Thomas shares makes


Central Heating sensor manipulation


We used to have a gas-powered central heating system that had no external controls other than radiator valves. It had a sun-exposed exterior temperature sensor on the west side of the house that caused it to refuse to heat in the evening.

I wanted to fix the issue and control it via Home Assistant, and since only the newer models had bus control, I resorted to manipulate the outside sensor readings and designed a man-in-the-middle device.


The central heater was meant to be always on, even though no floor heating was installed. It had a built-in clock which it used to lower the water temperature a bit during the night.

The system did not have thermostatic valves on the radiators, but with minor tweaking to its PID values produced comfortable temperatures.

The house did not have a north-side to install the exterior temperature sensor onto, so it was mounted on the west side. My first idea was to replace that temperature dependent resistor with a digitally controlled resistor, and feed it the temperature value from a wireless thermometer placed in the backyard shade.


photo of the print
  • Arduino Uno based interface board
  • perforated pc-board based point-to-point wired prototype
  • MAX31865 temperature dependent resistor interface
  • MCPxxx SPI digital potentiometers, optically isolated and powered by CR2032 battery
  • Dallas one-wire temperature sensors on the water-in and -out feeds.

The Arduino runs a simple mainloop program I compiled and uploaded using platformio.

It employed a simple ASCII based request/response stream protocol to trigger I/O actions.

  • Set the digital potentiometer values
  • return the resistance reading of the outside thermometer.
lib_deps =
    Adafruit MAX31865 library


I created a python daemon that I ran in a docker container using systemd. I used Paho-MQTT and tried to leverage asyncio.

I also used python3-serial to talk to the Arduino, and the UliEngineering module to convert the resistance value to temperature.

I created a helper tool to perform calibration of the digital potentiometers when the device was wired in a 'loopback' mode. It obtained a MAX31865 resistance reading for each of the possible potentiometer values. The values were dumped in a text file and used to translate a resistance value to the best matching potentiometer setting.

Home assistant integration

The python daemon exposed the temperatures on MQTT topics, and listened on a MQTT topic for the outside temperature it should emulate towards the heater.

MQTT template sensors were used to bring this project into my Home Assistant instance!

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