Is my wiring sufficient to change to a WiFi thermostat for my boiler?

I have a single stage oil boiler, it pumps hot water to four zones, and is controlled by an Argo relay control box. All my t-stats are standard Honeywell T87F’s (the circular, manual t-stats w/ mercury). I would like to replace one of the Honeywells with an Ecobee Smart SI Wifi t-stat.

I’d like to ask if my existing wiring is sufficient to connect the Ecobee. My current t-stat wire has 3 leads: red, white and green. The red and white wires are connected on both ends, but the green wire is not in use. I know that the Ecobee needs 24VAC in addition to the normal wiring. Is having one spare wire enough to hook up the 24VAC or do I need two spare (additional) wires?

Also I’d like to ask exactly which terminals should be connected. I’ve attached some diagrams to help make this clear. FYI I’m an audio guy and I do audio wiring but electrical wiring is not so familiar to me.

Currently the white and red wires connect from the T-T terminals of zone 4 on the Argo box to the Honeywell t-stat.

My confusion lies in how to connect the 24VAC. The Ecobee has a terminal for Common, but no terminal for 24VAC. (see attachment)
You can see on the right side of the Argo box two terminals, one above the other: the top one is labeled Common, and the bottom one is labeled 24VAC. I’m not sure if I should connect the Common terminal on the Argo box to the Common terminal on the Ecobee? What is the Argo’s 24VAC terminal used for?

I asked a contractor about this and he said “it’s better to have a 4-wire connection, I should come and run another wire to the Argo”. I just don’t know if I NEED to pay him to run a 4th wire for this hookup - or can I do it myself with my three wires??

Can you post a pic of the boiler terminals? You should be ok using the spare green wire you have and connect it to common on the boiler and common on the thermostat

I don’t have any pics now - the house is a vacation place… Curious though - what would the boiler terminals show you?

The mode you are setup as, uses the T87F as a thermostaticly controlled SPST switch.
The common wire of the Argo goes to the common of the Ecobee © Use the Green Wire for this.
The problem is that the Argo does not label the 24V side or the control side for the T 1 T connectors.
Can you see the PC trace go from the other 24V connector in the Argo to one of the two terminal posts labeled T 1 T ? It goes to one, but I can’t tell which. That’s the one that goes to the other 24V lead of the Ecobee (RH) Use the Red wire for this. And of course the other T 1 T terminal goes to the 1st stage heat (W) Use the White wire for this.

You will not hurt anything if the two wires are reversed in the Argos T 1 T, the Ecobee thermostat will not power up. Just reverse the T 1 T wires on the Argos.

Then apply the same strategy to the other Thermostats.

You should check the current draw of the Ecobee thermostats and make sure the Argo can power the number of thermostats you want to attach. 1VA is not very much power. Roughly 40 mA.

I can’t see the trace from the manual (that’s where I created this attachment) but I should be able to see it on the board when I go to the house next. Thanks for explaining the connections. Previously, I didn’t understand the difference between the two T-T terminals, so I really appreciate your help here.

FYI I am only changing out one thermostat. So hopefully the 1VA will be OK.

Just curious - Do you have any idea why a contractor would say I need four wires for this hookup?

Typical setups are:
Heat only = 2 Wires (Red, White wires are standard wiring)
Heat, Cool, Fan = 4 Wires (Red, White, Green, Yellow wires are standard wiring)
1 Wire extra for 2nd stage each (Heat, Cool)
1 Wire extra to power thermostat.
1 Wire extra if you have separate heat/cool transformers.

The latter three options are not wired with same color wires all the time … usually a function of the wire colors available.

Some of the newer thermostats do a lot more over a few wires … it’s a digital bus as opposed to dedicated wires for control.

Great info, thanks again.