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    coldone's Avatar
    coldone Posts: 6, Reputation: 1
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    #1

    Jan 3, 2009, 10:01 PM
    Cold air from gas furnace
    My gas heating system no longer works properly. When heat is turned on, the furnace seems to start fine, however after some time the burners turn off and the furnace keeps blowing cold air, effectively cooling down the house back to where it started. The time it operates OK is always different. It may work fine for an hour or 2, or it may kill the flames as soon as after 5 minutes.

    I called the heating company before the New Year's; the guy inspected the furnace, cleaned the burners (they were indeed dirty), took $160 and left. I did not notice any difference. Then I replaced the original thermostat with a modern one--no difference again.

    Simply turning off the termostat does not turn off the fan for considerable time, like 30 minutes or so. To stop the cold air from blowing in, I flip the circuit breaker switch.

    The house was build in 1996; control board in the furnace -- White Rogers 50A50-471. The repairman mentioned that I might want to replace the control board, but it seemed OK to him, and he did not seem convinced about what's going on at all.

    Any suggestions?
    coldone's Avatar
    coldone Posts: 6, Reputation: 1
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    #2

    Jan 3, 2009, 10:30 PM

    Additional details: the unit is Trane XE 80. I believe it has electric ignition.
    wmproop's Avatar
    wmproop Posts: 3,749, Reputation: 91
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    #3

    Jan 3, 2009, 10:57 PM

    Well, they say its hard to stop a train,, its also hard to make them run right
    KC13's Avatar
    KC13 Posts: 2,556, Reputation: 99
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    #4

    Jan 3, 2009, 11:48 PM
    No heat combined with continuous blower is probably the result of an open limit device.
    coldone's Avatar
    coldone Posts: 6, Reputation: 1
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    #5

    Jan 4, 2009, 11:41 AM
    Quote Originally Posted by KC13 View Post
    No heat combined with continuous blower is probably the result of an open limit device.
    Hmm, and which one is that? Is there any way to check/test it?

    This morning I examined the furnace more closely. The indicator light flashes slowly, when there is no call for heat and flashes fast when there is. That part seems to be OK. What happens is that at some point gas simply shuts off, then the ingiter (?) heats up, trying to start the burners, but as far as nothing comes out, it turns off, too. The fan keep blowing, though...

    By the way, on the outside of the furnace box there is a plastic dial, resembling a clock. What is that one for?

    Thanks!
    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
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    #6

    Jan 4, 2009, 12:47 PM
    Look on the wiring diagram on the back of the blower door to locate the limit control in the wiring. Then use a meter to see if it is a open or closed circuit. NOTE: It might have a reset button in the center of the control.
    coldone's Avatar
    coldone Posts: 6, Reputation: 1
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    #7

    Jan 5, 2009, 10:51 AM
    Quote Originally Posted by hvac1000 View Post
    Look on the wiring diagram on the back of the blower door to locate the limit control in the wiring. Then use a meter to see if it is a open or closed circuit. NOTE: It might have a reset button in the center of the control.
    The board has a specific LED code for that control, but I have not seen it yet. Instead, I noticed that when the gas stops flowing, the LED on the control board starts to glow continuously. The description for the continuous glow says "Internal control failure (Please replace)". I guess this calls for the board replacement.
    hvac1000's Avatar
    hvac1000 Posts: 14,540, Reputation: 435
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    #8

    Jan 5, 2009, 11:37 AM
    That could be a good possibility but KC13 gave you at tip as to another possible cause.
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