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  1. #1
    Join Date
    Aug 2007
    Posts
    339

    Confused Fan Limit control with heater on Lennox unit

    Hello,

    I'm new to the forum and I've been in the field for about 4 yrs now but I ran into something that I never seen before. On this lennox unit there was this fan limit switch with a heater. I call lennox tech support and the guy on the phone act like he did not know what it was and what it was used for. Could some one please explain to me what's its function and is it me or is lennox tech support the most complicated people to deal with for information? And also do anyone think that they over control their units?

    PS:

    This was a 80% upflow 100,000 BTU furnace. I can't find the model # but I know the control was a old Honeywell part and the insert size that goes inside the furnace was 8" long.

    Thanks

  2. #2
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    the heat is infact a time delay to bring the blower on within 30 os seconds after the burner fire. It also makes sure the blower does not turn off when the burner cycle is over till it cools down and breaks. Then if the heat is still there the temp limit keeps the blower on the it cools off enough to shut the blower off. All of the older lennox units had a fan limit with a heater in them

    te limit will also work by temp if the heater were to fail.

  3. #3
    Join Date
    Aug 2007
    Posts
    339
    Thank's, but could you tell me some of the ways you know when one is going bad? I was watching the furnace cycle and it look like the blower was coming on to fast. I mean soon as the burners came on the blower came on at the same time. The customer was complaining the the registers was blowing out cool air. But I notice that the cold air return was not connected to the furnace, and that the basement was cold. I would think that this would affect the performace of the fan limit switch is this true? We all know (hopefully!!) that it is very important that the cold air return needs to be connected to the furnace to make it run properly to give it the correct temp rise ?

  4. #4
    Join Date
    Nov 2001
    Location
    east kansas
    Posts
    8,032
    The RAD shall be connected to the furnace. This is to prevent the blower from robbing the combustion air required for complete combustion. And the blower not back drafting from the flue.

    If the blower is starting when the burners light, are the burners not lighting when they first get a call?
    Beware of advice given by some guy on the Internet.

  5. #5
    Join Date
    Aug 2007
    Posts
    339
    Yes the burners come on and thing seconds later the blower will come on. I also noticed that the burners would short cycle off and on. This a indirect spark igniting furnace.

  6. #6
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    bumb up your post count and apply for pro statis
    all the info that can be given has been given
    if you are a diy sorry call a pro to fix it

  7. #7
    Join Date
    Aug 2007
    Posts
    339
    WEll as I stated before I have about 4yrs in the field and this was my first run in with this type of fan limit. I'm no DIY!!! I'm a person just like you who's trying to gain knowlege in the trade to make myself a better tech. I'm sure when you first got into the business you've asked questions about things you ran into that you did not understand! And personally I don't like DIY or hacks!! Mybe again they might be ok because as long as they are around there will be more work for everyone to go behind them to fix their mess to make MONEY!!!

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,048
    The burners are short cycling?.
    You might have 2 problems occuring at the same time.
    Thats an old furnace your working on.
    If the burners aren't cycling on high limit, fix the flame sensing problem first.
    If the blowers coming on in less then 30 secs, the control is bad.


    Thats more info then we are suppose to give on the open resi forum.

    Which is why tinknocker said to get your post count up and apply fro pro memebership.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    Join Date
    Aug 2007
    Posts
    339

    Smile Thanks!!

    Quote Originally Posted by beenthere View Post
    The burners are short cycling?.
    You might have 2 problems occuring at the same time.
    Thats an old furnace your working on.
    If the burners aren't cycling on high limit, fix the flame sensing problem first.
    If the blowers coming on in less then 30 secs, the control is bad.


    Thats more info then we are suppose to give on the open resi forum.

    Which is why tinknocker said to get your post count up and apply fro pro memebership.
    Thanks, Beenthere I hope tinknocker and yourself did not take my message in the wrong way I just want it to be known that Im a certified tech with a number of awards and certifications but that does not mean a thing unless you are out in the field as long as some of the guy's like maybe yourself and tinknocker has to experience some of the old stuff out there! I know a lot of book smart people and once they are out there in the field they are lost. Also I think its not fair for all of these HACKS & DIY people out here messing up the industry while people who do the right thing get the short end of the stick!! I love this web site and it has helped me a lot to become a better tech and to solve problems with the help of people like tinknocker and yourself. With that said, I would like to say THANKS!!!!

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,048
    No problem.

    but, I warn you, you need a thick skin to be on this site.

    Take care.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  11. #11
    Join Date
    Mar 2005
    Location
    Suppy NC
    Posts
    4,513
    sorry wasnt trying to insult or upset you. Just following the rules.

    give the model number of the unit you are dealing with. It is important because some of the older lennox units have other things that can cause your trouble. G-12=16=20q

    get your count up as fast as you and apply. You will be overwhelmed by the info you can get in the pros section.

  12. #12
    Join Date
    Feb 2008
    Location
    Dallas & Longview, TX
    Posts
    629
    Quote Originally Posted by cook42 View Post
    Hello,

    I'm new to the forum and I've been in the field for about 4 yrs now but I ran into something that I never seen before. On this lennox unit there was this fan limit switch with a heater. I call lennox tech support and the guy on the phone act like he did not know what it was and what it was used for. Could some one please explain to me what's its function and is it me or is lennox tech support the most complicated people to deal with for information? And also do anyone think that they over control their units?

    PS:

    This was a 80% upflow 100,000 BTU furnace. I can't find the model # but I know the control was a old Honeywell part and the insert size that goes inside the furnace was 8" long.

    Thanks
    Maybe I'm reading your post wrong but are you asking what a fan limit switch is? I'm not a professional and I know. Seems like kinda basic stuff for a pro not to know. Since some of the pros are buying your story- the limit switch turns on the fan at an adj. temp. after the burner has fired. Then it shuts it off after the burner is off and the temp drops. It has a high temp limit that will shut off the burner if reached. There is a spring wrapping around it that can go bad. This is a replaceable part and insert length (8") if you don't have the numbers off it.

  13. #13
    Join Date
    Mar 2008
    Location
    Grundy Co. IL
    Posts
    773
    Also, maybe I missed it, is it the old cam-stat style, or honeywell? If it's the cam-stat, it's time to upgrade to the honeywell.

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