Need to Add a C Wire

[larryjones]

Hi, I would appreciate any help on this. I have a 130 yr old house with a 5 yr old Buderus boiler running 3 zones. Two of the zones are fine and working on basic stats. The 3rd zone (upper floor) had a new wire run during recent renovation (by me) and I have since discovered that the wire is pinched or cut somewhere. To avoid running a new wire through my recently renovated and painted house I decided to purchase a Honeywell Visionpro stat with the equipment interface module. Problem: In hooking up the EIM near the boiler I realized that I need a "c" wire but I only have "r' and "w" coming from the 24V transformer. The EIM needs constant power in order to work wirelessly with the stat on the upper floor. How do I rectify the problem (i.e. add a "c" wire) without damaging anything or creating more problems for myself? Thanks in advance!

 

[Jadnashua]

The 24vac transformer has four wires to it:
- 24vac hi, low (C)
- 120vac hi, low (neutral) (well, most boilers run on 120vac, but it could be 240vac...doesn't matter as long as the output is the nominal 24vac).

Find the transformer or check the electrical terminal strip to find the 'other' side of the 24vac transformer output. A thermostat takes a 24vac signal, in, when it calls for heat, it closes a switch, running that 24vac back to whatever. Inside of that control, it will have the 'other' side of the transformer connected to complete the circuit and power it up. You have to find that 'other' side of the transformer, and run it to your thermostat so that it will have a complete circuit to power itself.

 

[Sponsor]

 

[larryjones]

I've posted a few photos of the transformer and the EIM. I wasn't exactly following your lead I'm a carpenter by trade so bear with me. Can I run an 18-3 wire from point A to point B to accomplish the goal? Thanks!

 

[larryjones]

 

[Jadnashua]

It appears that you have the manual folded up there. Either in that, or pasted inside of the cover, there should be a wiring diagram. Find the transformer. One of the outputs will go to the R terminal, the other C (or common). There are two sides to a transformer, the input and the output, each has two wires. On the input (primary), will be the 120vac and neutral. On the output will be 24vac (well, it's usually higher than that, but not over say 32vac). One of those leads on the secondary will go to the R terminal, and what you want to find out is where the other goes, to hopefully, a place where you can access it easily. What makes it impossible to just follow the wires is that the transformer is soldered onto the circuit board, so you can't just follow the traces on the board to see where it is connected and then tap into it since they're on the back side. It MIGHT be the A terminal, but without seeing the wiring diagram, that's only a guess. You wouldn't want to attach anything there until you were sure or you might fry it.

If you can post the model number of the unit or take a good picture of the wiring diagram, we can probably figure this out, if you can't on your own. Or, maybe a link to the manual on line, if you can find it.

There are other ways to accomplish this, but it takes another transformer and an isolation relay, and you don't want to go there unless it's necessary.

Again, though, all the thermostat is, is a smart on/off switch, and all it is doing with the current R and W wire terminals is connecting them together when it wants heat, and opening them when it doesn't. To power the thermostat (dumb ones don't need power!) requires a complete circuit with R being one part, and C being the other.

 

[larryjones]

Ok thanks, Jim. I'll give it a go in the morning and see if I can make any headway. Much appreciated.

 

[Stuff]

That zone control board is one of the unfriendly ones that does not have a terminal for C. Probably because the transformer is only 15va so does not have much to spare to power any other device.

So follow the instructions for the EIM where it says "Remove jumper(s) if using separate transformers." - Instead of a jumper buy a 24vac transformer and feed it to either sets of the R and C terminals. Then use RH and W to go to the TT terminals on the zone board where the thermostat used to attach.

 

[larryjones]

Thanks, Stuff. I linked a 24vac. Is this what I need? Also linked the wiring diagram for the EIM (page 6).

I have red and white from the existing boiler setup per the photo.

1. So run red to "RH" and white to "W" on the EIM....

2. Then, use the plug-in tranformer to run a line to "C" after pulling the jumpers?

https://www.supplyhouse.com/Venstar-ACC0410-Add-a-Wire

https://forwardthinking.honeywell.com/related_links/new/69-2758.pdf

 

[Stuff]

Yes, page 6 without the red jumpers. Two wires from the low voltage side of the transformer - R and C

What's the Venstar add-a-wire for? I thought you would be abandoning the cabling going to the thermostat?

 

[larryjones]

I need constant power and control from the boiler to the EIM so it can then communicate wirelessly with the stat on the upper floor.

 

[larryjones]

Need constant 24vac to the EIM to power the unit, right? My problem now is that I have only R and W going to the EIM and it has no power. If I add a "C" via the Venstar plug in that gives me the "C" that I need?

 

[Stuff]

Yes, you need a constant 24vac at the EIM.
No, the Venstar won't do what you need as it does not create a C. It is used for when a remote thermostat doesn't have enough wires. It needs a real C at the furnace/boiler/air handler and assumes you have the C wire to run to the thermostat. What the Venstar does is free up a wire by combining Y and G at the furnace into one wire and then you install a diode at the thermostat to re-create (split out) Y and G from the single wire.

 

[larryjones]

I'm a little slow but I think I got it now.

#1. Pull jumpers.

#2. Run newly purchased 24vac to R & C on the EIM.
https://www.amazon.com/gp/product/B075PN6NCV/ref=ox_sc_act_title_1?smid=A3TYKI3C4ZGTOB&psc=1

#3. Run the pre-existing red and white from the boiler setup back to RH & W on the EIM.

 

[Stuff]

Yes, that should do it. Then hook up wireless module to EIM and your new thermostat should be ready.

 

[larryjones]

Very much appreciated. Thanks for bearing with me!