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Lennox G23 popping limit switch after a few minute of firing

JCtester Member Posts: 2

Looking at a friend's older Lennox G23 furnance.


1. Vents appear to be getting good air output

2. Intake vents not blocked

3. filter clean

Here are the symptoms.

When the furnace kicks on for hear, everything appears normal.

The burners appear to be working fine and the blowers turn on.

After a minute or so the burner will kick off and the limit switch has been tripped.

The blower stays on indefinitely.

The burners will not come back on until the limit switch is manually pressed to reset, then the same cycle repeats itself.

Any ideas what could be causing this limit switch to constantly pop open? It appears to be getting good airflow so I would not think it is over heating, but maybe?

There are no LEDs on the control board to review.

Thanks in advance...


  • Slimpickins
    Slimpickins Member Posts: 339
    Get a pro

    Time to call a professional immediately. Could have a cracked heat exchanger and a dangerous situaltion on hand.
  • Steve Whitbeck
    Steve Whitbeck Member Posts: 669
    limit switch

    Is this limit located above the burners and senses if there is excess heat ( roll out ) The only manual reset limits are usually roll out switches. If this is the case your furnace is not venting properly and the burners are rolling out.

    This is a very dangerous condition. Call a PRO.

    It is time to replace the furnace more than likely.
  • JCtester
    JCtester Member Posts: 2
    switch popping

    Thank you for the replies.

    This appears to be quite an old furnace.

    The switch I am talking about is mounted just to the left of the burners.

    It has two red wires going to it. It is quite small and is almost like a little breaker switch button. When it opens you can press it down to reset.

    I was looking at a diagram for what I believe was a newer version of the furnace because i saw where it mentioned a roll out switch and a limit switch.

    This appeared to be more of a limit switch just by how it looked and the placement?

    Then again, I thought I read a temperature limit switch resets itself when when the temperature gets back down to a certain point. This particular switch has to be pressed manually to reset.

    I obviously do not want a dangerous situation but times are tough for them (probably why they asked me to look instead of a pro) and I don't think they can afford costly repairs or furnace replacement. Is there a way I can visibly check for roll out?

     See attached picture.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,853
    That is a manual reset roll out switch...

    and if it is tripping on a regular basis, it indicates a dire need for service on the fire side. Could be something as simple as a good cleaning and fuel pressure adjustment, but if it is old, it most probably has a cracked heat exchanger and may be in need of replacement.

    It may not necessairily be "rolling out" that is causing the issue. It may be reflected radiant energy from a dirty heat exchanger that is causing the problem.

    I realize you are only trying to help out a family in distress, but you are now a part of the chain of command, so make darned sure that you do the right thing and document it as much as possible, just in case...

    If in doubt, contact the Comfort Institute and see if one of their trained/approved partners in the area can help this family out. Expect them to tell the consumer they need a new furnace. Most furnace have cracked heat exchangers after 5 years of operation. The crack may not be in a spot that can create a dangerous situation, but they spread fast... If its cracked in such a position that it is blowing forced air onto the burners, it is a dangerous situation and needs to be addressed immediately. If in doubt, ask the local utility to become involved. They usually have access to emergency intervention funding for situations like this.

    CO is nothing to be taken lightly...

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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