Which style of water closet typically uses the least water in its trap?

• A. Flush Down Water Closet
• B. Standard Gravity
• C. Dual-Flush
• D. Commode

Answer: (A)

Water in the trap serves to seal out sewer gases, so different toilet designs manage that trap water differently. The flush-down (washdown) style directs the flush straight downward and uses a shallower trap, which means less water sits in the trap to maintain the seal. In contrast, standard gravity toilets rely on a deeper trap and siphon action, which requires more water in the trap to create the siphon and move waste. Dual-flush systems aim to reduce total flush water, but the trap seal itself isn’t necessarily minimized compared to the washdown type. A commode is a general term and doesn’t specify a trap-water design.