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 rvmeush 02-29-2012 08:05 PM

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.

 Tator1076 02-29-2012 08:16 PM

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.

 Tator1076 02-29-2012 08:17 PM

Yes I'm been drinking:drink:

 hvac5646 02-29-2012 09:33 PM

Quote:
 Originally Posted by Tator1076 (Post 867014) 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!

 rvmeush 02-29-2012 09:36 PM

Quote:
 Originally Posted by Tator1076 (Post 867014) 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.

 hvac5646 02-29-2012 09:49 PM

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.

 rvmeush 02-29-2012 10:16 PM

Quote:
 Originally Posted by hvac5646 (Post 867098) 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.

 how 02-29-2012 10:35 PM

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.

 Missouri Bound 02-29-2012 11:08 PM

Quote:
 Originally Posted by rvmeush (Post 867124) No is the simple answer I was looking for. Thanks.

Then you have come to the wrong :censored: place.

 biggles 03-01-2012 04:55 AM

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.

 hvac5646 03-01-2012 07:11 AM

Quote:
 Originally Posted by Missouri Bound (Post 867157) Then you have come to the wrong :censored: place.
Why do you say this?:huh: Kind'a harsh...no?

 Missouri Bound 03-01-2012 07:39 AM

Quote:
 Originally Posted by hvac5646 (Post 867259) Why do you say this?:huh: 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.:whistling2:

 beenthere 03-01-2012 09:58 AM

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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