When should the main drain valve be opened during the flow test?

• A. The beginning of the flow test
• B. Midpoint of the flow test
• C. End of the flow test
• D. Never opened

Answer: (A)

The test relies on an unobstructed, fully purged line so you can see the true discharge and pressure when the system is flowing. Opening the main drain at the very start lets air trapped in the piping escape and establishes a continuous flow path as the pump comes up to test pressure. This purge ensures the readings reflect actual system capacity rather than the effects of air pockets or initial pressure transients.

If you wait to open it or do it later, air remains in the line, which can skew flow and pressure readings and make the test unreliable. Leaving the main drain closed would prevent the line from being purged altogether, and never opening it would defeat the purpose of the flow test. Opening at the beginning gives you a clean, accurate baseline for the test.