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Discussion Starter · #1 ·
Hello and thanks in advance for your answers! We are installing a Honeywell thermostat model number RTH6580WF. We are pretty sure this issue isn't a bad unit because we have the same problem with the second one.

The digital multimeter verifies that the C wire (blue) at the wall has just under 28 volts. But we get no display. No lights. Nothing.

There was no problem with the old thermostat. We have an electric, forced air system. American Standard unit, no terminal block, no circuit board, just wires coming off the transformer The blue wire is connected to the red wires at the transformer. We verified that it's the same blue wire at the transformer as the one at the wall. The blue wire is connected to the C terminal on the thermostat. Y,G,W, R jumpered to RC....all connected as they should be.

Any ideas why no display? Too much voltage?
 

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How are you using your meter. Normally the R/red at the furnace or thermostat is hot 24-28 volts and the only way to get a reading from it is to the C side of the transformer. C never has a reading at the thermostat so something is wrong with your process. Normally R is red/hot, W/heat, Y/Cool, G/Fan and if you have an extra wire then it goes to C on the transformer and then to C on the thermostat. Most thermostats are battery operated and need no C wire. If it does need a C then you need to sort out your wires. Often they could be spliced somewhere between the furnace and upstairs and have a color change so just because it is Blue upstairs does not mean it is the same wire. C never has a voltage reading, same as a white/neutral wire for your 120 volt house wiring never has voltage.
 

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Discussion Starter · #4 ·
When we started this process the blue wire was just wrapped around the bundle on both ends, at the wall and at the furnace. We connected it to the C terminal at the thermostat and ohmed it to verify it was the same wire. We then connected it with a wire nut to the red wires off the transformer. We have no circuit board or terminal board at the furnace.

Multimeter on the blue wire at the thermostat shows just under 28 volts. I'm failing to understand why it won't provide power to the display on this thermostat. (there is no place for batteries, it's a wifi thermostat).

Thanks for your help.
 

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post a pic of the transformer and elec wiring of the furnace. I suspect you got hot and C mixed up on it. elec furnaces sometimes have universal transformers that don't always use R and C conventional wiring colors/terminology. I have seen multitap ones because they are 220/208/110 volts type. You need to figure out what the C is on that transformer and not go by a wire color. You should be able to go from one side of it to ground to find the hot/R and the other should be your C. then follow it from there. usually you can follow back from the sequencers as 1 side of them has to go to C also. click Go Advanced>manage attachments and follow from there to post pics in your reply
 

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a little bit. the transformer has 2 sets of windings. primary/high voltage 220 input and the secondary/24 side which may be the hidden side on the top of it as you are holding it. you need to follow the wires from that side to see which are the low voltage. one of them should go to the R as hot and the other is C. don't know which color wire they use on yours. the C should also follow backwards to the sequencers which turn on the elements in a daisy chain. the other side of the sequencers goes to W from the tstat.
 

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Discussion Starter · #10 ·
We hooked up the blue wire to the low voltage R wire. At the Tstat, the blue wire is connected to C. Now we have 27 volts on RH and C at the stat.

I'm not sure I follow everything you said....we are stuck on "why doesn't the blue wire provide power to the display if it carries enough voltage?"
 

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modern tstats and electronics are polarity sensitive and newer furnaces have the C grounded to the furnace chassis. I suspect the tstat is polarity sensitive and locks out when it sees voltage at C.
 

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Discussion Starter · #12 ·
Awesome!! Got it. We were close but just couldn't cross the finish line. It's working now. We did exactly what you said and it worked perfectly. Thanks a million!
 

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Honeywell, ac, heater....I know from just doing this yesterday. the transformer needs to be connected to the c and rc or rh. which ever its called, on the honeywell i have it was rc. 2 wires from transformer, 2 connections to heater/ac unit. c and rc...

then theres 4 wires , 2 for heater, 2 for ac......

side note, I may have to post this somewere else, but, Can i put my 120/24 volt transformer in a junction box and cover it with a wall face plate?
I got bigger problems too. maybe i better make a new post....
 

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how many wire are connected to thermostat. you need at least 3 wire, 2 for supply and one for heat, old thermostat needed only 2 wires
 

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carmusic said:
how many wire are connected to thermostat. you need at least 3 wire, 2 for supply and one for heat, old thermostat needed only 2 wires
Modern ones only need 2 also if they run on batteries. That is if you only wanna have control of your heat
 
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