C Wire for Smart Thermostat (again)

bgwesttex

First thanks for the many existing threads concerning adding a c wire for a smart thermostat for a radiant heat system. I think I am on the right track, but want to verify.

Existing equipment:

Weil Mclain 550-223-798/0797wm boiler
Honeywell v8043e1012 zone valve (2 red and 2 yellow wires)
Three wire (red, white, green) thermostat wire

Proposed Plan:
Add external 24 v power supply
Terminate wire on hot 24 V power supply to RC thermostat connection (red wire)
Terminate wire on Neutral 24 V power supply to C on thermostat connection (green)
Terminate wire from yellow zone valve wire to W thermostat connection (white)

I have read about the possible need for a relay on the Weil McClain units. Will that be required on this setup? If so, where does the relay need to be installed?

Jamie Hall

That should work. The reason many of us --including me -- recommend a relay in the mix is that it completely isolates the thermostat wiring and its power supply from the zone valve and controller wiring. All the thermostat and its power supply is aware of -- if I may use the term -- is the coil of the relay. All the zone valve and any controllers is aware of is an on/off switch -- the relay contacts. So no one gets in a fight...

Teemok

The wisdom of a relay and separate transformer is clear. I've come around to understand it is always the best advice to give. Sometimes it can be done without a relay but knowing exactly how to do it, when it's ok and when it's not, is not generally agreed on. A fuse with a lower rating than the transformers max load is a prerequisite if you choose to not use a relay. Don't overload the controls transformer. Without the right fuse you are an accidental wire short away from a damaged control.

EdTheHeaterMan

Teemok said:

The wisdom of a relay and separate transformer is clear. I've come around to understand it is always the best advice to give. Sometimes it can be done without a relay but knowing exactly how to do it, when it's ok and when it's not, is not generally agreed on. A fuse with a lower rating than the transformers max load is a prerequisite if you choose to not use a relay. Don't overload the controls transformer. Without the right fuse you are an accidental wire short away from a damaged control.

Speaking of damage control, there is a running joke about frying a control transformer: You will let the Factory Installed Smoke out of the transformer. So here is the all new and improved Smoke Keeper:
I don't really sell this. Just pick up the necessary items at the local auto parts store. 3 AMP is the usual size for a 40VA transformer.