Illustration: Harry Bates
All the joints in PVC piping whether it's used for the drains and vents in house plumbing or as a part of a central vacuum system, are literally welded together with a chemical solvent. The solvent cement, applied to both of the pieces being joined, temporarily melts a thin layer of PVC, which resolidifies in the seconds after the pieces are put together and forms a seamless, waterproof, airtight seal. The process is simple but unforgiving of error—once the weld sets, the only way to change it is to cut it off. Here's how to keep mistakes to a minimum.

1. Measure and cut. To ensure that the pipe will be fully seated in its fittings, measure the distance between the shoulders on the fittings at each end of the pipe. Transfer that measurement to the pipe and make a straight cut with a hacksaw. Scrape the inside of the cut smooth with a utility knife.

2. Prime. Spread cement primer over the outside of the pipe and the inside of the fitting where they'll overlap. The primer softens and cleans the plastic.

3. Dry fit. Assemble everything to make sure the pipe is the correct length. On fittings such as elbows, position is critical, so make a reference mark from the pipe onto the fitting.

4. Glue. Apply the cement to the primed areas on the inside of the fitting and to the outside of the pipe.

5. Push and twist. Slide the pipe into the fitting until it bottoms out against the fitting's shoulder, then give it a ¼-inch turn to spread the glue and speed its cure. If you're attaching a fitting, push it in with the reference marks slightly out of register, and turn the fitting to align the marks. Hold for 30 seconds, then wipe up any excess cement with a dry rag.
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