Recently, adapters have been offered for sale on the Internet that allow 2”ID Schedule 40 plumbing pipe to connect to 2” OD Schedule 20 central vacuum tubing, facilitating the installation of the piping system (from the sharp 90 behind the inlet to the power unit intake) with the more commonly available Schedule 40 plumbing pipe. There are several issues relating to the use of this material for a residential central vacuum system, which I believe outweigh the benefits.
The first is the simple fact that all fittings made for use with Schedule 40 plumbing pipe have been designed to carry slow-moving liquid waste, not the fast-moving air and dry debris that a central vacuum system conveys. This means many of the fittings are going to have a less than ideal bend radius, and have not been designed to take into account the venturi effect that can pull debris into unused branch lines at tee connections. Only a few of the many types of DWV fittings available are suitable for vacuum system usage.
There is also a design element of PVC plumbing fittings that makes them more susceptible to clogging than vacuum fittings. The area where the inside of the fitting hub meets the nominal interior diameter of the fitting does not have a flat shoulder (as vacuum fittings do) but a beveled shoulder. Unless the pipe is beveled to match, there will be a gap where the pipe bottoms out in the hub which can catch debris and lead to a clog.
In addition, the common availability of Schedule 40 pipe and fittings at home centers (one of the main advantages given by promoters of the adapters) makes it easier for improper installation to occur, as opposed to central vacuum fittings usually sold by specialty retailers having the expertise to instruct on the proper fittings for certain applications.
Another purported advantage is that the larger interior diameter of 2” DWV (being actually two inches instead of the 1 7/8” interior diameter of vacuum tubing) reduces friction loss, providing for greater airflow. In most residential installations, the friction loss is inconsequential. There may be a perceptible difference in airflow with larger systems (especially using a power unit generating high CFM), however, the disadvantage which I believe can outweigh this benefit is the fact that the air will be traveling at a lower velocity, making it possible for dirt to remain in risers and horizontal lines instead of being carried to the unit. Commercial multi-user systems are laid out so that each section of tubing is large enough to support the required number of users, but small enough to ensure adequate velocity (40 feet per second being the minimum). Residential single-user systems are designed to work optimally with vacuum tubing, nothing larger.
Finally, professional installers use specially designed vacuum tubing cutters which leave a clean, square edge on the tube with no deburring needed. There are no such cutters available for Schedule 40 PVC, meaning the only way to cut the pipe is with a miter box or hacksaw. Even if the cut is square, it will need to be deburred to avoid catching lint and forming a clog.
In summary, the reasons not to use Schedule 40 PVC for vacuum systems are that the fittings are not designed for conveying air and debris, that there is a greater potential for improper installation, that the larger diameter will reduce the air velocity and cause debris to remain in the pipe, and that it is more difficult to properly cut the pipe, as is critical for vacuum system installation. I believe the small additional expense of sourcing proper tubing and fittings for the installation of your vacuum system will more than repay itself in a more effective, trouble-free central vacuum that will serve you for many years to come.