WassersteinThermostat C-Wire Common Wire Add-On Kit
Step-by-step illustrated procedure

As you'll read in this article series (links at page bottom), there are several ways to get a C-wire to bring power to a smart thermostat such as the Nest learning thermostat.

Here we use the Wasserstein C-wire add-on kit to illustrate how you can add quickly, easily, and successfully add a C-wire or Common wire when needed.

While the process of adding a C-wire is pretty simple and easy, in trying out several methods and several C-wire kits we found that the hook-up wasn't always as easy as promised.

Remarkably, even among several of these kits, instructions for the wiring connections were inconsistent, not using the same connections, and some kits did not work at all, either because the wall adapter, a low voltage transformer, was defective or because it was too easy to buy the "wrong" transformer kit.

But we had quick and easy success installing Wasserstein's 24VAC transformer and C-wire adapter to power a Nest wall thermostat that had been giving occasional trouble by running low on its own internal battery.

Shown below are the contents of the Wasserstein C-wire add-on kit:

In the box are a 24VAC transformer and wire and an instruction sheet that gives step by step hook-up details.

Pro's:

• The kit worked easily and immediately when we followed its instructions.
• The AC adaptor, Class 2 poer supply, model GPU4124004 10WAOO specifies
  
  Input: 120VAC 60Hz 17W - this is a 17-watt transformer
  
  Output: 24VAC 410mA - which we found adequate to power the Nest room thermostat

Con's:

• The instructions are printed in microscopic font that are illegible without a magnifying glass. Alternatively you can go to the company's website wasserstein-home.com to find more-easily read installation steps.
• The instructions suggest routing the C-wire from the transformer inside of a wall cavity and up to your wall thermostat - a completely un-realistic procedure for most homeowners.
  
  Unless the electrical receptacle is immediately below the room thermostat in the same stud-bay, routing a wire inside the wall cavity will require more-extensive cutting and drilling to route the wires.
  
  We simply ran the C-wires up to the thermostat as shown in photos below.

First we plugged the Wasserstein AC Adaptor in to a wall receptacle and used our digital DMM to confirm that its output voltage was in-range. We found about 29VAC - compare this with the inadequate output voltage

at our description of a different C wire kit that did not work:

Nest C-WIRE KIT FAILURE, DEFECTIVE AC ADAPTER TEST

Watch out: as you'll notice below, before running the C-wire bundle up to the thermostat we un-plugged the AC Adapter from the wall to be sure it's power was OFF.

Below: Next we ran the wires from the AC Adapter/transformer along the top of our baseboard trim, then

up the wall

and then to the thermostat. Notice that our Nest Thermostat was on and live and calling for heat.

Watch out: before removing the thermostat and connecting the new C-wires as per Wasserstein's instructions, we went to the heating boiler and turned off all electrical power to the boiler.

If you're not sure how to do that,

see ELECTRICAL POWER SWITCH FOR HEAT

Tip: before running the new C-wire bundle into the thermostat we loosened the screws holding the thermostat to the wall, and we cut a small gouge in the drywall at the edge of the thermostat so that when we later tightened it in place the thermostat would rest flush against the wall.

Notice that the original thermostat wires remained at their original connections:

• Original Red thermostat wire remained connected to Rh on the Nest Thermostat
• Original White thermostat wire remained connected to W1 on the Nest Thermostat

Now we connected the C-wire bundle wires (red and white) according to the Wasserstein instructions.

• The C-wire kit wire bundle's black wire connected to the C terminal on the Nest Thermostat.
• The C-wire kit wire bundle's red wire connected to the Rc terminal on the Nest Thermostat

With all of our thermostat wires now connected, we level and tighten the thermostat back onto the wall, and then we're ready to plug the AC adapter back into the wall receptacle (below).

Now we simply re-connect the Nest Thermostat to its base,

and we'll see that the smart wall thermostat detects just what wires have been connected.

Click to enlarge the photo above and you'll see that the Smarty-Pants Nest Thermostat has detected exactly the two wires we just connected, the black C wire and the red Rc wire that we showed in our earlier photograph.

If when re-connecting the Nest thermostat to its base you do NOT see that it has detected the wires that you connected, then there is a connection problem (or you forgot to plug-in the AC adapter).

Since all of the thermostat wire connections look good, we let the Nest continue ... it displays its technical information and then

we see that the Nest thermostat display looks perfectly normal, sensing room temperature at 72 degF.

If we turn the temperature up (rotating the thermostat outer bezel to the right or clockwise) you'll see the temperature number go up accordingly and when the heater turns on, the thermostat display will be orange in color.

...

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