In this video, This Old House plumbing and heating expert Richard Trethewey shows how to replace an old bathtub drum trap with a new PVC P-trap.
1. Use reciprocating saw fitted with a metal-cutting blade to cut out the old iron drum trap; be careful when lowering the trap, it's filled with water.
2. Loosen the old threaded adapter that's attached to the existing drain line using two large pipe wrenches; use one wrench to hold the pipe, and the other to turn the adapter.
3. Once it's loose, remove the adapter with pliers.
4. Brush pipe dope onto the threads of a PVC male adapter, then wrap the threads with single-strand wicking to provide extra protection against water leaks.
5. Insert male adapter into the existing iron fitting, and tighten the adapter with pliers.
6. Apply pipe dope and wicking to the existing iron male fitting, then install a threaded PVC female adapter, tightening it with pliers.
7. Dry fit (no glue) the PVC P-trap and PVC pipes to ensure they fit together properly.
8. Measure last piece of PVC pipe, and cut it to size using a PVC cutter or hacksaw.
9. Use a pocketknife to scrape off any rough burrs from the just-cut end of the PVC pipe.
10. Dry fit together all the parts one final time, then use an indelible marker to draw alignment marks onto the pipes and fittings.
11. Disassemble the PVC pipes and fittings, then clean them with PVC primer.
12. Permanently glue together the pipes and fittings with PVC cement, making sure you line up the marks drawn earlier in Step 10.