Short definition
A reducer is a fitting that joins two pipes of different sizes. Available concentric (both pipes share the same centerline) or eccentric (pipes offset). Common in residential repipes where a 3/4-inch main steps down to 1/2-inch fixture branches, and in DWV connections where new pipe meets existing pipe of a different size.
What it is
The basic shape is a coupling that’s wider on one end than the other. Concentric reducers are symmetrical and used in most residential transitions. Eccentric reducers offset one centerline, useful for pump suction (eliminates trapped air pockets that disrupt pump priming) or for keeping the bottom of a horizontal run flat.
In threaded systems, “reducing bushing” is a related fitting that inserts into a larger female fitting to give a smaller female outlet — a different solution to the same size-change problem.
Common variants and what a reducer is not
- Reducer vs. reducing bushing. Reducer is a stand-alone coupling-style fitting; reducing bushing inserts into an existing same-size fitting.
- Concentric vs. eccentric. Concentric centers both pipes on a shared axis; eccentric offsets.
- Reducer vs. reducing tee. A reducing tee branches with one or more outlets a different size than the run.