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Wiring a modern Honeywell RTH9585 thermostat to a Honeywell M644 valve actuator

PEvans Member Posts: 116
edited December 2022 in Thermostats and Controls
This is for a steam heating system with two zones controlled by Honeywell M644 valve actuator.

The valve actuator is a reversible motor type with two circuits, one to open the valve and one to close the valve.

Edit: this is sometimes referred to as a "power to open and power to close" zone valve. Other times referred as "Series 20".

Is it possible that when the RTH9585 thermostat is configured in the setup for steam heat that one of the terminals is energized when the heat is off?

This is made a little more complicated in my case because the thermostat I'm removing is a Chronotherm with a heat anticipator relay that controls the "close" circuit. It looks like the "close" circuit wire is connected to an unmarked terminal on the thermostat.


  • Jamie Hall
    Jamie Hall Member Posts: 23,668
    Looking over the manual, I don't see any terminals which do that. You can, however, use a SPDT relay. Energize the coil with a call for heat. NO contacts to power open, NC contacts to power close. You may als need a transformer.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • PEvans
    PEvans Member Posts: 116
    Honeywell customer support (855 733 5465 and very helpful btw) says for this thermostat when it is configured in setup for steam heat the Y terminal is energized when the thermostat is not calling for heat. I think this is like an old style mercury thermostat like a T87F. No, this is not in the RTH9585 documentation.

    So for my system the "close" circuit of the valve actuator should be completed through the Y terminal on the thermostat base.

    In my case this does not work. The thermostat requires the full 20-30VAC between R and Y to work, so even if the valve closes when the wires are jumped there might not be enough voltage for the thermostat to work properly. My thermostat might also be defective.

    I'll post this in the other thread where it is more relevant, but Honeywell also points out that the trade thermostats (TH models rather than RTH models) are more configurable; that is where you have options to set CPH directly. So I'm going to try a TH8321WF if I can get one.

    So maybe I won't need to go the relay route, though thanks Jamie.
  • PEvans
    PEvans Member Posts: 116
    edited December 2022
    So there is a further complication for a modern Honeywell thermostat running "power to open and power to close" zone valves.

    The TH8321WF documentation is pretty clear that it supports operating a "power to open and power to close" or "Series 20" zone valve, with the Y terminal connected to the valve's "close" circuit.

    On the phone Honeywell explained that internally the thermostat connects R to W to energize the "open" circuit but Rc to Y to energize the "close" circuit. This makes a little sense as they are sort of hacking the thermostat's cooling system features to do this.

    The thermostat comes with a jumper connecting R and Rc and it has a single C return, so as for one transformer, and the thermostat wants the same AC source to operate the "open" and "close" circuits on a zone valve.

    My old Chronotherm setup had a second transformer and 2-wire pair for the clock which I had connected to the new thermostat at Rc and C with the jumper removed. I also suspect the wires on one of the transformers were reversed so R and Rc were not only separate sources but also opposing AC phases.

    So I abandoned the clock transformer's red wire and repurposed its white wire as a C wire for the valve transformer. I also restored the R to Rc jumper in the transformer. It all seems to work now -- the valve both opens and closes.

    The TH8321WF settings for steam in the installer menu are system type = radiant and CPH = 1.