Is a jumper wire necessary between TH/TR and the left-hand END SWITCH?

If so, what does it do?

As requested, here is my best approximation of a wiring diagram:

As it is now (note the missing jumper wire in the zone valve for T2):

Current system wiring diagram

As it was before I fussed with stuff:

Original system wiring diagram


I have a 3-zone heating system in my house, with a boiler at the heart. I also have a 2-zone air conditioning system. That's not immediately pertinent, but will become important later (I think).

I'm trying to convert an available wire in one of my Google Nest Thermostat 2020's to work as a C-Wire.

FIGURE 1 Thermostat  T2

And it is controlled by a Honeywell zone valve with wiring that looks like this:

FIGURE 2 Thermostat  T2 valve

I have already converted a spare wire for another zone valve to power a second Google Nest Thermostat 2020. The wiring at the thermostat was identical to FIGURE 1, with a spare green wire. The zone valve controlling that zone looks like this (FIGURE 3), after my rewiring, with annotations based on the subsequent diagram (FIGURE 4):

FIGURE 3 Thermostat  T3 valve


Honeywell zone valve wiring diagram

I don't want to brag, but this has been working out perfectly for the past couple of months. That could be because of my ignorance, and perhaps I'm actively deteriorating my wiring or parts in this zone and it just hasn't caused a defect in the system yet, but ignorance is bliss anyway (just kidding... that's why I'm trying to educate myself; my ignorance is dangerous!).

So, I thought, "I can repeat this process for the other thermostat!" But, of course, as is clear in FIGURE 2, there is no green wire coming from the relay or going to the thermostat at this valve. I believe, given its placement on TR, the black wire from the relay would serve as a C-Wire in this case (FIGURE 2).

Is that the wiring in this valve (FIGURE 2) looks to be at odds with the wiring in FIGURE 3. In FIGURE 2 I've got a white wire from the thermostat going to TH/TR, which also has a red wire coming from the relay. And I've got a red wire from the thermostat going to TH. The yellow wire from the relay is going to the left-hand END SWITCH terminal, while the blue jumper wire is going to the right-hand END SWITCH.

I do not understand the purpose of the jumper wires. Is one way "right" (not going to break stuff) and the other "wrong" (going to break stuff)? Or are they both irrelevant and ignored by the valve?

Now, behind all of this, there is a third thermostat controlling the final zone of heating in the house, with red (Rh)/white (W1) wires coming from the boiler system and red (Rc), yellow (Y - cooling), green (G - fan), blue (currently in C, but see below), orange (disconnected), black (disconnected), and white (disconnected) wires coming from the air handler for the air conditioning system (call back from the beginning!!). That thermostat has its blue wire inserted into its C port, but it reported that it was getting no power from the C-Wire. Turns out I had blown the 3-amp fuse in the air handler when installing the thermostat (I crossed some wires). Anyway, the zone valve for that thermostat's heat looks like this, which is where the relay coming in to FIGURE 2 comes from (note that the red/white/jumper configuration is the same as in FIGURE 3, divergent from FIGURE 2):

FIGURE 5 Thermostat  T1 valve

  • Can you try drawing a wiring diagram of all this...? Commented Jan 13, 2022 at 2:32
  • I'll give it a shot! I did rewire Figure 2 last night and got that thermostat to work. Ended up removing the jumper wire. Still not sure what their purpose is, but they're not on the Honeywell zone valve wiring guide! Commented Jan 13, 2022 at 11:17
  • @ThreePhaseEel, I added the diagram (actually, 2, one showing the current state, one showing the initial state). Some of the connections are hard to see, but show up if you click on the images to enlarge them and zoom in. Commented Jan 14, 2022 at 17:27
  • 2
    Please change your "Update" into an answer, because replacing a blown fuse may be an answer that helps others. To bring light to the "remaining question", please ask it as a new one. We do have rules that there should only be one question per post exactly for this reason.
    – FreeMan
    Commented May 5, 2022 at 13:10
  • 1
    Yes, please make your update an answer -- I'll swing it a +1 if you do, even Commented May 6, 2022 at 1:36

2 Answers 2


In your case, the RED WIRE should go to W, and the WHITE should go to R (on the thermostat). The spare BLACK WIRE should go to C on the thermostat, and, on the other side, connect it to TR on the zone valve.

I know it's weird, but it's common for the red and white wires to be swapped.

Leave the jumper there, don't change anything else.


In the spirit of trial and error, I removed the jumper wire today. The valve stopped relaying the call for heat to the boiler. So I suppose the jumper completes a circuit in the valve to transfer the message to the transformer of the boiler that the thermostat is calling for heat!

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