Please help me locate the equivalent of the "C" wire

[controlIt]

This is for connecting a hot water driven (gas) furnace/AC unit combination to a CT30. There is no terminal with a "C" connector. The wires going to the thermostat originate via wire nuts connected to a box containing the transformer, pump relay and fan relay so I can tap into any of the wires coming in or out of these elements (see attached wiring diagram). Question is, which one? I am thinking maybe one of the wires from the transformer but I don't know for sure. The original thermostat is a standard Y.R.G.W mercury thermostat. Thanks for your help!

[blaine]

There is always a C wire, it is one side of the 24VAC and the other side is the R wire, but sometimes it is difficult to locate. I recommend getting a 16VAC or 18VAC adapter, and connecting it to the wire going to the thermostats RH terminal and a totally seperated wire going to the thermostats C terminal. That puts 2 wires on the RH wire which is OK.

Recommended C wire voltage 16V to 24V, AC or DC

ADAPTER:

www.mouserelectronics.net Part # 802STA4116, specs 16VAC @ 250ma, about $11.00, phone 1-800 346-6873

www.digikey.com Part # T1129-P5-ND specs 16VAC @ 250ma, about $11.00, phone 1-800 344-4539

http://www.mpja.com/18VAC-20VA-Plug-Tra ... /17560+PA/ 18VAC W/Terminals and wall screw, about $4.00, phone 1-800-652-6733

http://www.sciplus.com/singleItem.cfm/terms/5103 Part # 27671 ADAPTER, specs 18VAC/100mA, about $2.00, phone 1-888-724-7587



TRANSFORMER

17VAC doorbell transformer available at Home Depot

[controlIt]

Thanks for the answer Blaine. I need a bit of clarification please...

First of all, I failed to mention that I have plenty of extra wires in the bundle the leads to the thermostat.

Unless I absolutely have to, I'd rather not get an adapter. It's a rental house and the furnace is up in the attic which means I would need to tap into the "W" and "B" wires in the diagram to power it, which are 110V and in that environment I'd really try to avoid that. I mean if I have to, I will, but I think that would be my "last resort" if I can help it.

This said, if it is as you say "the other side of the 'R' wire", than I think based on the diagram, we are talking about the "Br" wire in the diagram. Would you agree? I could easily tap into that and feed to one of the extra wires in the bundle.

[blaine]

BR off of the transformer goes to the 2 relays which makes it the common (C)wire. Also it is from the opposite side of the transformer from the R wire (to the thermostat). The problem is often finding it.

[controlIt]

Should be pretty easy for me to tap into it at the point where the Br wire is twisted into the blue wire going down to the condenser coil outside (the wire nut is represented by the second box underneath the second letter "A" in "cabinet penetrAtions" in the diagram). Thanks for your help!