DWV fittings are used to vent high efficiency furnaces, primarily in 2 and 3 sizes (the most commonly found on OEM equipment). Occasionally, the OEM will supply a 2-1/2 fitting. Unfortunately, DWV fittings do not come in 2-1/2 and the contractor must use Schedule 40 fittings for the entire installation, or install a 2-1/2 to 3 fitting and complete the installation with 3 DWV fittings. DWV fittings are available in long turn elbows in order to have as free a flow as possible in venting applications; Schedule 40 fittings are not available in a long turn elbow.