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  1. #1
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    Limit switch issues

    You will have to pardon me but I do not remember the model numbers of the furnaces. The first one was a 19 year old Lennox 80% natural gas furnace at my uncle's, it had been blowing cold air for a couple of hours and I found that the limit switch was tripped and would not automatically reset. after I removed the limit switch from the furnace while I was holding it in my hands it reset and would operate at that point. the furnace was operating with a 85 to 90 ∆T, the manufacturer wanted to see 45 to 75. I replace the limit switch and lowered the manifold pressure in order to achieve a 70 degrees Fahrenheit temperature rise. The second furnace is a Rheem high efficiency from 2011, the customer had been running the furnace with an extremely plugged filter and 42% of the heat registers closed. The limit switch was tripped and when I removed it from the furnace it measured 235 ohms of resistance. I went inside and removed the filter and opened all the registers and then went back out to the garage 5 to 10 minutes later. At that point the limit switch measured OL so I ordered a new one. Our install manager went back two days later and the old limit switch measured just fine, he replaced it anyways because of the previous issues. Has anyone else had issues like these with a limit switch not wanting to automatically reset but then resetting later?

  2. #2
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    Quote Originally Posted by jperigo View Post
    I replace the limit switch and lowered the manifold pressure in order to achieve a 70 degrees Fahrenheit temperature rise.
    Did you have a combustion analyzer on this unit when you lowered the manifold pressure?

    If not, how do you know you didn't change the combustion performance on this system to the point where you are now producing high levels of carbon monoxide (CO)?

    You are masking the problem, not fixing it.
    Last edited by rundawg; 11-27-2019 at 04:55 PM. Reason: typo
    Instead of learning the tricks of the trade, learn the trade.

  3. Likes CEAS-AC-TECH, R600a liked this post
  4. #3
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    ^x2.

    Limit switches aren’t “bad.” They wear out because of low air flow, often from inadequate ducts for the equipment.
    AOP Rules: Rules For Equipment Owners.

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    Questions should end with a question mark "?" Statements end with a period "."

  5. #4
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    WTH does a delta T twice the recommendations have too do with the high limit?

  6. #5
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    Did you measure the pressure or just blindly lower it?
    Quote Originally Posted by jperigo View Post
    You will have to pardon me but I do not remember the model numbers of the furnaces. The first one was a 19 year old Lennox 80% natural gas furnace at my uncle's, it had been blowing cold air for a couple of hours and I found that the limit switch was tripped and would not automatically reset. after I removed the limit switch from the furnace while I was holding it in my hands it reset and would operate at that point. the furnace was operating with a 85 to 90 ∆T, the manufacturer wanted to see 45 to 75. I replace the limit switch and lowered the manifold pressure in order to achieve a 70 degrees Fahrenheit temperature rise. The second furnace is a Rheem high efficiency from 2011, the customer had been running the furnace with an extremely plugged filter and 42% of the heat registers closed. The limit switch was tripped and when I removed it from the furnace it measured 235 ohms of resistance. I went inside and removed the filter and opened all the registers and then went back out to the garage 5 to 10 minutes later. At that point the limit switch measured OL so I ordered a new one. Our install manager went back two days later and the old limit switch measured just fine, he replaced it anyways because of the previous issues. Has anyone else had issues like these with a limit switch not wanting to automatically reset but then resetting later?
    Sent from mars using Tapatalk

  7. #6
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    Thread Starter
    Rundawg, yes I had a combustion analyzer and verified proper low CO levels.
    Kdean1, never used the word "bad" just asking if anyone else has had limits not reset initially but then reset later.
    Pecmsg, a high Delta T has a lot to do with the limit, low airflow or extremely high manifold pressure that will cause a high Delta T could also trip the limit.
    R600a, did you assume that I didn't measure the pressure or just blindly post?

    Listen guys, I was just making a quick post asking a question if anyone else has had these similar kind of limit switch issues with them "wearing out" but then showing up normal later. Some of you make it very "undesirable" to ask a simple question on this site.

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  9. #7
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    Quote Originally Posted by jperigo View Post
    Rundawg, yes I had a combustion analyzer and verified proper low CO levels.

    Listen guys, I was just making a quick post asking a question if anyone else has had these similar kind of limit switch issues with them "wearing out" but then showing up normal later. Some of you make it very "undesirable" to ask a simple question on this site.
    Sorry if you thought we were being critical of your abilities.

