An eccentric reducer joins two pipe sizes with the centerlines offset, so one side stays flat. On pump suction piping, pump makers say flat side up, so air can't pool at the top. On a plumbing drain, the code bars reducing the pipe size in the direction of flow, so a reducer isn't how you step a drain down.
What is an eccentric reducer?
An eccentric reducer is a size-change fitting with offset centerlines, which leaves one side of the pipe running straight through. A concentric reducer is the other kind. It tapers evenly from all sides, so both pipes share one centerline.
The offset is the whole point. Hold the flat edge up and the top of the pipe stays level across the size change. Hold it down and the bottom of the pipe stays level instead. Which one you want depends on what's trying to collect: air rises to the top of a full pipe, and solids settle to the bottom of a drain.
Eccentric reducers come in drain, waste and vent materials too. Spears, a PVC fitting maker, lists eccentric reducer couplings in its Schedule 40 and Schedule 80 DWV fittings catalog. So the fitting exists in drain systems. What matters is the rule for using it.
Which way does the flat side go on pump suction?
On the suction side of a pump, the flat side goes up. Armstrong's instructions for its base mounted pumps say to "Use eccentric reducers on suction connection with flat side uppermost." Armstrong also tells installers to "Arrange piping to eliminate potential air pockets."
Goulds, a Xylem brand, says the same in its NPE pump manual: "If larger pipe is required, an eccentric pipe reducer (with straight side up) must be installed at the pump." The manual adds that "Suction pipe must be at least as large as the suction connection of the pump."
The reason is air. Armstrong ties the fitting to avoiding air pockets ahead of the pump, and a flat top keeps the top of the pipe level through the size change.
One limit on that answer. These are pump-maker instructions for suction piping, so they cover a pump, not a drain. Your pump's own manual controls if it says anything different.
Can you use a reducer on a drain line?
Not to make the drain smaller as the water flows. Section 704.2 of the plumbing code that Phoenix adopted (the 2024 International Plumbing Code) says "The size of the drainage piping shall not be reduced in the direction of the flow."
The code lists three things that don't count as a reduction:
| Fitting | The code's wording |
|---|---|
| Closet flange | "A 4-inch by 3-inch (102 mm by 76 mm) water closet flange." |
| Closet bend | "A water closet bend fitting having a 4-inch (102 mm) inlet and a 3-inch (76 mm) outlet provided that the 4-inch leg of the fitting is upright and below, but not necessarily directly connected to, the water closet flange." |
| Offset flange | "An offset closet flange." |
Note the condition on the closet bend. The 4-inch leg has to be upright and below. It's not a blanket pass for any 4 by 3 bend.
So when a drain gets a different size, the larger pipe goes downstream, which makes the fitting an increaser. The code itself uses that word in Section 706.3, which covers installing "combination fittings, side inlets or increasers." And Section 706.2 sets a second rule: "The fittings shall not have ledges, shoulders or reductions capable of retarding or obstructing flow in the piping."
Phoenix did amend its 2024 plumbing code, in Ordinance G-7397, effective August 1, 2025. The city's amendment list covers Section 704.1 (drain slope) but not 704.2 or 706.2, so the model wording above is what applies in Phoenix. Other Valley cities adopt their own amendments. I only read the International Plumbing Code text, so a project designed under the Uniform Plumbing Code needs its own check.
Which way does the flat side go on a drain?
The code sections above don't say. They set the size rule and the no-obstruction rule, and neither mentions which side of a fitting is flat. I also didn't find a fitting maker's drain-line instruction that settles it. Spears' catalog lists the fittings but doesn't give an orientation.
Don't borrow the pump rule for a gravity drain. The pump manuals are written for a pump inlet, and nothing in them covers a drain. Whatever fitting goes in, it still has to follow Section 706.2: nothing in it may hold back flow. If the fitting's maker prints an orientation, follow it.
What to do about it
- 1Pump or booster suction pipe: if the pipe is larger than the pump's inlet, put the eccentric reducer right at the pump, flat side up, unless your pump's manual says otherwise.
- 2A drain that gets smaller downstream: that's a flag. Only the closet flange and closet bend cases above are carved out. Anything else should be corrected or reviewed with your inspector before the walls close.
- 3A drain that slows or backs up at a size change: a camera shows what's sitting in the line. See how a sewer camera measures pipe slope and what drain pipe slope the code requires.
- 4Choosing drain pipe and fittings: the material matters for how you glue the transition. ABS and PVC join differently.
- 5Access for later: drain changes should leave you a cleanout you can reach.
- 6Air showing up as sputtering at the faucet? That's a supply-side problem, not a drain one. See air in pipes and sputtering faucets.
If the line in question is a plumbing drain, the licensed plumber who pulls the permit is the right person to answer the orientation question for your exact fitting. HQ Plumbing & Air is licensed and insured, and estimates are free.