If you don't plan on having the door triggerable by players at all, just don't include a trigger volume. If you are, however, in the trigger volume's keyvalues, there's a field called Start Enabled (or Start Disabled, I'm not sure and dont have hammer rn). Just use that to disable the trigger volume by default, and when the timer's setup time reaches 0, have an output fire OnSetupTimeFinished, targeting the trigger volume, with an Enable command
Also make sure the Touch Opens flag int he flags tab is unchecked