It is quite normal for mechanical timers to heat up slightly. There is usually a small motor or coil inside that is permanently energized to operate the time mechanism. This generates some heat, especially if the clock is active for many hours.
The heating becomes critical if:
the timer becomes significantly hot (not just warm to the touch),
there is a burning smell or discoloration on the housing,
a loose contact can be felt or the clock crackles when moved.
In these cases, you should disconnect the timer from the power supply immediately and replace it if necessary.
Common causes are
Overload: The connected appliances exceed the maximum power limit of the timer (usually 3500 W / 16 A).
Continuous operation of heavy consumers such as fan heaters, kettles or heat lamps.
Poor contacts on old or inferior models.
Moisture or dust in the housing, which can lead to insulation problems.
Look at the rating plate or in the timer's instructions. It usually says there:
Max. 16 A / 3500 W at 230 V
Make sure that the sum of the connected devices does not exceed these values. You should be particularly careful with appliances with heating resistors - they often draw a lot of current.
Check the appliance output and compare it with the technical data of the timer.