[AWK]

It depends on the moment of start warming. Reaction needs a few minutes at RT. Warmin before addition of acetic anhydride will diminish yield. Warming after adddition... decomposes minute amount of phenol ester.

[Discosucks]

We didn't do any warming until the recrystallization .

It went ,

mixed 4 aminophenol with purified water  .

mixed well .

Added acetic anhydride gradually while stiring with magnetic bar (noticed a increase in the temp here )

The grey 4 aminophenol then changed to a nearly pure white solid .

also the amount of solid increased ( from 10g to 12g but this was still wet ) so something has hopefully happened

Just dont understand the need for the reflux 

[AWK]

This is just RSC procedure. But you asked about warming. This done (short warming) at the end of your reaction probably increase yield slightly.

[Discosucks]

Ok great .

Thanks for your help AWK