Help with adding a C wire for a Smart Thermostat

ColonelSanders

Friends,
I am trying to replace my current hall thermostat but it only has an R and W wire. I have inluded a wiring diagram of my system in the hope that you could tell me what I need to do to get a working C wire. Any thoughts?

Many regards,
Colonel Sanders

Tinman

It needs to come from the same source as your R wire at the thermostat. So, the transformer would be the answer.

Eric_32

What he said ^^

You'll have to snake up a wire to the thermostat for C terminal which will connect to the 2nd screw on that transformer.

ColonelSanders

Thank you. Does a way exist that doesn't involve snaking a new wire? Maybe with a "Common Maker" product or something similar?

ChrisJ

Agree with others, your common needs to hook with the white wires at the transformer.

As far as if there's a way to do it without snaking a new wire, I'd be grabbing my poke saw and mud pan.

Robert O'Connor_12

Why not first check to see if there is another wire under the insulation cover of your existing thermostat wire feeding the T-stat. when I purchase low voltage wire, I always buy minimum of 4 wire. Hopefully for your sake the original installer did too.
Good luck!

Robert O'Connor/NJ

ColonelSanders

Thank you everyone, I greatly appreciate your advice. The customer support representative from my wifi thermostat company has suggested I use a step-down transformer and an Isolation relay to avoid snaking a new wire. Is this good advice?

Also, I just checked the insulation cover and regrettably there were only 2 wires. It was worth a shot though!

ChrisJ

ColonelSanders said:

Thank you everyone, I greatly appreciate your advice. The customer support representative from my wifi thermostat company has suggested I use a step-down transformer and an Isolation relay to avoid snaking a new wire. Is this good advice?

Also, I just checked the insulation cover and regrettably there were only 2 wires. It was worth a shot though!

Sure, if you have a place to put the extra transformer and relay. Though, it will be two more items wasting power in the system, albeit not much it's still a tiny increase as well as more things to fail.

hot_rod

I would like to see someone try one of the "add a common" kits

If fishing a new wire is s hassle try a kit and report back

Dave H_2

Colonel,

Try using a different smart stat that doesn't require the C wire.

Lux just came out with one that can be battery powered or C wire. I have not used it yet, but like the idea of making it real easy for the retrofit.

Also, I just installed an EcoBee 3 and it had the kit in the box with the stat. Again, I didn't have to use the kit since we had the extra wire.

Enjoy

ColonelSanders

The plot, like Grandma's gizzard gravy, is thickening!

On Robert's advice, I did a more thorough exam of the wires on the back of the thermostat and buried deep down was an unconnected red wire!

In the basement, I found that the 3 wires were then spliced into 4 low voltage wires (see new diagram)—2 of which were connected to an (old?) transformer above the boiler. Can I use this red wire as my common as is or do I need to connect it to the "white terminal" on the other transformer?

Again, thanks for your help and for your patience. When it comes to HVAC, I am just learning the ropes (wires).

ColonelSanders

Anyone?

Jamie Hall

You can use any wire... for anything. One thing to do, though, before starting to do that, is to mark both ends of the wire the same. Suppose, for instance, you consider that unused red wire. Mark it at one end -- perhaps tape around it, or paint, or nail polish. Or something. Then find the other end in the basement. Make sure that a) it really is the other end of the same wire, b) it really is connected through, and c) it really isn't connected to anything else. Then mark it the same way.

Now you have the old red and white thermostat wires -- and a nice wire to use as the common wire. No problem.

Now wire it up exactly as is shown on the instructions. I would use the same transformer, though, provided it has enough capacity, rather than the old one. Using two different transformers is subject to a variety of interesting electrical problems, most of which will burn out your fancy thermostat, or the valve controller, or both.

ColonelSanders

Jamie,
Thanks for your help. How do I determine if the transformer has the proper capacity? The white terminal is currently connected to two different valve assemblies, as shown in my initial diagram.

Regards,
the Colonel

Jamie Hall

Thermostats take very little power to operate, so it is likely that there is enough. One should be able to find on the valves and the thermostat the power requirements, though, and add them up and compare it to the rating on the transformer.

Eric_32

sweet deal!! a hidden wire.. lol the color doesn't mean a thing if both ends are not connected to anything.

Connect it as mentioned above and let us know how you make out...

ColonelSanders

Oh my garsh. I re-routed the secret red wire to the white terminal on the transformer and hooked up the thermostat according to the instructions. It worked perfectly as soon as I powered it on. Hopefully now I'll be saving a pretty penny on my heating bills AND I have learned a little something about my system in the process. Thank you to everyone for your guidance.