Short answer: Generally, you should not drink boiled water before taking the Pachkan. Pachkan is the vow you take first, after which you follow the rules for the fast or upvas, including when and what you may drink. Once Pachkan is recited, boiled water may be consumed only within the time and rule limits of your tradition (Shwetambar or Digambar), not before you have taken the Pachkan. If your practice is Navkarsi or Sajala Upvas, the boiled-water rule applies after Pachkan and within the prescribed hours.

Key point:

• Take Pachkan first (before sunrise or as guided by your local sangh) and then follow the water/food rules for that upvas.
• Differences may exist between Shwetambar and Digambar practices, so follow your community’s exact wording and timings.

You can read more here and here