What typical feature helps in maintaining water flow in systems without a sewer connection?

A trap primer is specifically designed to maintain water flow in plumbing systems, particularly in situations where there is no sewer connection. Its primary function is to provide a small amount of water to the trap of a plumbing fixture, ensuring that there is always a water seal present to prevent sewer gases from entering the living space. This is crucial in areas where the plumbing may not be actively used, which can lead to traps drying out and losing their seal. By ensuring a constant water supply to the trap, trap primers help maintain proper function and hygiene in the plumbing system.

Other devices listed, like backflow preventers, deduct meters, and pressure regulators, serve different functions in plumbing systems. A backflow preventer is designed to prevent contaminated water from flowing back into clean water supplies. A deduct meter is used for billing purposes to measure the amount of water not discharged to the sewer system, and a pressure regulator is meant to control water pressure within the system. These devices do not directly address the issue of maintaining water flow to prevent trap drying and gas escape, which is the primary concern in systems lacking sewer connections.