PVC DWV fittings have a 'tight' elbow and a 'sweep' elbow. Is one for vent and the other for drain/waste? In general, is there a handbook that describes these fittings and explains why you would use one over the other?


If you are going by the International Residential Code, you are limited on what types of fittings you can use for sanitary drainage (see table 706.3 from the 2015 IRC below), but there are no restrictions on the fittings that can be used for venting as long as you maintain sufficient slope. Similar restrictions likely exist in the Universal Plumbing Code, but I am not as familiar.

The reason that certain fittings are restricted is to allow sufficient transition to help prevent solid waste from clogging in the drain (note that some of the restrictions are lifted for 2" and smaller pipe sizes because these pipes cannot carry any sizeable solids).

Table 706.3 from IRC


One discussion sweep 90 vs short radius 90.


From the above discussion the sweep 90 is used for DW piping when significant waste water is involved and especially when solids are in the waste. A specially important case is when liquid waste is transitioning from a vertical run to a long horizontal run so that the momentum of the bulk liquid is not absorbed by slamming into a short radius turn and generating turbulence. This speeds the flow through a low slope horizontal run and keeps solids suspended.

The short radius 90 is used for vents which are only conducting air.


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