Thermostat wiring question

Hi guys im planning on purchasing a honeywell t6 zwave thermostat.

When i investigated the current wiring the dumb thermostat is not hooked up to a c wire. However it looks like whoever installed simply snipped the blue wire a kittle ways back.

What is the best way to test that this blue wire has the power i need for thermostat.

If the answer to above is yes if once i pull the ild plate off depending on if there is any extra slack in the cable. Would it be proper to use a wago connecter to extend it and wire up the new thermostat?

Picture attached.

Thanks in advance.

You should have 24v between RC/RH and C. With an AC multimeter test between that red wire and your blue.


Equally important, you need to verify where the other end of that blue is... Is it connected to anything? If yes, what?

If extra length is necessary, a Wago will do the trick fine if it fits in the wall cavity (Wago 224s can take wire as small as 24, and t-stat wire is 18). A small wire nut would work perfectly fine too.


@cwwilson08 You need to pull the front panel on your HVAC in the basement and look to see if the blue wire is hooked up to C there. If not you need to hook it up. Then if you have the extra length in the wall, Strip the blue back at the thermostat.


FYI, this thermostat will work without being powered by the air-handler; i.e. without a C-wire. When it is powered by batteries, the T6 is an LSS device, and sips power very conservatively. So batteries will last over a year.

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I figured that was where this one going. Fwiw the blue wire does appear to have 24 v. However i will climb up in the attic as well to confirm this.

Thanks to everyone for chiming in. I knew id be provided with proper information.

Will do the final confirmation and let you guys know if i fry anything.

Many thanks again.

Thanks @aaiyar i considered just doing this but if that blue wire has juice i would like to try to use it. Battery only will be for the i cant get it to work plan.

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Pair without batteries so it repeats. After it's paired put batteries in it. It can trigger rules switching from main to battery of power goes out.

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You really do need to have the C wire hooked up correctly. I have faced this personally. Without the C wire connected you have to use batteries. Under battery power the tstat will go into power save mode when not active (it states this in the manual). I had to send every command to the tstat twice. The first command wakes up the tstat and the second 10 seconds later sets it. This is not acceptable and does not work very well. I ran a new cable to my tstat and connected the c wire and now it works as it is supposed to. My ac tech checked it all and I hooked it up correctly.


And the situation you describe is a perfect example of how differences in z-wave setups can make a big difference in end-user experience. In my opinion, LSS/FLiRS devices should always communicate with the controller via a beaming repeater. This ensures the sleepy device receives commands sent by the controller. Your experience suggests your LSS device had a direct route to the controller.


The C wire is similar to the neutral in a normal household circuit. The power (24V AC in this case) comes from the red wire connected to Rc and jumped to Rh. The C wire completes the circuit back to the transformer and allows current to flow which powers the thermostat. If the c wire is properly terminated at the HVAC end, your meter should read 24 VAC across the R and C wires. Not really sure what it would read if the blue was disconnected..

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Hi everyone. Figured id close the loop and follow up.

I finally got up the nerve to do this. It is funny how things i used to not worry about cause me anxiety. So after much procrastination i just jumped in. Took like 10 minutes.

I used the mounting plate because the holes lin3d up perfectly with the old stat holes. Looks good. Seems to be working. Its 105 f so i wont be calling for heat atm lol. Here is a couple of pics. Thank you to everyone who chimed in. There really is a great wealth of experience and knowledge here.

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Glad that worked out for you.

General suggestion - I would recommend exposing less conductor. While there's no danger of a short in your setup, there's no real need to expose more than what is necessary to insert in the terminal block.