Can be used in either direction. Wide side: inner diameter is 2", outer is 2-1/4". Narrow side: inner diameter is 1-7/8" and outer is 2-1/8". Central vacuum pipe fits into it to extent the exhaust. Central vacuum tubing is thin wall PVC, not SCH40. But SCH40 can be used, the 1.5" ID has a 2" OD that fits into the coupling like vacuum pipe does. SCH40 is three times thicker so the exhaust might be constricted a little, but with no adverse affects.