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Mystery Temperature Spike

In late 2022 I noticed a weird behavior in one of the temperature sensors in a HomeAssistant home automation system I run where I live. The sensor is a AHT20 temperature and humidity sensor (this Adafruit one in particular) hooked up to an ESP32 microcontroller board running Tasmota - a pretty standard setup a little home automation board.

The strange behavior I noticed was an occasional large spike in the temperature readings. When the room temperature was around 65°F (this was in a drafty old room in the US in December), it would suddenly increase up to about 90°F, stay there for a little while, and then return to the same regular temperatures:

Screenshot from 2023-06-18T21-46-26.png

Very strange! It wasn't a huge problem because I don't have any systems that rely on that temperature measurement, but it was a weird thing to see on my temperature plots and also made it a little harder to see the regular daily temperature variations I'd expect to see for that room.

Given that I mostly wired up this sensor board by self, it was easy to assume I had a shoddy connection somewhere or an unreliable power supply. There was a corresponding jump (down) in humidity at the same time as these temperature spikes, so some kind shared failure mode (like a loose wire or a faulty voltage source) seemed likely.

Screenshot from 2023-06-18T21-46-04.png

I poked around a little bit, but never found a satisfying explanation of this temperature spike, despite it happening a handful of times per week. At the end of the day, it was more of an inconvenience than a problem so I mostly just left it alone.

Finally, on December 18th, I figured out what the issue was. While working from home that day, I happened to look over at the sensor and saw it as bathed in the low December light!


Immediately the temperature spike issue became clear:

Easy fix: I moved the sensor so it is now behind the TV and permanently shaded and the temperature spikes were going for good!