Ejet:
Pilot sets take-off data on the thrust Rating System page of the MCDU. The bleed options are ECS(environmental control system) on/off and AI (Anti-ice) Off/ENG/ALL.
The aircraft configures the engine beed valves, pack valves and anti-ice valves automatically.
If you've asked the engines to not supply air for take off, they won't until either 500' all eng operating or thrust reduction after an engine failure.
If the APU is on it will provide bleed air for pressurisation until the engines take over at the above stated alt except if you have requested anti ice ALL. In this case the engines will provide the bleed air for anti ice and the pack valves will remain closed as per the above logic. (APU cannot provide wing anti ice as the demand is too high).
simples
