The pneumatic bleed air after it has been cycled through the air conditioning packs and distributed into the cold air plenum is too cold for distribution to the cabin zones by it self. During normal operation, all packs are programmed to provide air of a temperature required by the zone calling for the coldest temperature. To prevent other zones from being too cold, pack 2 and 3 also supply temperature regulated hot air which comes from the output of the primary heat exchanger and a tap off prior to the heat exchangers, to the hot air manifold. Hot air is then ducted to the hot air zone trim valves which modulate to maintain the selected temperature for each zone by mixing the air leaving the cold air plenum with the hot zone trim air.
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