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I have a Laars gas boiler that has 3 heating zones; basement, 1st floor, and 2nd floor. I want to replace the old mercury honeywell tstats on the 1st and 2nd floor with newer honeywell RTH9580 wifi model for both. The 2nd floor is a piece of cake as the current tstat and wiring there also consists of the central cooling control. So on the 2nd floor I have all the necessary wires for the new tstat there to work with (red, white, yellow, green, and blue). On the 1st floor however, the current tstat only controls the heat, and there is only the 2 wires, red and white. I have a clear shot to the basement in the wall behind the 1st floor tstat, and can run new wires from there to the boiler, no problem. My dilemma...once I get the wires there (and do I run 5 wires and just use the red, white, and blue?), how do I hook up the blue wire? To the boiler itself? To the wires connected to the zone valves, and which one? Or to the transformer, and how? I've taken pictures of my entire system, can someone tell me the best method (proper preferred, not necessarily easiest) to accomplish this?

I've added some more pics to included the transformer specs and the boiler schematic

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marked as duplicate by Tester101 Oct 22 at 16:41


You should include a photo of the schematic as well, since it's difficult to follow wires through multiple pictures. –  Tester101 Feb 8 at 14:11
I posted 8 pics. They are not visible? –  Evan Feb 8 at 15:24
The photos are great, but without a schematic it's just a bunch of wires. –  Tester101 Feb 8 at 15:50
I don't have a schematic. I did mark on one of the pics the wire that comes from the thermostat in question. Can I just run the c wire from the thermostat to the transformer? –  Evan Feb 8 at 19:52
You might be able to connect the C wire directly to transformer, but you'll have to make sure the transformer can handle the additional load. It's common for there to be a schematic inside the service cover, or with the boiler documentation. –  Tester101 Feb 9 at 0:51

1 Answer 1

You will want to use the wire labeled "Y" in your schematic as the "C" wire for your thermostats.

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Thanks. The only issue...the "Y" wire isn't labeled. Either is the "R". I've attached another pic that has a more detailed view of the transformer face. One question though on this...is the transformer in the schematic the one on the wall above the boiler, or is there a transformer within the boiler electronics? –  Evan Feb 12 at 0:41
There is probably only one transformer, i.e. the one on the wall. Do you have a multimeter? Measure the voltage between the red wire on the transformer and "R" on the wire going to the thermostat; it should be 0. Then check between the white wire on the transformer and the "R" wire; it should be 24 volts. If so, the white wire on the transformer is "Y" in the schematic, and "C" for your thermostat. –  longneck Feb 12 at 3:39
I had to table this project due to family matters, but now i'm back on it. Thanks for the reply....I tested the wires as you suggested and came up with opposite results....the volts between the red transformer wire and the R wire is 24, and between the white transformer wire and the R wire is 0. Conversely, the volts between the red transformer wire and the W wire is 0, and between the white transformer wire and the W wire is 24. Any suggestions? Thanks everyone for your help on this. –  Evan Sep 15 at 23:41
