PID library, controller always ON

Since the PID is controlling a fan to cool a large water volumel, I set the windowsize to 30000

The setpoint is 25

Current PID output value:
PID: 6323.52

Current temperature
tank_: 25.5

When the PID approaches the setpoint it turns OFF the relay for something like 1s - 2s, the time it takes to call the pid() function again. Even when the PID get to the setpoint 25 it keeps this behaviour...