Guest 1/2 x 3/8 Reducing Union

The John Guest 1/2" x 3/8" Reducing Union from Reliance Worldwide Corporation joins a 1/2" tube to a 3/8" tube inline, transitioning cleanly between two line sizes without tools or threads. It is well suited to larger filtration and water conditioning installations that mix tubing diameters, providing a tidy, leak-free bridge between a higher-flow supply line and smaller distribution tubing.

Whole-house and higher-flow point-of-use systems often run 1/2" supply tubing that must step down to 3/8" for certain components, valves, or downstream runs. This reducing union handles that transition in a single fitting, using a push-fit collet on each end to grip and seal the respective tube sizes. Combining the size change into one body avoids stacked adapters and keeps the junction compact.

Bridging Two Sizes

• Connection: Tube x Tube, 1/2" to 3/8"
• Straight reducing union for inline size changes
• Corrosion-resistant polypropylene construction
• Push-in assembly grips and seals each tube automatically
• Demountable for easy servicing and rework
• Reusable across installs and repairs

To install, cut both tubes square and free of burrs, then push the 1/2" tube into the larger port and the 3/8" tube into the smaller one, seating each fully past the collet. A gentle tug on each side confirms the grip, and the internal O-rings create watertight seals at both ends automatically. Because polypropylene resists corrosion and the chemicals in treated water, the union holds up to continuous service without scaling or rusting, even in the larger flows typical of these systems. When servicing is needed, depress the appropriate collet and withdraw the tube undamaged, then reconnect once the work is complete. For reliable, reusable size transitions in residential and light-commercial water treatment plumbing, this John Guest reducing union is a dependable solution that keeps mixed-diameter runs neat and secure.

Within the John Guest range, this reducing union pairs naturally with the larger 1/2" fittings as well as the 3/8" components that share the same collet system, so a layout that mixes sizes stays fully interchangeable. Running a larger supply line where flow matters and reducing only where a component requires it is a smart design approach, and this union makes that transition clean, quick, and entirely tool-free.