year old. Sensor has been cleaned many times. Bought parts for my unit as a package and it was included.

Evap fan capacitor- measured outside the +-5% uf.

Filter- just because. I replaced every 2 months unless needed sooner.

Limit switch- Again, part of the package i bought. 2 wires/2 screws. Not very difficult.

Cleaned A coil- I do this yearly because I can.

See I told that you wouldn't like the words. This profession is an on site in person job.

If you're offended by those words... you will never make it in this profession.

Good luck.

So still no opinion on what i did wrong or if you diagnosis is different than mine? PS. I was on site and in person

I guess I should have just paid hundreds of dollars for a company such as your to do the same steps i did and come to the same conclusion. No thanks.

@Andrew M,

Some HVAC "professionals" are just, well blowing hot air. Their comments are mostly self aggrandizing--not particularly helpful, nor especially accurate, but never in doubt. You won't need to spend much time on houzz.com to figure out who to turn to for good information and who to turn to if you'd like a dose of abuse.

“Oh would some Power the gift give us to see ourselves as others see us." (Robert Burns.)

Andrew M, Everyone has an opinion. When I said this profession is an on site in person job... I was referring to the fact that it's not me asking how or what to do to fix something. If it's just a money problem? So you are getting into this profession to make money... and your argument is this?: I guess I should have just paid hundreds of dollars for a company such as your to do the same steps i did and come to the same conclusion. No thanks.

You can't expect others to do what you won't do yourself. (How you go broke as an HVAC tech, if that is your plan.) Parts changers do not last in the HVAC business... the argument you provide is just that...

I am not here to provide you a guess, anyone can do that --just as I said.

If someone has an attitude problem, I am not a shrink. I fix air conditioners and furnaces, I don't fix people and their perceptions. That 'fix' that I provide does not include guessing from a forum board.

You don't have to like it. That's life for you, so better get used to it.

The home builder has such knowledge ask him what's wrong let him guess... everyone knows how valuable builder grade knowledge is. To some people everything looks like a nail, and contrary to popular myth I have installed, serviced, maintained, & fixed 1000's of air conditioners and furnaces over the course of my 27 year career in HVAC not using a single nail...

I service the Katy, Texas area.

@Austin Air Companie,

Andrew is pursuing your same trade, made it clear he's had some PM experience, but no training yet. .He came to this forum looking for help. What he got from you was your typical, bully-like behavior instead.

You don't just bully those who disagree with you, you bully the original posters, too. Your "Pro" label needs to be revised that your "Pro" status is that of professional bully.

Charles as I have said many times before... there are trade schools that teach. This isn't the place to learn something that could have detrimental effects to the safety and well being of those that live in the said dwelling.

It makes not one bit of difference, that this person is pursuing this trade. He needs to learn this valuable lesson. If he amounts to anything above a 'servicer' (maintenance tech)-- he will look back upon this week in his life as a changing point... or he will look back at this moment as his HVAC career path downfall.

How many HVAC techs have I seen come and go over the course of my 27 year HVAC career?

Back when I was a kid growing up, they called this 'wisdom'. Now? not so much. You'd rather call me a bully, without actually thinking about the situation?

Hi Andrew,

I want to commend you on how you identified the problem and went about to verify the root cause. I am confident you will be come a very good HVAC tech. Good luck in your career.

Andrew - Yes, there is variation in switch setpoints due to manufacture and aging effects. If you raise the fan speed the temp at the limit switch should drop some (due to higher mass flow) and you may be able to use the new limit switch without bumping against its setpoint.

HU.. thank you for the suggestion. I did change from medium to high speed and had the same outcome.

Mike..appreciate encouragement. Thank you.

Charles..same to you for the encouragemt. Much appreciated.

Found tnis on yelp for Austin Air Company in Katy, TX. Not a surprise.

“The guy was rude and condescending to my wife and neighbor I referred him to. Not sure of his skills but I like doing business with someone who is more customer oriented.”

LOL Andrew,

The Truth? You're not interested in that, ironically enough. If your true intent is to to learn the HVAC business and work for 'people' -- you will find you have a long road ahead of you.

Are you going to lie to them like most techs do? You know the thing you despise now about calling service people to your own home?

I tell it like I see it. They call it honesty. --- So the rebuttal to that when people don't like the truth?

Just as you have shown, just as this particular person commented on Yelp. You don't like the truth I have provided you with so you lash out and try to demonize the situation at your own detriment.

You will never forget this situation if you have any serious career involvement in HVAC going forward. You complain when people lie to you don't you?

Brought to you by: "Honey do these jeans make my butt look fat?" Lie, lie, lie... all to butter people up and refuse to tell them what you really think the 'TRUTH' is.

Clearly you haven't truly thought this situation through have you?

Ray,

Somehow you turned a simple question by one poster into a crusade for the truth with you leading the charge as a caped crusader (For truth, justice and the Raymerican way!) They asked a simple, straightforward question about a temperature limit switch which they suspected was defective.

Instead of answering the OP's question, you decided to use the opportunity to berate them. If you were indeed concerned about sharing the truth you would have simply pointed out--based on your self-described "illustrious career"-- the "truth" that sometimes new components fail rapidly after installation (e.g., infant mortality,) sometimes they are defective right out of the box (e.g., quality control issues or damage in shipment) and that the majority are subject to wear out at the end of their useful life expectancy. You could also have used the opportunity to educate the OP in how to test the switch. Doing so would be giving back to your profession by helping to grow the next generation of HVAC techs.

Austin Air..other than berating me you haven’t hinted to what i did wrong or could have done better. At this point your words mean nothing ro me. ive looked at some of your replies on here and you’ve done the same thing to others. i have no respect for people like you. If you are not interested in helping someone that comes on here for information then just skip the thread and move on.

https://www.manta.com/c/mtcg5tf/austin-air-companie

HVAC isn't something 'learn' on the side from forum board.

A gas furnace is nothing for you to 'play around with'. You like to call this me 'berating you'.

From your very first post: Ive recentlly done the following:

-replaced limit switch.

-replaced ignitor.

-replaced pressure switch.

-replaced flame sensor.

-replaced evap fan cap.

Of which didn't fix your issue. I told you this was 'part changer' mentality.

You say this is berating you. Of course it's the truth. You would rather pass the buck and blame me for your lapse in judgement. Everything I have told you about this business is true and accurate... yet you want to take the path of feeling like I am berating you? You want the truth or you want a lie. If you want a lie... put me on ignore. I tell it like it is, even when it doesn't benefit me.

The truth of the review you posted? This person hired me to move an electric air handler into the attic because? The closet it was in prevented servicing of the system. Everytime the unit broke down she would have to pay to have the system removed parts changed then placed back.

She chose not to replace the duct work at the time the system was moved. The duct work a few years after this job was done (to her specification, what she was willing to pay for) the duct work fell apart. She felt she shouldn't have to pay again, that's not the way HVAC service and repair works. HVAC systems don't last forever, you're never done. 30 year old duct systems don't last forever, but she left that info out of the review.... You have a clear picture how this business is 50 shades of hard work.

You are looking to get into this business to work for people by just replacing things that don't need replacing?

Good Night & Good Luck.

Here's a recent review of my company... click to enlarge if necessary. It's not always about replacing everything... who has $30K to spend on air conditioning? BUT... if you don't replace everything a day will come that you will have to address more problems.

Charles, of course you are 'entitled' to your opinion... but what comes with freedom for one... also includes me.

I am entitled to my opinion as well. That's how a 'free' society works... you don't have to like it. You're entitled to that as well.