For most modest to moderate remodel jobs, you will tap into an existing drain/vent line rather than run new pipes all the way from the main drain and out through the roof. Consult with a plumber or with your building department to make sure that your new project will be drained and vented properly. The following pages show some of the main methods for tying into existing lines.
T-ing in
T fittings are commonly used to tap into an existing drain and vent pipe. If the new plumbing will use smaller pipes than the existing plumbing, you can use a reducer T or install a reducer bushing in the T. Measure for cutting into the pipe carefully, so the new lines will be at the correct heights and can be sloped correctly. An important consideration is whether there is sufficient leeway for the existing pipe to be moved up or down—or side to side—to permit you to install the T. If it is solidly in position, you may need to use flexible fittings. Flexible fittings are also often the best solution when changing materials—for example, when tapping into steel pipe with PVC.
1. ASSEMBLE THE TIE-IN PLUMBING. In many tie-in situations you need to tap in for both the drain and the vent. If you are tapping into a stack that acts as both a drain and a vent, start by assembling a section of plumbing with a sweep T at the bottom for the drain line and a sanitary (or vent) T at the top. Note that the sanitary T for the vent is installed upside down, to help air flow smoothly upward. Glue carefully, so both tees are facing exactly the same direction.
2. MARK FOR CUTTING THE EXISTING PIPE. Hold the new pipe section in place, making sure the drain and vent fittings are at the correct height. (A piece of tape helps keep it from moving.) Mark the pipe for cutting.
3. SUPPORT HEAVY PIPE. If the existing pipe is cast iron or steel, it will be heavy and may be in danger of falling after it is cut. Support with a piece of blocking and a tightly screwed strap.
4. CUT THE PIPE. Cut steel pipe with a reciprocating saw that has a metal-cutting blade. Cast iron is more difficult to cut: Use a circular saw or a grinder equipped with a metal-cutting blade if possible; cutting with a reciprocating saw is very slow work.
5. INSTALL THE TIE-IN SECTION. Use flexible fittings to attach the new section. Tighten all the clamps firmly.
IF THERE IS ROOM TO MANEUVER. Often, especially in the case of plastic pipe, the existing pipe can be moved up and down and back and forth after it has been cut. In that case, you can simply install a T fitting. The cement hardens quickly, so first install it in a dry run, check that the Ts outlet is facing in the right direction, and make a reference mark on the pipe and the fitting. Then disassemble, apply cement, and reassemble.
Working in tight quarters
Sometimes you need to tie into a pipe that is in a tight spot, as in this case where a new drain line must be installed for a whirlpool tub. Here you may have a choice: tear open a wall, floor, or ceiling to give yourself plenty of room, or work more surgically.
1. REMOVE THE EXISTING PIPE. Cut the existing pipe with a reciprocating saw and then unscrew the old pipe stub using a pipe wrench.
2. TORCH IF NEEDED. If the pipe refuses to budge, spray with penetrating oil and wait a few minutes before unscrewing. Or, heat the joint with a torch, then unscrew.
3. SCREW IN A NEW FITTING. In most cases, a new threaded PVC fitting can be screwed into an old metal fitting. Here a male adapter is installed, so PVC pipe can be cemented from there on. Apply plenty of plumber’s tape or TFP paste to the threads, screw on by hand, then use slip-joint pliers or a pipe wrench to tighten the fitting.
Tapping into supply lines
When supplying water to a new fixture, you need to tap into the existing pipe, usually with a T fitting. Don’t forget to shut off water to the existing pipes before cutting or unscrewing them to tap in. If the pipes are movable after cutting, you can often simply install a T fitting. But if they are solidly in place, you will need to use repair couplings as well.
A SIMPLE T-IN. If you can move the cut pipes far enough, you can just pull them apart and then press them into each side of the T in a dry run. Pull the pieces apart, apply plenty of flux to the fitting and the pipe, and then solder.
1. MARK AND CUT. If the pipe will not move from side to side, assemble a T with two short nipples and hold it in place to mark for cutting the existing pipe.
Cut the pipe at each end.
Dielectric Fittings
If you are transitioning from galvanized pipe to copper, codes often require that you install a dielectric union, which has plastic innards that prevent ionization that can corrode the joint.
2. FLUX AND SOLDER. Apply flux to two repair couplings (which do not have stops, and so can be slid all the way onto the pipe), and slide them onto the two cut ends.
Position the new section and slide the repair couplings over to make the joints. Make sure the T is facing in the right direction, and solder the joints.
SNAKING AROUND. Where a supply pipe needs to cross another pipe or other obstruction, you may need to snake it around. Use a series of fittings that make as neat a route as possible. Often you’ll find that 45° street elbows (which are male at one end and female at the other) are your best friends. Assemble all the parts and check that they are facing in the right direction. Disassemble, apply flux to all the insides and outsides, reassemble, and do a bunch of soldering.
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