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Different color leads in transformers

I am replacing a 120v/24v transformer in an air handler. The secondary voltage (24v) leads in the old transformer are red and white. In the new one they are pink and brown. Which corresponds to which? Is there a way to tell the polarity and is this essential when replacing it?

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No, it doesn't matter. If you want to make sure hook up the leads to the 120 side and then turn on power and check the leads coming off the low voltage side to make sure you getting the 24v before hooking it into the board.

Posted on Jul 31, 2009

  • FNGHVAC Jul 31, 2009

    No, it doesn't matter. If you want to make sure hook up the leads to the 120 side and then turn on power and check the leads coming off the low voltage side to make sure you getting the 24v before hooking it into the board.



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1 Answer

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Your thermostat is probably wired directly into the furnace to supply the 24V to power the stat. You have to make sure you're getting 120V into the primary side of the transformer, and 24V out of the secondary side. If you've got voltage in and nothing out replace the transformer with one of the same voltage, and same or higher VA rating. If you don't have 120V in then start checking backwards starting with power in/out of the door switch, then back to the unit disconnect switch, and back to the breaker if necessary. If you've got 24V out of the secondary go check to see you've got 24V at the thermostat between the R and C terminals, if you do the thermostat is bad. As with any electrical wiring make sure you've got good connections on all wiring terminations. Good luck, if you need an explanation of how to check the power in and out of the transformer let me know!

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2 Answers


Usually that is because of a blown fuse in the air handler or fuenace which ever you have or the transformer that supplies the 24v to the thermostat, also in the air handler or furnace, has gone bad...
Check to see if you have 24v at the transformer and then flow it from there...


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rule of thumb from thermostat
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