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Old 02-29-2012, 09:05 PM   #1
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Question about "common" terminal on furnace controller


I'm trying to understand the function of the "common" terminal on the control board of my furnace. Is there a difference between "common" and ground? I thought that the common terminal was another name for ground, but when I measured the resistance between the common connection and the case of the controller I got a very high resistance reading - so apparently common isn't the same as ground. Could someone tell me what the common terminal is supposed to be used for? Thanks.

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Old 02-29-2012, 09:16 PM   #2
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Question about "common" terminal on furnace controller


Look at a transformer it has two wires. Pretend it a train going on one wire. It goes through all the other electrical parts to make them work. But the train need got back to its station ( other wire on transformer) To complete cycle of electric.

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Old 02-29-2012, 09:17 PM   #3
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Question about "common" terminal on furnace controller


Yes I'm been drinking
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Old 02-29-2012, 10:33 PM   #4
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Question about "common" terminal on furnace controller


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Originally Posted by Tator1076 View Post
Look at a transformer it has two wires. Pretend it a train going on one wire. It goes through all the other electrical parts to make them work. But the train need got back to its station ( other wire on transformer) To complete cycle of electric.
lol..what the heck kinda Mr. Wizard-Romper Room explanation is that????? ROTFLMAO

I can't even think straight enough to answer the post I'm laughing so hard!
Tator...gimme what ever your drinking dude!HIC-OP~****
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Old 02-29-2012, 10:36 PM   #5
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Question about "common" terminal on furnace controller


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Look at a transformer it has two wires. Pretend it a train going on one wire. It goes through all the other electrical parts to make them work. But the train need got back to its station ( other wire on transformer) To complete cycle of electric.
Thanks for the analogy - but what I'd like to understand is if the return "track" is supposed to be connected to the overall system ground - e.g. the metal case of the controller housing.
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Old 02-29-2012, 10:49 PM   #6
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Question about "common" terminal on furnace controller


No. It is not connected to the earth grd of the system.

It is like the neutral on a light circuit.

A return leg that the voltage returns thru to complete a circuit when it is energized.


That is over simplified but should answer your question.
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Old 02-29-2012, 11:16 PM   #7
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Question about "common" terminal on furnace controller


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No. It is not connected to the earth grd of the system.

It is like the neutral on a light circuit.

A return leg that the voltage returns thru to complete a circuit when it is energized.


That is over simplified but should answer your question.
No is the simple answer I was looking for. Thanks.
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Old 02-29-2012, 11:35 PM   #8
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Question about "common" terminal on furnace controller


I think we should offer complete cognitive amnesty to Mr. Rogers for his Hvac electricity/train explanation.

Perhaps in the morning when he's quaffing aspirin to get ready for work, he'll read what he wrote and he'll see the wisdom of separating drinking and social media.

On this side however, the comedic relief was welcome.
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Old 03-01-2012, 12:08 AM   #9
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Question about "common" terminal on furnace controller


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No is the simple answer I was looking for. Thanks.

Then you have come to the wrong place.
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Old 03-01-2012, 05:55 AM   #10
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Question about "common" terminal on furnace controller


with power up read the R Hot off the TR 24v to ground if you don't get 24V the TR and the board aren't grounded.Common wire on 24Vs is the same as the white/neutral on 115V wiring along with the ground wire serving the same purpose.
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Old 03-01-2012, 08:11 AM   #11
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Then you have come to the wrong place.
Why do you say this? Kind'a harsh...no?
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Old 03-01-2012, 08:39 AM   #12
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Why do you say this? Kind'a harsh...no?
I doubt there are any simple answers on this board, or any board for that matter. Everyone has an opinion, you and I included and simple answers just aren't the usual responses in answer forums. You ask the time and I tell you how to build a watch.
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Old 03-01-2012, 10:58 AM   #13
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Question about "common" terminal on furnace controller


The common terminal is a current return terminal, for devices such as thermostats that need 24 volts to work. And or the A/C condenser.

On some units it will be grounded, and on others it won't be.

It is very similar to a line voltage neutral.

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