Want to replace multiple 2-wire dumb heat-only thermostats with "smart" stats

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I’m looking for ideas on how to replace a bunch of two-wire dumb, heat-only thermostats with “smart” thermostats.

  • The existing wiring is two-wire, and it would be extremely challenging to run additional wires to most of the locations.
  • The current stats are KEL-T201 (powered by a small battery) to display the temperature or setpoint, and only have W and RH connections.
  • The stats control a whole-house radiant-floor heating system. I don’t understand all the plumbing involved (see picture), but the current stats are used to control valves connected to a manifold (the white cylinders in the upper right corner of the picture) when that area of the house needs to be warmer.

I would like to replace all these stats with something “smart” so the temperatures can be adjusted up/down on a schedule, and ideally, integrated in to openHAB.

I’ve thought about using something like the Fast-Stat Common Maker but I’m not sure what smart thermostats would be cost-effective to use with it? Also, it’s unclear if I’d need a separate Common Maker for each stat (which would add significant cost on top of each smart stat), or if it could be shared with multiple thermostats?

Open to any suggestions! Thanks in advance.

This looks perfect for My Central Heating Solution using Thermal Actuators

You’ve got the Thermal Actuators on your manifold already, so you’re already appropriately zoned.

Then you’ve got choices for temperature sensors to feed into it, either basic like my ESP8266/DHT22, or something different.

Do you want to be able to override the temperature from a unit on the wall, like you’ve got at the moment, or are you happy without per-zone on-wall adjustment, and either leave it to the automation, or utilise other ways (like I can use the openHAB app, or Google Assistant or Alexa)

If you’ve got any further questions on my setup that’s not in the linked thread, please feel free to ask!


Thanks for the reply Garry!

Good to know that there is at least some possibility of automation here.

There is definitely a desire to keep some sort of “local temperature control”, although I realize that complicates matters (especially with only two wires to each stat)… and that’s why the heating has remained un-automated for so long! Do you have suggestions on how to go about doing local control?

I looked further at the Fast-Stat Common Maker product and it does seem like it would require a separate “common maker” at each stat, since it utilizes something they call a “sender”. The installation instructions say:

The Sender is the smaller component and has purple, white, and black wires. It is installed inside the wall behind the thermostat.

I also saw a reply in another thread that referenced Velbus “glass panel” controllers but not sure if there’s any way to use them in a 2-wire situation.

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Sorry, no

These require a 4 core cable network.

Data High
Data Low

I think the 2 wire (2 core) Thermostats are just dumb contact closures, from a mechanical thermostat, or like yours, a locally powered device.

(Unless it is a digital 2 core system, which was designed by boiler manufacturers as an upgrade option, that makes use of the existing 2 cores)

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That said…

You could…

Simply remove the current thermostats in each room and attach a 10K3 thermistor in its place.

Then put all your logic in your boiler room.

For which I can guide you to a Velbus solution, but would need 1K ohm or 2K ohm thermistors.

Or you could research the EasyIO controller or Robot Electronics 4 channel DScript board boards which have 4 inputs for 10k3 thermistors and 4 relay outputs for valve control. (As well as other IOs)

There is an example heating controller DScript on their website.

You could then simply get openHAB2 to change the set points via UDP

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You could utilise your 2 wires as power - positive and ground, and then you could put something like an ESP8266 (such as a Wemos D1 Mini), a DHT22 temp/humidity sensor, a small OLED, and a rotary encoder into a custom-printed 3D enclosure.

*The Wemos will want 5v, but you may need to pump more voltage in to account for voltage drop over the wire runs, and put a suitable step-down converter at each end point to provide your 5v


Thanks for the idea! A home-grown thermostat is definitely intriguing… just not sure I have the time to develop it, but it’s an interesting possibility to consider for sure. The KONOz thermostats seem like they could work but I bet a custom solution would be cheaper (ignoring the cost of developing it first, of course.)

You could…

Use a Velbus VMB4AN with a 2K ohm thermistor at each location.

These units have the logic onboard for thermostats, with 4 set point options for each input.

You’d only need a 4 way relay to trigger the valve actuators.

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