These are holders for the DS18B20 temperature sensor designed to be attached to a water heater to monitor the water temperature. The first holder can be fitted with four 6x3mm neodymium magnets such that it attaches to the stainless steel tank right next to water heater's thermostat. With magnets one avoids messy heat resistant glues or tapes. Apply some thermal paste for the most accurate measurements.

This setup is typically used to make sure the water is heated to safe levels (+70C) frequently enough to kill off legionella bacteria. This is useful when the water is turned on only during the hours of the day with the cheapest electricity prices.

The design idea of the first holder is based on https://www.thingiverse.com/thing:5457784 by 3dprint4fun. I added curvature to fit the diameter of a 300l tank (50cm diameter), holes for magnets, moved the sensor closer to the tank (no space in-between), and lengthened it a little.

The second holder can be used to clamp a DS18B20 on the water heater's 15mm outlet pipe. This can be used to ensure that the water flowing out is mixed with the right amount of cold water to avoid accidental burns (55C or so).

I connected both my DS18B20's to a Fibaro Smart Implant, which sends the data to my Homey Pro via Z-Wave.

Print with something else than PLA due to high temperatures. I used PETG, which works up to 85C. My water heater is set to 75C, which is usually default from the manufacturer.