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Nest thermostat wiring on the low-voltage side of Weil-McLain WM97+ Boiler
BB2023
Member Posts: 3
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.
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.
0
Comments
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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.
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@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!
1 -
@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
2 -
@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!
1 -
@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
1
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