Which one of the following traps are legal?

Legal traps in plumbing must adhere to specific codes and standards designed to ensure they operate effectively in preventing the escape of sewer gases while allowing for the proper drainage of waste. Among the options provided, anti-siphon traps are considered legal.

Anti-siphon traps are designed to prevent siphoning, which can occur when there is a negative pressure in the drainage system, causing the trap seal to be drawn down and potentially allowing sewer gases to escape. These traps maintain a continuous water seal, safeguarding the interior environment from hazardous gases. The design effectively addresses issues of venting and pressure balance within the plumbing system, making it compliant with plumbing codes.

The other types mentioned may present regulatory challenges. For example, traps with moving parts may not be reliable over time, leading to potential failures that could compromise their effectiveness. Building traps are generally considered outdated and may not comply with modern plumbing standards, as they can create issues with venting and may not be as effective in maintaining the necessary trap seal. Therefore, the recognition of anti-siphon traps as a legal option reflects the need for plumbing solutions that ensure safety and efficiency in the drainage system.