APU service life can be extended with all packs on

[kuobin]

ERJ 190,in our manual it suggested that when on ground,turn both packs on instead of one will extend APU service life,Does anybody know why?

[Goldenrivett]

It depends on the APU controller.
Please see Bleed Pack Operation: The 737 APU (http://www.b737.org.uk/apu.htm)
"The number of packs to use on the ground. APU's which should run both packs have load compressors to supply bleed air. So two pack operation gives both cooler turbine wheel temperatures and a lower fuel burn."

Probably because you are off loading the "load compressor's" pressure by allowing the air bleed to escape through two packs.

By 1972, Lockheed's L1011s were using a load compressor on their APUs.
https://sites.google.com/site/thel1011projectwiki/aircraft-systems/auxiliary-power-unit-apu
"The free turbine driven generator and load compressor are unique to the L-1011 APU. A noticeable affect of this configuration is that there is no sharp increase in TGT when packs are turned ON, as there is on APUs where the air is bleed directly from the APU compressor."