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I have a Goodman HVAC system that is split into three zones. I wanted to upgrade to programmable thermostats, so I picked up three Honeywell RTH8580WF. Reading through the descriptions I noted they need a C wire hookup. I looked under the current thermostats, noted there wasn't a C wire, but there were several unused wires so I figured I could just hook them up at the zone board.

I opened up the zone panel, a Honeywell EMM-3, and after looking around inside I'm a little confused.

Zone Board

Unfortunately there wasn't a way to capture the labels on the wiring blocks, but here's a shot from the manual.

Zone Board Wiring Diagram

I've done enough wiring work in my life to understand the basics, but I can't figure out what the installer did with the C wire hookups for the thermostats (the blocks at the bottom). He apparently ran jumpers (the white wires) from the C terminal for thermostat 1, 2, and 3 and then connected them to the blue wire off to the left (which I think is supplying power to the HVAC board for some reason), and then is also connected to the heat relay terminal through the resistor. There's nothing in the installation manual about doing this kind of hookup.

I did some googling to see if there were other examples of EMM-3 wiring jobs out there and they all look pretty much like the wiring diagram, one wire per terminal.

Any thoughts on what's going on here? Should it be safe to just put an additional wire from the thermostat lines into the C block for each along with the white wire jumpers that are already there?

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Remove the jumpers, they're superfluous

It appears from the installation manual for your zone controller that the C terminals for the thermostats are connected internally to the C wire coming into the board from its 24VAC power source. This means that the white jumper wires between the zone C terminals and the blue C wire from the furnace can be removed altogether and your C wires hooked up in their place.

As to the 1K resistor? Someone thought this unit power-stole, I guess, and put that in there to provide a bypass load to prevent erratic operation. I'd remove it and keep an eye on the system -- if it doesn't malfunction after a week or two without it, I'd say whoever put it in was a member of a power-stealing cargo cult.

  • Ok, this is starting to make a lot more sense. A couple of other bits of info: – Dave Dec 13 '16 at 3:02
  • 1) The thermostats currently on the system are Honeywell T8400C, which I believe are power-stealing. So maybe that's why the resistor is there. 2) The blue wire is from the HVAC control board. It's labeled C and 24V Humidifier in the wiring diagram. I'm assuming the installer thought they needed to wire it to the panel to provide 24V to the C wire terminals on thermostat blocks. If I'm right, I'm assuming it's ok to just remove all the extra stuff (white jumpers, resistor, blue wire from the HVAC) since I'm switching the thermostats with ones powered through C wire. Sound correct? – Dave Dec 13 '16 at 3:08
  • @Dave -- the thermostats being power-stealing has nothing to do with the connections to the zone board -- the zone board has its own 24VAC R/C pair (going to the transformer terminal block on the zone board) so it doesn't need to power-steal at all. And yes, you can remove all the extra stuff. Since the zone board has its own 24VAC supply, it provides R and C to the thermostats independent of its connection to the furnace. – ThreePhaseEel Dec 13 '16 at 3:11
  • Thanks so much for the info. I'll try setting this all up in the next few days and report back. – Dave Dec 13 '16 at 13:36
  • Looks like everything is working after removing the jumpers/resistor and setting up the new thermostats. Thanks again. – Dave Dec 18 '16 at 19:52

protected by Community Jul 12 '18 at 1:49

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