Suntouch 500850 thermostat for wire floor warming needs switch controlled by Smartthings. Devices? momentary contact?

My first post so I’m not sure about posting a link or uploading images.

I frequently install floor warming wire under tile floors. Typically the heating wire leads are brought to an electrical box. Depending on the thermostat the the load wires are connected to terminals or wires on the thermostat. At least one temp sensor is under the tile (I always install a spare) The sensor leads are connected to the marked terminals. The power feed may be 120 VAC or 220 VAC depending on the heat wires. This one will do both. In the past I always placed a GFI in the feed but lately the GFCI is integrated into the thernost like this one.

There are typically several terminals for sensors and/or controlling relays for larger systems. This install does not need an extra relay. What is different with this thermostat is that it has one terminal labeled Home Automation and one labeled COM. According to the literature, including the installation manual, if the home automation terminal and the COM terminal have a short or 24 VAC applied it will switch the thermostat between the ‘Away’ temperature and regular operation.

It is a SunTouch thermostat, specifically it is a SunSet Comand Touch thermostat #500850. It is advertised that his thermostat has the ability to be controlled by home automation. They also have a programanal thermostat that has WiFi. The price for the WiFi thermostat ranges around $225 to $260 vs this one cost me $145.

I knew that I might need additional room to install whatever device would be required. To that end I installed another electric box. That box has 120 VAC and 2 pair of 18ga, wire from that box to the thermostat box. My preference is to install a Z wave or Zigbee device that will allow me to start the floor heat remotely several hours before guests are expected.

The installation instructions are online I will quote the section on home automation:
Home Automation System Rough-in Wiring

  • A short or 24 VAC applied between the Away and Com terminal will switch the thermostat between the ‘Away’ temperature and regular operation.

  • Pull 18 AWG to 24 AWG 2-conductor shielded wire from the home automation control to this control location.

  • Strip the wire ends to 1/8" long.

  • Sensor, relay and home automation connections are made to the terminal block by inserting the wires into the square openings and tightening the screws on the side.

  • Connect the sensor wires to the SENSOR terminals on the thermostat. These connections are not polarity sensitive.

  • For a SunStat Relay C3, connect 2 wires from the relay to the Com and Relay terminals on the thermostat. Ensure the com wire at the relay is the same conductor connected to the Com terminal on the thermostat.

  • Connect the Away and Com terminals to the appropriate conductors from a home automation system.

  • Refer to the instructions for the home automation control before making these connections.

My request is twofold, one is for suggestions on what device would work in this application. The other is to clarify my confusion about the shorting of the 2 terminals. Is it a momentary contact, or is it left on for one condition and off for the other?

I would be most happy with a battery powered device or I can tap into the 120 VAC in the box. This bathroom is as far from the hub as it can be without going outside. There are several Z-wave plus devices closeby. I will probably place a Z-wave repeater in the area. I prewired the house for cat 6 to avoid using WiFi, so while I could put an access point in the next room over, I would prefer to not use WiFi

What suggestions come to mind? I have seen other posts where people are looking for a thermostat for electric floor heating. Maybe this could be that solution. Under the right circumstances it might be able to control base heating. Most of these types of thermostats can use remote sensors like the ones under the tile, or use air temperature by changing the settings…

Thank you

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I’ve been looking to do the same. I called and they indicated the only thing this can control is the active/away state. I looked at contact sensors, but have not made any progress on this.

I would hook a ST controlled relay and run it’s NO contact to AWAY and COM contact to COM on the Suntouch controller. It sounds like it is expecting that short to remain as long as you want the floor to remain in “away” mode. Are you looking for a specific relay device? There are a lot of different options for you to choose from depending on if you want a Z-wave, zigbee or LAN device, the cost you want to outlay and how much work you want to put into it. You don’t say whether you are in the US or UK, so you may not be able to use these exact items but you should be able to find the EU freq equivalent. This would be by far the simplest.

This realy allows for control of a circuit with a different voltage than the one applied to the relay to control it.

I have the device you recommended in hand. I don’t have a NO or COM contact. I have Neutral, live lead, input and output for electrical supply, an input2 for switch/push button or sensor, and input1 for just switch/push button, and a terminal for the thermal sensor. I’m not sure where to go from here on wiring it up. I assume neutral gets tied in, then load (b) goes to Live Lead, then perhaps com goes to the input1 for switch?

I’ve included screenshots from both manuals with wiring details.
Also, US based to clarify.


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You want to use the setup for different load power supply.

The connections I’ve highlighted in Red should connect to the away and com terminals. Have you confirmed if it has to remain latched or is momentary?

It has to be latched. I have everything wired as shown above for LNQ1 and Q2. I’m not sure what would connect to l1 and l2 in this configuration.

I’m currently tripping the GFCI with this configuration.

Thanks for the help thus far!

Tripping the GFCI for what? I1 and I2 are inputs for the relay to be connected to switches (it’s in the manual for the relay).

Sorry I wasn’t more clear. The GFCI is built into the suntouch control. I have line and neutral connected, q input goes to away and q output goes to Com(A).

And you turn power off completely before you hook up the relay? Then turn power back on and the GFCI is tripping on the controller? What happens if you just disconnect the relay and short the two wires together (that’s what the relay is doing). That would be able to ferret out whether the problem is the relay or the controller.

And what about the load side? Is that connected too or did you disconnect it from the heating element?

I just tested with shorting the com and away and the grci did not trip. It seems to trip when the device is triggered to turn on or off from the app. I’m currently using the custom device handler by erocm123

Wait…you said that it trips when the relay is connected, not triggered. That’s a different situation. What are you powering the relay with? You are sure that you have the Live in L and the neutral in N. Do you have them swapped?

I have them wired to a wirenut bundle to the live and neutral from the box and the live and neutral from the suntouch unit.

That might be your problem. I bet your GFI would trip even if you activate the relay while Q1 and Q2 are disconnected from the relay.
You can also try swapping the Q inputs.

Yes, that is what occurs. What would be the corrective action?

The load being the suntouch, should be connected to the relay? Did I just completely miss that?

No, look at the diagram that you posted. It should be wired like that.

Thanks for the help. I have not ordered the device. The thermostat and floor warming are operational. How do I get the device to latch? Are there settings in the device setup or does that have to come from the device handler? Are there products that default to latched? I am in the U.S.

Sorry, I feel like I’m missing something quite obvious. I’ve looked at the wiring schematic for the relay, and believe it is wired correctly. I have Neutral and line both coming in from the wall to a wirenut bundle. The attached line and neutral on the back of the suntouch are included in those bundles. I’ve added one line and one load wire between the bundle and the respective contacts on the relay. I then have q1 and q2 connected to home and away. What am I missing?

Home and away? Should be common and away. I don’t see a home anywhere.

But which do you have going to Q :arrow_down:and which do you have going to Q :arrow_up:? You might try swapping those.

Sorry, the 2 Q’s are going to com (a) and away. I’ve tried.swapping them, no change.