So I decided to install a smart thermostat. My house did not have a c-wire and I have a heat only system.
Some things to go over that if you don’t have a background in HVAC won’t make sense but once you read it does. Working IT we have the same kind of issue with acronyms that don’t translate well into normal speak.
- TSTAT is just short for Thermostat – I don’t know why it took so long for me to get this.
- c-wire means common wire this is essentially constant power source similar to the rc or rh but not switched.
- The terminals on your boiler and tstat
- RC is Red Cooling – this basicly means its a switched hot wire
- RH is Red Heat – this basucly means its a switched hot wire
- C Common – this is a constant on hot wire
- W is the return path or if you are familiar with electrical this is your neutral
So I had 2 options, run a special power extender kit which would have looked really clunky because I have 4 zones and would have needed to mount 4 power extenders or install an step down transformer and an isolation relay which I thought was overkill.
I found that I had a 3 wire cable run, Red White and Green. Green was not connected to the thermostat or the controller on the boiler. Green is generally for the fan, which I don’t have it being a boiler and not forced hot air.
I tried doing some research but there was much confusion including on Ecobee website that says you need a transformer and an isolation relay. This is the case only if do not have a 3rd wire you can use as your common.
So where do you plug in this third wire? There was a lot of confusion when looking this up because TACO in order to have a less complex system decided not to put a dedicated common on each zone tstat terminal and instead decided to do a single common which can be used on all 4 zones. Even more confusion came when people tried to connect the wires to the 24VAC terminal on the left and it didn’t work for reasons I won’t get into here.
See the terminal with COM near it? That’s where you common wires go. Not all boards say that and on some of them it is placed differently.
mythoughtcube 
It looks like you are using the same TACO relay as I am. I’ve been struggling to add the common wire to my system. I just bought 2 nest thermostat’s a 3gen the e. I added the wire to the com connector and ran new 3 wire thermostat wire to the nest. I have a constant 24v between the Rh and C wire, but the nest throws an E79 error. I have the white thermostat wire on the leftmost terminal in the taco and the red on the right. and the 3rd connected to the com. Is this how yours is set up?
Sorry I don’t check this site as often as I used to. I have it reverse. I will upload a picture of my wiring in case it helps anyone else.
Use the COM screw (not the 24VAC screw) to connect 3rd wire to TACO SR 504 controlled by NEST thermostat C wire. If 3rd wire is NOT available, then wire the 24VAC screw (not the COM screw) with a 220 Ohm 5 watt resistor (Youtubers’ indicate both 1000 Ohm & 500 Ohm also may work) bridged to the Neutral(white) connection. If more than one zone is being controlled by more than one ECOBEE or smart thermostat, contact TAC0 technical support: 4OI.942-8OOO
Somehow I never found this when I was installing an ecobee into my Taco 504 relay 2 years ago. I ended up adding a plug in C wire but could have just connected to the COM terminal. Looking at adding two more ecobees for the other zones and this will be super helpful. Thanks!