Which fitting is acceptable for connecting two horizontal drain lines?

The combination wye and 1/8 bend is the correct fitting for connecting two horizontal drain lines because it allows for a smooth and gradual change in direction of the wastewater flow. This particular fitting is designed to direct flow without creating significant turbulence or backlash, which can occur with sharper bends. By providing a less abrupt transition, it minimizes the likelihood of blockages within the drainage system.

Using a wye allows for better flow characteristics, especially in horizontal applications, as it allows liquid to flow efficiently into the main drain line while maintaining the direction of the water without excess resistance. The inclusion of the 1/8 bend further aids in directing the flow at a slight angle, which is especially useful for maintaining a consistent slope required in plumbing installations.

In contrast, other fittings may not provide the same level of efficiency or may not be suitable for the application. A standard elbow fitting, for example, causes a sharper angle and potential turbulence that could impact drainage performance. A sanitary tee is typically used for vertical connections where it allows for venting and not ideally suited for connecting two horizontal lines. An offset fitting, while useful in achieving horizontal adjustments, does not provide the same direct connection and flow advantages as a combination wye and 1/8 bend.