    You have to remember that many posters on this forum (including techs) have done minimal troubleshooting when they ask a "why isn't this working" question.

    It is always in your best interest to provide the readers of your thread all the details of your troubleshooting. That way we don’t have to speculate on what has, and has not been done, in an attempt to answer your questions.

    Unfortunately, there is no such thing as a simple answer to every questions.
    Instead of learning the tricks of the trade, learn the trade.

  10. #8
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    Quote Originally Posted by jperigo View Post
    Rundawg, yes I had a combustion analyzer and verified proper low CO levels.
    Kdean1, never used the word "bad" just asking if anyone else has had limits not reset initially but then reset later.
    Pecmsg, a high Delta T has a lot to do with the limit, low airflow or extremely high manifold pressure that will cause a high Delta T could also trip the limit.
    R600a, did you assume that I didn't measure the pressure or just blindly post?

    Listen guys, I was just making a quick post asking a question if anyone else has had these similar kind of limit switch issues with them "wearing out" but then showing up normal later. Some of you make it very "undesirable" to ask a simple question on this site.
    If you want help I would suggest to apply for Pro Membership. You should always have the proper information included if you don't want to get "beaten up."

    High limit switches can fail but usually they are just doing their job. If they don't reset on their own I would replace them but that is only a symptom usually. In a case like above you should include model and serial#. Combustion readings before and after if applicable, gas pressure entering and manifold pressures, delta T, static pressure. Is there a dirty evap coil? What is the fan setting on?

    If you don't include the information what would you expect them to think?
    Signature removed Violated rule #15

  11. #9
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    Sorry. Just the way you worded it was a bit worrying. I am glad you were on top of it.

    Now that I understand yes I have had Rheem fan housing safeties fail intermittently. I have also had regular high limit safeties stick open but reset as I removed them.

    Sent from mars using Tapatalk

  12. #10
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    Quote Originally Posted by kdean1 View Post
    ^x2.

    Limit switches aren’t “bad.” They wear out because of low air flow, often from inadequate ducts for the equipment.
    And/or clogged filters.

  13. #11
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    The limit failed because OF the high Delta T. Find and Correct the cause of the high deltaT

  14. #12
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    Quote Originally Posted by rundawg View Post
    Sorry if you thought we were being critical of your abilities.

    You have to remember that many posters on this forum (including techs) have done minimal troubleshooting when they ask a "why isn't this working" question.

    It is always in your best interest to provide the readers of your thread all the details of your troubleshooting. That way we don’t have to speculate on what has, and has not been done, in an attempt to answer your questions.

    Unfortunately, there is no such thing as a simple answer to every questions.
    Quote Originally Posted by pageyjim View Post
    If you want help I would suggest to apply for Pro Membership. You should always have the proper information included if you don't want to get "beaten up."

    High limit switches can fail but usually they are just doing their job. If they don't reset on their own I would replace them but that is only a symptom usually. In a case like above you should include model and serial#. Combustion readings before and after if applicable, gas pressure entering and manifold pressures, delta T, static pressure. Is there a dirty evap coil? What is the fan setting on?

    If you don't include the information what would you expect them to think?
    Allow me to add:

    This is an open forum. That means there is no keeping out the search engine bots that are tasked with indexing the net. As a result, just any old searcher can read what we post in these open forums. We are therefore willing to limit ourselves, as a community of professionals, to what we post in the open areas of this site.

    The answer to this?

    Get your stuff together and use the link in the bottom of this post, "how to become a professional member."

    It's free. We just want the committee to vet those to whom we open the entire site, as nearly as we can, to allow that entree.

    The search engine's bots can't access the Pro areas.

    Lastly (and this is not directed at you so much as others who want to post about an issue)....

    ALWAYS post the model and serial of the equipment, and what diagnostic testing you have done, AND the actual numbers.

    No one posting here should say "subcooling and superheat were good," or "voltage was good," or "amperage was good"....you get the idea. Those assumptions can lead to bad advice.
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  15. #13
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    The biggest problem is more techs read this sub forum than they do the pro forums. What really needs to happen is have the tech to tech chat sub forum made a pro forum but also highlighted and as easy to find as this one. I find there is a ton more traffic here than any of the pro forums. What would make the most sense, in my eyes, is to have this topic put right below as pro tech to tech chat with an asterisk by it.

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