Upgrading an old thermostat to a smart system featuring an HTTP server on a Raspberry Pi and a mesh network of ESP32 thermometers.

I have a pretty old heating system with an old Honeywell thermostat that is mechanical rather than electric. It works well but can only track the temperature in the main room. It’s pretty hard to control the temperature in the bedroom on the second floor when the thermostat is located on the first floor in a cold room. It will never know when to stop the heating system.

How does the Honeywell thermostat work?

The old Honeywell thermostat is genius and dumb at the same time. It has a glass tube with a silver liquid inside. Mercury is typically found in older mechanical thermostats like the Honeywell Round T87 and rectangular T8090. It has a flat spiral spring, and the tube is on top. The spring bends with cold, and the mercury in the tube falls and closes the electrical circle. When it heats, it unbends, and mercury goes to the other side of the tube and opens the circuit. It’s straightforward, reliable, and fault-proof because it’s physics. However, this system is outdated, and modern thermostats use electronics.