What device periodically adds water to the trap in a drainage system to prevent odor and sewer gas from entering a building?

A trap primer is the device designed specifically to periodically add water to the trap in a drainage system. This is crucial in maintaining the water seal that prevents unpleasant odors and harmful sewer gases from entering a building. A trap, whether a P-trap or another type, relies on a certain amount of water to create this seal. Over time, water in these traps can evaporate, especially in infrequently used fixtures. The trap primer ensures that water is automatically replenished, thereby maintaining the functionality of the trap and ensuring sanitary conditions within the structure.

The other options do not serve this purpose. The P-trap is a type of plumbing fitting that retains a water seal but does not add water itself. A drain cleanout provides access to the drainage system for maintenance purposes and does not affect the water level in traps. The vent pipe allows air into the drainage system to prevent vacuum conditions but is unrelated to water replenishment in traps. Thus, the trap primer is essential for the effective operation of the drainage system by ensuring that traps remain effective in blocking sewer gases.