Air Conditioner Fan STUCK ON while thermostat off/disconnected


Old 08-21-15, 09:39 PM
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Air Conditioner Fan STUCK ON while thermostat off/disconnected

I have an armstrong air tech 80 which has been working really well for me in both seasons. However I just noticed that it started leaving the FAN on even if the thermostat was at the proper temperature for a while with no AC turned on. So I played with a few wires and it seems that even without the leads from the thermostat (I power cycled the unit and still stuck) the fan turns on and keep running. The only way I can turn it off is using the powerswitch mounted outside of my unit. Or if the door is open and the black power switch is unpressed. Other than that it is ALWAYS on regardless of temperature. Any help would be greatly appreciated.

Removing the leads from the thermostat.
Reseting the thermostat.
Power Cylcing the unit using the power box. (which the leads use a switch to turn on).
Tapping the components with the back end of a screw driver to see if anything is loose and changes behavior.
Once I return power I hear an immediate hum and then 5-10 seconds later the fan starts up.
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Old 08-22-15, 03:40 AM
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Could it be a stuck relay? Try to find the schematic. I had one recently marked IFR. Indoor Fan Relay.
Old 08-22-15, 06:28 AM
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What is the model number?
Old 08-22-15, 06:41 AM
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Here are the pictures of my thermostat. I'm not sure what the pieces are because I don't have a manual since I'm the 2nd owner of the home.

I have the Armstrong Air Ultra V Tech 80.

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Old 08-22-15, 06:46 AM
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Could it be the transformer has gone bad? How does the transformer work? If there is no power to it? AKA the Thermostat leads are disconnected will it still provide power? Or does this provide power and just at varying degrees with it on/off? Once I flip the switch where the transformer is it will turn off but will stay on regardless of the feeds on there on/off.

THIS IS THE HOUSE FAN. Sorry just to clarify since it wasn't clear in the title!

Last edited by Heather Steck; 08-22-15 at 07:52 AM. Reason: clarification
Old 08-22-15, 08:08 AM
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That would be called the blower.

I see a transformer in the first picture but it's not connected so it isn't your system transformer.

Of course your blower shuts off when you remove power. Does the blower come on anytime the power is restored or does it come on when it's called for and then just won't shut off ?

What is the part number on that control board ?
Old 08-22-15, 08:14 AM
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The transformer supplies voltage (24VAC nominal) that runs the thermostat, furnace control board, and contactor (in outside unit). Without the transformer, nothing would work. The switch that is mounted above the transformer turns off power both to the transformer and the furnace.

My guess (like Pulpo) is that the fan relay is stuck in the ON position. Without a circuit diagram I can't say for sure, however I suspect that the part with the OMRON label in your third picture is the fan relay. Try gently tapping it with something to see if it starts working properly again.
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