A plumber is inspecting a problematic trap seal in a residential bathroom. The toilet is connected to the same branch drain as the lavatory, and both fixtures are properly vented. However, the lavatory trap seal is frequently lost. Upon inspection, the plumber notes the lavatory trap arm is excessively long. Which plumbing principle is primarily being violated, leading to trap seal loss?

Question: A plumber is inspecting a problematic trap seal in a residential bathroom. The toilet is connected to the same branch drain as the lavatory, and both fixtures are properly vented. However, the lavatory trap seal is frequently lost. Upon inspection, the plumber notes the lavatory trap arm is excessively long. Which plumbing principle is primarily being violated, leading to trap seal loss?

Answer options:

• Hydrostatic pressure principle
• Atmospheric pressure balance ✅ Siphonage due to excessive trap arm length
• Capillary action

Correct answer: Siphonage due to excessive trap arm length

Explanation: An excessively long trap arm can lead to siphonage, where the water flowing through the drain creates enough negative pressure to pull the water out of the trap seal. This is a common cause of trap seal loss, even with proper venting at the stack. While atmospheric pressure is involved, the specific mechanism here is siphonage facilitated by the long trap arm.