Hello,
come in an Astra H 1.6 petrol, have changed thermostat housing, engine radiator, coolant,
if you start the engine and let it at idle the engine reaches 99C deg
but radiator is at room temperature.

You accelerate a little and the radiator start to become hot.
The problem is the radiator has these pipes:

1 I------------
2 -------------
3 ------------O
4 ------------RT
5 -------------
where:

I is water intake
O is water output
RT is radiator Temp Sensor
--- is the core of the radiator

part 1 from 3 is HOT
part 4 to 5 is COLD

the radiator Temp Sensor doesn't get hot even with engine at 105C deg
and radiator fan DOES NOT COME IN.

I stoped engine at 113C deg.

At what temperature does an 1.6 petrol must start the fan with AC off?

There is water circulation in every pipes.

I read water temperature in the hidden menu of display, it's the value that ECU read from
the thermostat temperature sensor.