Introduction

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.

Hardware

• 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
    https://github.com/PaulStoffregen/OneWire.git
    https://github.com/milesburton/Arduino-Temperature-Control-Library

Software

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!