Installing a Smart Thermostat Without a C-Wire on a 4-Zone Boiler System

Nicholas Siakotos
taco thermostat

When I decided to install a smart thermostat, I quickly ran into a common issue—my house didn’t have a C-wire. If you have a heat-only system, this is something you might encounter too.

Understanding the Basics

If you’re not familiar with HVAC systems, some of the terminology can feel like alphabet soup. Working in IT, I’m used to acronyms that don’t always translate well into everyday language. Here’s a quick breakdown of the key terms:

  • TSTAT – Short for "thermostat."
  • C-Wire (Common Wire) – A constant power source for the thermostat.
  • RC (Red Cooling) – A switched hot wire for cooling.
  • RH (Red Heating) – A switched hot wire for heating.
  • C (Common) – The constant power wire.
  • W (Heat Call Wire) – The return path, similar to a neutral wire in electrical systems.

My Options for Adding a C-Wire

Since I didn’t have a C-wire, I had two choices:

  1. Use a Power Extender Kit (PEK) – This would require installing four separate power extenders (one for each of my four heating zones). It would have been a messy and clunky setup.
  2. Install a Step-Down Transformer & Isolation Relay – A more complex approach that felt like overkill for my situation.

The Better Solution

After checking my wiring, I found that my thermostat already had a three-wire cable:

  • Red (R)
  • White (W)
  • Green (G)

Interestingly, the green wire wasn’t connected to anything at the thermostat or the boiler controller. In most systems, green is used for the fan, but since I have a boiler (not forced hot air), I don’t have a fan.

This meant I could repurpose the green wire as my C-wire, solving the problem without needing an extra transformer or relay.

Finding the Right Terminal for the C-Wire

This is where things got confusing. My boiler system uses a Taco SR504 four-zone switching relay:

(Insert first image of Taco SR504 4-Zone Relay here)

I initially struggled to find where to connect the newly repurposed C-wire because Taco doesn't provide a dedicated common terminal for each zone thermostat. Instead, they have one shared common terminal for all four zones.

To make matters worse, many guides suggested connecting to the 24VAC terminal on the left side of the board. However, this doesn’t work for reasons I won’t go into here.

Instead, the correct location is here:

(Insert second image showing the Common Terminal on the Taco SR504)

See the terminal labeled COM? That’s where your common wire goes.

Connecting the Thermostat

Once I connected my repurposed green wire to the COM terminal, my thermostat powered up correctly without any extra hardware.

(Insert third image showing Thermostat Wiring on Taco SR504)

Not all boards clearly label this terminal, and on some models, it might be in a different location. If you're installing a smart thermostat on a Taco SR504 or similar zone controller, be sure to find the correct common terminal instead of using the 24VAC terminals directly.

Final Thoughts

If you’re looking to install a smart thermostat and don’t have a C-wire, check your existing wiring first. If you have an unused wire (like my green wire), you can likely use it as a C-wire without additional components.

Hopefully, this helps clear up some of the confusion around C-wire solutions on multi-zone boiler systems! Let me know in the comments if you have any questions.