'Silly' Question: Adding a C wire on two thermostats


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Old 10-24-14, 08:12 AM
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'Silly' Question: Adding a C wire on two thermostats

Hi all,

I successfully installed a honeywell RTH6580WF wifi thermostat, replacing an older two wire 'dumb' stat. I followed this video:
C Wire - How to power the thermostat via an external transformer - YouTube

Background:
Previous owner had a 5 conductor wire coming out of boiler, with 2 conductors used per 'zone', the fifth conductor unused. I moved the terminus of this wire into a junction box, and left one stat untouched, the other one I switched to wifi as described here...

The first stat:
Basically, I ran a 5 conductor wire from this new stat to that basement junction box. I ran a two conductor wire from a wall wart to that same box in the basement. In the junction box, I paired the two wall wart wires and two conductors coming up from my boiler to the 5 conductor running to the new stat.

At the stat, I connected white-white; black to Rh (following existing thermostat wiring). I wired a wall wart and connected it to C and Rc (with jumper to Rh that came from factory). This stat works fine!

My question:
I have a second honeywell RTH6580WF wifi thermostat that I want to replace another 'dumb' stat with. I have a new 5 conductor wire run to it. I need to get power to this one as well. I know I can buy another wall wart and connect it in similar manner as the one above, but I am wondering: Can I use ONE wall wart for bother stats?

this is the wall wart:
Elk TRG2440 24VAC, 40 VA AC Transformer with PTC Fuse - - Amazon.com


this is the stat:
Honeywell Wi-Fi 7-Day Programmable Thermostat - Programmable Household Thermostats - Amazon.com
 
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Old 10-24-14, 10:35 AM
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I wired a wall wart and connected it to C and Rc (with jumper to Rh that came from factory)
That should be set up to work with no jumper. The transformer is connected to C and Rc only.

If THAT thermostat works ok with the jumper out.... then you can use the same 40 watt transformer to run both thermostats BUT the jumper must also be left off the second thermostat.
 
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Old 10-24-14, 11:16 AM
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what does that jumper do? Thermostat doesnt control an AC--just boiler

...hmmm.... Thermostat instructions seem to imply that the jumper is needed? Or maybe the added power from the wall wart will satisfy this without the jumper?


(page 14 at:
https://customer.honeywell.com/resou.../69-2718EF.pdf
)
Remove metal jumper only if you have both R and RC wires
 
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Old 10-24-14, 11:29 AM
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You are adding a new transformer to the thermostat and there is already one in your boiler.

You do have an R and Rc wire. The R wire was existing and now you are adding an Rc wire from the new transformer.
 
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Old 10-24-14, 12:07 PM
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is this the sort of thing that I would know immediately if it works/doesnt. Or is it the kind of thing where it might work but then burn out the thermostat in a day/week/month/year/etc?

Thanks for your help on this!
 
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Old 10-24-14, 02:17 PM
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If there was a transformer conflict.... one or both would get hot and possibly burn up.

The R connection is the terminal where the 24vac comes in and is switched to the outputs for fan, heat, A/C, etc.

The Rc connection is 24vac to the clock or WiFi section. That's why in your case you don't use the jumper since you aren't using the transformer of the heat unit.
 
 

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