High flight detent.
The weather here to-day was BEAUTIFUL (bound to get a bite).I actually live north-west of Brizzy At an altitude of nearly 600 feet(above the humidity boundry)and on a clear day can see Caloundra. Anyway with regards water separators (fitted to Boeings Fokkers )the bypass isn't external,that is it's built-in.When you look down the front of one you will see a pointed area.reach in and push it (less than 4 psi to move it from memory,different pressures for different models but they all do the same)otherwise if the seperator 'froze'up you would have no air at all.The temp probe for the control of 'anti-ice is generaly near the drain(incidently feed back onto the heat exchangers)so will begin to modulate prior to freezing"in theory"
A common prob experienced by crew is "thumping"in regards to airflow from packs.A lot of the time a water sep change cures it but not always !!!
Cheers