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Old 11-19-2010, 07:31 AM   #1
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Thermostat wiring question


Quick ? re- thermostat wiring
I just purchased and attempted to install a Honeywell Prestige 9321.

My old thermostat has 5 wires, (2 for my boiler)Basement, (3 for the a/c unit)attic/outdoor.

It looks like the new thermostat requires an additional wire from the common side of the transformer, the literature recommends the transformer of the a/c unit.

Does it matter which unit provides the common side to the t-stat?

Would failing to have that this common wire connected keep this model t-stat from functioning?

I have installed quite a few different t-stats in my home and have never had one not function in the way this one is.

I can jump across the different circuits and all the systems will function, just seems the t-stat is DOA.

I'm pretty sure I have the wire in place to add the common leg from the a/c or the boiler. It would be much simpler to add the common from the boiler, if this will not create an issue.

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Old 11-19-2010, 08:11 AM   #2
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Thermostat wiring question


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Originally Posted by tgeb View Post
Quick ? re- thermostat wiring
I just purchased and attempted to install a Honeywell Prestige 9321.

My old thermostat has 5 wires, (2 for my boiler)Basement, (3 for the a/c unit)attic/outdoor.

It looks like the new thermostat requires an additional wire from the common side of the transformer, the literature recommends the transformer of the a/c unit.

Does it matter which unit provides the common side to the t-stat?

Would failing to have that this common wire connected keep this model t-stat from functioning?

I have installed quite a few different t-stats in my home and have never had one not function in the way this one is.

I can jump across the different circuits and all the systems will function, just seems the t-stat is DOA.

I'm pretty sure I have the wire in place to add the common leg from the a/c or the boiler. It would be much simpler to add the common from the boiler, if this will not create an issue.


Yes the Common(24volt neutral) is the 24volt side of power all 24 volt circuits return on to complete the circuit,
With the emphasis on completing the circuit,Its akin to using the Positive side of a battery #1
to go through a switch and on to the circuits load, and rather than returning to the Negative side of power on Battery#1,
we grab neutral or Negative off of Battery #2 in another device! that doesnt complete any circuit!
RED is the "hot leg" or" positive leg" (L-1) of the 24 volt control circuits power from the transformer
and enters the thermostat on RED or R.Common or C is the "neutral leg" or "common leg" of the 24 volt control power,
its "common" as all the 24volt circuits that transformer powers, returns to it to complete the circuits path,
thus common to all the circuits.As a result the RED off transformer A and COMMON off of transformer B
wont power the stat as required, it may say RC and RH red heat and red cool, in that case with 1 transformer RC and RH are jumpered, other wise if 2 transformers,RH is heat and RC is for cool.


Last edited by Blaise; 11-19-2010 at 08:15 AM.
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Old 11-19-2010, 03:41 PM   #3
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Thermostat wiring question


Yes, it does matter. That thermostat wants it to be from the cooling transformer, no your boiler.
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Old 11-19-2010, 04:08 PM   #4
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Thermostat wiring question


Thanks Beenthere, thanks Blaise.

I have it correctly wired up and functioning properly.
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