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  1. #1

    Fan limit switch issue.

    Ok I just replaced an old camstat 57E-25T-110-20 Fan limit switch because it was not staying closed when the furnace was cold from not running. Basically the limit switch was doing the opposite of what it was supposed to. It never hit the high limit when the furnace was running it just had issues staying closed when the weather got real cold and furnace was not running.
    Example: at night I drop the thermostat 6 degrees but in the morning when the thermostat calls for heat the switch won’t make contact and fails to close, thus no heat. It would eventually start working again but struggled. I measured the voltage as it was trying to make contact and it goes anywhere from .500v-26v eventually settling out at 26 volts and kicking the gas value in gear. Once it on it’s good to go and will run until the thermostat is satisfied but if it sits to long then it does it all over again. I have gotten around this by not dropping the heat at night.

    After all this I decided the switch was bad and ordered a 57C-05T-120A which is considered the replacement for this switch. The only real difference is the old switch is not adjustable and has a static limit set at 200 degrees F. The new switch is adjustable and was pre-set at 200.

    Here's my new issue: The new switch has no problem kicking on when it’s cold but now it’s hitting the high limit and shutting off the burners after about 10- minutes. So basically I traded one problem for another. I find it strange that the old switch never hit the high limit and actually only worked when the burners remained on but the new sitch is hitting the high limit within 10 minutes. Any thoughts on the issue?

    Thanks.

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
    Posts
    6,743
    Change your filter? Call a pro to diagnose and correct.

  3. #3
    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

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    Last edited by jpsmith1cm; 01-31-2013 at 05:16 AM. Reason: non AOP member

  4. #4
    Join Date
    Sep 2008
    Location
    Western PA
    Posts
    25,551
    access_max

    This is the Ask Our Pro's forum. In order to post a response here, you must have verified qualifications and have been approved by the AOP Committee. You may ask a question by starting a new thread.

    You can find the rules for posting and qualifications here.

    Additional infractions may result in loss of posting privileges.

  5. #5
    Join Date
    Feb 2011
    Location
    North East Ohio
    Posts
    772
    It's not the switch that is hitting the high limit. The limit is doing what it is intended to do. You need to call someone and have your system looked at.

  6. #6
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,003
    Limit Switch = A device in a furnace that is used to sense excessive and potentially dangerous and damaging temperatures at or near the heat exchanger of the furnace. The switch should disable the burner (heat production) until the dangerous condition no longer exists.

    Your old switch could have failed or became erratic in it's operation due to continually sensing temperatures beyond it's designed setting and constantly opening.

    Have your system checked over by a qualified HVAC professional to determine the reason(s) for the furnace heating up past it's designed high limit capabilities.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  7. #7
    Join Date
    Jan 2006
    Location
    Powell River, BC, Canada
    Posts
    763
    Quote Originally Posted by firecontrol View Post
    Limit Switch = A device in a furnace that is used to sense excessive and potentially dangerous and damaging temperatures at or near the heat exchanger of the furnace. The switch should disable the burner (heat production) until the dangerous condition no longer exists.

    Your old switch could have failed or became erratic in it's operation due to continually sensing temperatures beyond it's designed setting and constantly opening.

    Have your system checked over by a qualified HVAC professional to determine the reason(s) for the furnace heating up past it's designed high limit capabilities.
    Ding, ding,ding! We have a winner! It's not unusual for a furnace to be cycling on its high limit and the occupant not know it. I have found this several times over the years. I had one that was in a crawlspace and the customer never went down there due to mobility issues, just had his kid go down& change the filter. I found the problem when he called us with no heat and the installer wouldn't come out until after the weekend. Unit was just under 3 years old when the limit failed. It was over fired, the blower speed was too slow and the customer had several registers closed 'to save money'.
    Where are you? Are you done yet? I got ONE more call for you.....

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