Nest thermostat wiring on the low-voltage side of Weil-McLain WM97+ Boiler

BB2023

Hello everyone,

I have a Weil-McLain gas-fired water boiler model WM97+ for heating system that has 3 zones.

Now, I'd like to install a Nest thermostat to only Zone 1 for space heating. My thermostat wire is 18/3 - red, white, and green. I currently connect a white wire and a red wire from my thermostat to 2 low-voltage terminals of Zone 1 connection on the boiler (see the photos please). So, I have a green wire left.

The schematic wiring diagram of my system is exactly like page 72 of this manual.
https://www.weil-mclain.com/sites/default/files/field-file/WM97+ Series 2 Boiler Manual.pdf

On the low-voltage connection of the boiler, however, there are 2 terminals that say the "R and C 24 VAC." Per the description and wiring diagram on page 71 and 72 of the manual, it says "24 VAC for thermostats when required. For the thermostats that require a continuous 24-volt power source, connect the thermostat power input to the 'R-C 24 VAC' terminal of the lower voltage terminal strip."

My questions are:
1) Can I just connect my green thermostat wire to the "C" connector? Will that be all I need to do to power the Nest thermostat without accidentally breaking my control board?
2) What about the "R" terminal on the "R C 24 VAC" that will be open if I connect the green wire to only the "C"? Do I need to add the 4th wire to connect from the "R" of the "R C 24 VAC" to the thermostat "R" in addition to my red and white thermostat wires that are already connect to the Zone 1 terminals?
3) On the cover of the low-voltage side of the boiler, it says "DO NOT supply 24-volt power to the thermostat circuits. If using 3-wire zone valves, use relays to provide dry contacts to the WM 97+ control module thermostat connections." So, do I need to use any relay in my case?

Thank you so much in advance for your reply.

HVACNUT

What's the VA rating on the transformer? 40 or better, you can use the C terminal. Less than 40, a separate transformer and relay are needed. And if you decide down the road to add another Nest, you'll definitely need the separate transformer and relay.

BB2023

@HVACNUT - Thank you so much for your reply. The VA rating on the transformer is 50 VA. However, after reading so many posts about the impacts of the power-stealing Nest thermostat, I decided to build an isolated circuit with a new 24-V transformer and an electromagnetic relay that will supply the power to my Nest thermostat and provide a dry contact to my zone 1 valve controller on the low-voltage side respectively. These are the products I use:

1) Newhouse Hardware 40TR 24-Volt 40vA Wired Door Bell Transformer for Powering Multiple Smart Doorbells and Thermostats

2) 90-380 Appli Parts Heavy Duty Switching Fan Relay 24 VAC Coil SP Normally Open an SP Normally Closed AC and heating Furnace blower relay Universal fit multi position bracket APFR-380

Not sure if I'm over engineering my system or the relay is too oversize for my low-voltage controller. But the solution with the relay seems to be working to operate Zone 1 heating successfully. I'm very excited!

Thanks again!

EBEBRATT-Ed

@BB2023

You did the right thing. Maybe it would have worked with the c wiring coming off the boiler control. But your fix is the best way. We get so many posts on this forum with people trying to mickey mouse a nest hook up and they go to much expense and time and trouble. I have posted a drawing I made of how to hook up with a separate relay and transformer which they never want to do. But it is simple and trouble free

BB2023

@EBEBRATT-Ed - Your feedback is really appreciated and that made me so happy! I learned it a hard way last year before I came up with this fix. As incorrectly suggested by many YouTubers, I used a cheap external 24 VAC adapter that is widely available on Amazon to feed the 24 VAC power directly to Nest without installing a proper C wire. I think that extra power might fry the control board and cause the internal process of my heating system to mulfunction because a circulator pump of my Zone 1 heat mysteriously stopped working one day after I used Nest for about a month. I had to replace the circulator pump and removed Nest from the system. It was a costly mistake. Luckily the control board and the rest of the heating system were not damaged.

Any way, the good fix that I implemented this year can be illustrated by a diagram and a photo as shown below. I also bought an ABS junction box with a cover for the relay as well as labeled all the wires with the white Scotch vinyl electrical tape for my own reminder. I learned a lot from this DIY project.

Thanks again!

EBEBRATT-Ed

@BB2023
I like to take a Junction box and mount a Rib relay RIBU1C relay on the box. If you are familiar with those relays they mount with it's own plastic nipple to a box. It saves having to find an enclosure for the relay and also put a transformer on the same box either nipple mounted or face mounted where a blank cover would go on the box.

Others on this forum use a Honeywell "fan center" which is a relay and transformer package. The rib relay and transformer can be bought for around $40

But to each his own and your method is as good as any