Thermostat won't power on

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ttsmith

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OK, this is driving me crazy. I'm hoping folks can get past my poor descriptions here and maybe offer some advice.

I was hoping to add a new Honeywell wifi thermostat which of course requires the common c wire for power. My house had ancient two wire thermostat control but did have a newish Honeywell thermostat existing. The furnace had a Honeywell unit with a blank hookup for c, so I decided to just run new 18-3 wire and hookup the red, white, and c to power the new thermostat.

Trouble is, the thermostat won't power on. I've done the following to troubleshoot:
1. I verified that I'm getting 26 VAC between red and c and about 23 VAC between red and white, at the screws of the thermostat base.
2. I switched back to the old thermostat. Everything works fine with the batteries in, but doesn't power on with the batteries out.

So...it's acting like it's getting no power from the c wire, but the voltmeter is showing about 24 volts there. I Haven't dealt with these much, and I'm not really sure what to check next.

Any advice? Thanks very much!

Travis
 

Reach4

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Are there two R terminals (Rc and Rh)? Are those jumpered together?

If that situation is as called for in the instruuctions, then I would call Honeywell. Based on your symptom, I expect they will send you a replacement, or at least help with some more troubleshooting help.

Also, get a magnifier to make sure that your wires are to the right terminals and not an adjacent terminal. The terminals are usually pretty small and closely spaced.
 
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ttsmith

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There is only one R wire, but the two R points are jumpered together at the thermostat which is what is called for.

I'll try calling Honeywell. At first I was convinced the thermostat was difective but when I swapped back to the old one and it wouldn't power on without batteries either, it made me suspect I was overlooking something. Thanks again.
 

Dana

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Having a measurable voltage isn't an indication that it will deliver sufficient power. You probably have a loose connection or a broken wire, which may have enough contact to measure a voltage at the high imput impedance of a voltmeter, but not enough to deliver the amount of current required to run a thermostat.
 

Reach4

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I guess I would measure the voltage from each R terminal to the C terminal. They should read the same of course, but it might be worth a brief check.
Having a measurable voltage isn't an indication that it will deliver sufficient power.
For an electronic thermostat, having about 24 VAC across the right two terminals should activate the LCD screen, I would think.
 

Dana

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A thermostat needs more current even to power it's temperature sensor (usually a~ 10K ohm thermistor) than the LCD screen, and WAY more current than a voltmeter draws. Even a fully disconnected wire in a 50-100' wire can get sufficient inductive pick-up to measure 23-26 VAC with a volt meter, but won't deliver enough current to power anything.
 

Reach4

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I am thinking he measured the voltage at the terminal screws on the terminal strip of the thermostat, rather than at just the disconnected wires.
 

JerryR

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More than once I've just wired an external 24vac or a 16.5vac transformer to a wifi thermostat and it comes alive. A few times that's all the wires on the thermostat. Ive used it just to report via wifi the room temp and not control AC at all.

So if you have 24vac between the Rh to C terminal the thermostat should come alive.
 

Dana

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Only if it can supply enough current at 24VAC will it come alive. It doesn't take much connection to have a measurable voltage, but if the connection has a high impedance that voltage collapses when the load is applied.
 

jadnashua

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Some thermostats will work fine with an external 24vac supply, some will require an isolation relay...you need to carefully read and understand the installation instructions.

The 'C' terminal's wire may or may not actually be connected back at the furnace...have you checked there to verify it is properly installed and tight?
 
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