Voltage to thermostat (r-wire) question


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Old 02-28-16, 09:09 AM
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Voltage to thermostat (r-wire) question

I am trying to install an Ecobee3 programmable thermostat which requires 24VAC to operate. I have a Carrier Comfort 92 heating system.

The current thermostat operated fine, but wasn't programmable (Carrier TSTATCCBAC01-B, http://esmithair.com/wp-content/uplo...stallation.pdf). That thermostat had the following wires: W, O, G, R, C, Y. It should be compatible.

When I check the R-wire, I am only reading 5VAC. (According to the manual for this old thermostat, it also requires 24VAC so I am unsure how it is running on the 5VAC being provided.)

Any ideas on what may be causing this before I open up the furnace to look at the Furnace Control Board? Any help is greatly appreciated.
 

Last edited by db9908; 02-28-16 at 09:28 AM.
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Old 02-28-16, 11:08 AM
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Checking the R wire to what ?

You should be checking between R and C.
 
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Old 02-28-16, 11:14 AM
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This would be the code you would follow. No need to jumper Rh to Rc.

W -----> W1
O -----> O/B
G -----> G
R -----> Rh
C -----> C
Y -----> Y1
 
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Old 02-28-16, 03:01 PM
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Hi, PJmax! Thanks for the reply. That is exactly how I wired it up and I was checking R to C which gave me the 5 VAC reading. From what I can tell looking at the diagram for the Carrier Comfort 92 it should be outputting 24VAC to the thermostat so I am confused as to why I'm only reading 5v there. I suppose I'm going to have to open the furnace to get access to the control board to take a reading there to verify output as the next step.
 
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Old 02-28-16, 03:05 PM
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If you did the wiring with the air handler on you probably blew a protection fuse on the control board. It's usually a 3 or 5 amp ATC automotive type fuse.


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Old 02-28-16, 06:19 PM
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Thanks, Pete! I will take a look at that tomorrow after work. Would fuse prevent the system from operating at all? Would it make the R-C read 5v?

The part that is really blowing my mind is that although the old thermostat documentation states that it is a 24V system, it still runs even though R-C only reads 5V. When I plug the old thermostat back in the whole system runs like a champ.

Weird.
 
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Old 02-28-16, 06:32 PM
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No.... if the fuse was blown..... the old stat would not work either.
Are you sure the meter is set for AC voltage..... not DC voltage ?
 
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Old 02-28-16, 06:38 PM
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Yup. I have pretty good experience working with electronics. This is why I am so baffled.

Also, another indication I receive of low voltage is that when I wire up the Ecobee3 I hear a clicking sound but nothing comes on. The documentation for that system says this is an indication of low voltage.
 
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Old 02-28-16, 06:57 PM
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Your next step is to check at the control board. 24vac should be found between the blue and red.

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Old 02-28-16, 07:12 PM
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I will take a reading there after work tomorrow and post what I get. Thanks for the diagram! I appreciate all the help!
 
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Old 03-01-16, 03:44 PM
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Alright, Pete! I am even more confused now! Haha! Red to Blue on the Furnace Control Board reads 27VAC. Any ideas?
 
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Old 03-01-16, 04:28 PM
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Do the colors on those two terminals match the colors on the stat end ?
Possibly you have a splice in the line.
 
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Old 03-01-16, 06:05 PM
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Good call. Didn't even dawn on me. The Blue wire on the Furnace Control Board is a Black wire at the thermostat. Looks like I am going to have to try to trace the cables to see what is in between. Are there any big caps or anything in there I need to worry about?

I still don't get how the thermostat is operating, though, since the manual says it requires 24VAC. Just bought the house a few months ago, so finding all these oddities is fun and aggravating!
 
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Old 03-01-16, 10:43 PM
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The red carries 24vac to the stat. The stat switches that red wire directly to the Y terminal for A/C or the W terminal for heat or the G terminal for fan.

The only thing that requires the C connection is the electronics in the stat itself. Most stats use batteries and don't require the C connection. Most old mechanical stats didn't require a C either unless it had a clock in it.
 
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Old 03-05-16, 07:52 AM
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Thanks! I'm going to try to trace out the wire today to see where and why it is splitting off.
 
 

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