I have a Honeywell TH6320R1004 and am trying to set it up to work with a second transformer (just added a Taco ZVC403 to an existing Taco SR503). The second transformer was needed to power the zone valves which was too much for the SR503). I'm hoping someone familiar with these Honeywell thermostats can help out.

The problem I'm having is that I can't get the call for heat from the Honeywell to register with my Taco ZVC.

The Honeywell is set to "2H / 1C heat pump" mode and had been using the Y terminal to call for heat. I reconfigured the R jumpers to split R+Rc from Rh. The Rh is now powered by the new ZVC. The circuit in question is in green in the attached image. Note, the Relay's 1 & 2 terminals are always closed in heat mode. The Relay Coil is energized in cool mode disabling the circuit, and I have the thermostat set to "O/B terminal controls valve in cooling".

I get 24v across the thermostat Rh and Y terminals whether or not heat is being called for. I also get 24v across the ZVC zone terminals.

Is the Y terminal closed with Rh when heat is called for in 2h/1c mode? I've also tried the G terminal (fan) with no luck. Just looking for what Equipment terminal on the thermostat is energized with a call for heat.

Any idea what the problem is? Thanks in advance for your help.

UPDATE I also removed power to Rh and then tested ohms across Rh and Y and Rh and G when calling for heat. Nothing. I tested across Y and G and found continuity. It made me want to jump Rh and G and see if Y is then energized when there is a call for heat, so I did and IT WORKS - the circuit is energized and the ZVC received a call for heat on the Honeywell zone.

Seems like a crazy workaround, but perhaps I'm not the first to discover this. Now the questions:

  • What is the right way to do this.
    • With O/B terminal controls valve in cooling mode, how else can I get the Honeywell to call for heat? There does not seem to be any Equipment terminals on the thermostat which energize on Call for Heat.
  • When in cooling mode, will Rc (from one transformer) now connect to Rh (the 2nd transformer) via G and create a short? The circuit on Y will be open so nothing should happen on it.
  • Anything else you can think of?

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  • @ThreePhaseEel Correct. The W2 Aux E terminal is set up for that. In cool mode, the W terminal energizes the relay coil to disable both the Tekmar (call for geo heat) and this circuit in green. What I am looking for is how the Thermostat can be used in Heat mode to signal the Hydronic Pump. Is there any other terminal or method?
    – mseifert
    Oct 27, 2019 at 23:29
  • What provides cooling in this system? Also, is the geothermal system the sole source of heat? Oct 28, 2019 at 0:33
  • @ThreePhaseEel There is no cooling in the system. However, the switch to Cool Mode on the thermostat Powers the Coil in the Relay, de-activating the Hydronic Pump circuit.
    – mseifert
    Oct 28, 2019 at 1:20
  • Are you OK with ripping a bunch of stuff out and redoing it? What is the SR503 even doing in this setup? Furthermore, can you check your work on how the Tekmar 256 is wired up, as the diagram surrounding it makes absolutely nil sense? (Right now, R from the GSHP is being shorted to C via the Tekmar's boiler contacts...) Oct 29, 2019 at 1:32
  • 1
    @ThreePhaseEel - I'm sure you're right about the mis-labling on the Tekmar - it's obvious now that you point it out. I need to open it up to verify. The SR503 has a relay whose coil is energized when the Geo Unit receives a call for heat. It then activates zone 1 and turns on the 120v Geo Loop Pump. The SR503 also has a relay for stage 2 heat. It used to also fire the Hydronic Loop pump, but I've migrated that to the new ZVC. I am open to rewiring, but everyone who has looked at it wants to start over at a cost of $4-$5k.
    – mseifert
    Oct 29, 2019 at 5:27

1 Answer 1


I was able to get the system working by switching the thermostat from 2h/1c Heat Pump mode to 2h/2c Conventional Heat mode. The Honeywell's 2h/1c heat pump mode didn't support 2 transformers (and there wasn't a 2h/1c mode of any sort that did).

I had to swap the Y and the W wires on the thermostat (since it was set up that cool controls O/B which is opposite from conventional heat mode).

My biggest concern now is that the Em-Heat function is not available with this mode but is replaced by Stage 2 Heat and it looks like I can't control when Stage 2 Heat comes on.

With Em-Heat, I had Droop Control and set it to 5 degrees, but this is not an option with Stage 2. I've read that Stage 2 automatically comes on when there is a 2 degree drop, which isn't going to work. So I think I need to spring for a new thermostat.

The Honeywell THM5421R1021 EIM (using thermostat THX9421 or THM5421R1021) seems to have a heat pump mode supported with two transformers. Anyone have suggestions of a better thermostat solution?

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