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Thread: fan limit combo

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  1. #1
    Join Date
    Apr 2003
    Location
    New York
    Posts
    700

    fan limit combo

    fan limit control L4064. I was sent on a follow up to replace fan limit switch. The tech that was there before said this prevent the blower from running.Although we are in ac mode. This was in a package unit Lennox GCS16-653-75-51 SN 5691HO1913. Was reading the ninstructions on the control it says must remove jumper on control when used on low voltage circuit.If anyone can explain this please explain or point me in the rght direction so i could fully this control. Any replies are thanked in advance. Oh i repaced control and removed jumper and ac is running.

  2. #2
    Join Date
    Dec 2007
    Location
    Southern, CA
    Posts
    520
    Assuming yer talking about a Honeywell control, I believe they were first made when furnaces were wired so if the hi limit tripped, the line voltage to the control transformer was cut. So it saved running another power wire or jumper to the switch. It also helps sell transformers and misc parts, cause everybody at some point forgets about it, hooks up hi volt for the fan, low for the safety, powers it up and blows an odd number of low volt items in the process

    It is a 2 contact switch that responds to heat picked up by the long probe on the back. Power for the fan comes through a relay with N.O. and N.C. points. In heat, power from the relay N.C. points is avilable to the fan limit. Once it warms enough and the dial rotates, the low speed fan points close and it runs. It prevents the heater from putting out as much of a cold blast of air at start up. If theres a problem the dial continues to turn until the N.C. hi limit points are opened. And the gas valve power is cut and shuts off. The design was never changed even though nobody wires their units that way anymore.

  3. #3
    Join Date
    Feb 2004
    Posts
    2,458
    Quote Originally Posted by jaybee View Post
    fan limit control L4064. I was sent on a follow up to replace fan limit switch. The tech that was there before said this prevent the blower from running.Although we are in ac mode. This was in a package unit Lennox GCS16-653-75-51 SN 5691HO1913. Was reading the ninstructions on the control it says must remove jumper on control when used on low voltage circuit.If anyone can explain this please explain or point me in the rght direction so i could fully this control. Any replies are thanked in advance. Oh i repaced control and removed jumper and ac is running.


    See next post
    Last edited by mike3; 08-22-2008 at 10:31 PM.

  4. #4
    Join Date
    Feb 2004
    Posts
    2,458
    Quote Originally Posted by jaybee View Post
    fan limit control L4064. I was sent on a follow up to replace fan limit switch. The tech that was there before said this prevent the blower from running.Although we are in ac mode. This was in a package unit Lennox GCS16-653-75-51 SN 5691HO1913. Was reading the ninstructions on the control it says must remove jumper on control when used on low voltage circuit.If anyone can explain this please explain or point me in the rght direction so i could fully this control. Any replies are thanked in advance. Oh i repaced control and removed jumper and ac is running.

    With jumper in place the limit side uses 120 volts and is wired to open line side of transformer. With jumper removed limit side is used for 24 volt circuit and opens low voltage side of transformer.

    As far as the a.c is concerned: The heat speed goes thru the relay (n.c) to the fan switch & the cool speed goes thru the relay (n.o)

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