PVC or CPVC - Choosing the Right One
Polyvinyl chloride is the most common drain pipe material in modern residential plumbing. Strong, durable PVC is usually white (except for grey electrical PVC conduit) and available in a broad range of sizes. Its popularity comes from its affordability and versatility, and its ability to handle both warm and cold water. It can't be used for hot water applications, however, which would warp it and eventually cause degradation, contaminating the water supply. As a result, PVC is mostly used in drain lines for sinks, toilets, bathtubs, etc. This is where CPVC comes into play.
With extra chlorine added to the material, the durability of PVC is greatly increased. This improved material, called Chlorinated Polyvinyl Chloride (CPVC) makes the PVC pipes able to withstand degradation from hot water use, making it safe for drinking and bathing water applications. This makes the cost of CPVC pipes slightly higher than PVC.
PVC and CPVC pipes are graded by their thickness and labeled under one of two Schedules (SCH)—an SCH 80 pipe offers a thicker wall of .179” (4.5mm) while the SCH 40 is .133” (3.4 mm) thick, thus making an SCH 80 pipe stronger. The nominal pipe size stamped on its wall specifies the outside diameter of all the schedules. Inside diameter varies depending on the thickness of the wall.
PVC and CPVC pipes and fittings are mended together using a primer and “cement”, which isn’t really a cement but a solvent, since typical cements (eg. contact cement) are adhesives used to bond materials together. The PVC/CPVC solvent “cement” (Fig.1) softens the two PVC surfaces, blending and fusing them into one.
Another product used to prepare the surface prior to applying the primer is PVC cleaner (Fig. 1), which removes grease, oil, and other dirt from a surface. While the cleaner is not required for the grey PVC, it’s usually necessary for white PVC.
The PVC primer (Fig. 2) usually comes in a purple hue, but is also available in clear form for jobs that need to appear nice and clean. The purple color is simply a dye added to make it easier to see where the surface is coated. Priming is a chemical process that occurs when the solvent is applied to the surfaces you'll be connecting. Several pieces can be primed at once as long as a) they'll be assembled within the next five minutes, and b) none of the coated parts get dirty before cementing.
If it takes more than five minutes to get to a piece, or one gets dirty after being primed, reapply the primer before moving on. Each surface has to be thoroughly coated, but look out for and wipe off any runs, as the excess solvent could degrade the PVC.
Safety Warning: The fumes from PVC solvents are extremely unpleasant but also carcinogenic. The procedure must be done in a well-vented area. Wear a pair protective gloves—direct contact can cause severe skin dehydration.
Making the Modifications
The work at hand at this point is the replacement of a deteriorated or damaged section of the pipe.
Step 1 - Measure
Determine the length of the section to be replaced and mark it at both ends (Fig 3).
Step 2 - Make the First Cut
If using either a manual saw or an electric saw, a blade with fine teeth should be used to cut the pipe at one end and as perpendicular as possible (Fig. 4).
Step 3 - Determine the Necessary Coupling
If both ends can be spread apart after the first cut to create a 1” (25mm) minimum gap (Fig. 5), two regular PVC couplings can be used for reassembly. If not, a PVC coupling will be needed at the bottom end and a flexible coupling (Fig 6) at the top. Those rubber couplings are commonly referred to as Fernco Couplings or Mission Couplings.
Step 4 - Make the Second Cut
The bad section of the pipe can then be cut at the other end as precisely as possible. The goal is to make a clean, perpendicular line.
Step 5 - Smooth the New Edges
All burrs have to be cleaned off by making a small 1/16 inch (1 mm) bevel at the ends of both pieces remaining, inside and out (Fig. 7). The inside bevel will keep dirt and other objects from getting caught while flowing out. The outside bevel will make it much easier for the fittings to slide in and sit in position. A utility knife can easily accomplish this.
Step 6 - Cut the Replacement Pipe
The new replacement section is cut to the same length as the sectioned piece, minus 1/4” - 1/2” (6 - 12 mm) minimum (Fig. 8).
Step 7 - Smooth the Patch Edges
All four perimeter edges (both ends) of the new section should be slightly beveled, as previously done in step 5.
Step 8 - Clean and Prime
Both ends of the new section and one end of each PVC couplings can now be cleaned and coated with primer. Take care to coat only the surfaces that will come into contact with each other, and avoid using too much and causing runs or spills along the pipe, as this could eventually degrade it.
Only one end should be treated with solvent if you'll be using a flexible coupling at the other end.
Step 9 - Apply Flexible Coupling (If Necessary)
If you'll be using a flexible coupling, it should be slipped onto the top part of the severed pipe and slid up, out of the way (Fig. 8).
Step 10 - Apply Cement
All primed parts you're ready to work with can now be coated with the cement, keeping in mind that the assembly should be completed within five minutes of applying the primer.
Step 11 - Attach the Fittings
Without delay, bring each end of the new section together with its respective PVC fitting (except for the flexible coupler), applying firm pressure to slide them into place while twisting into a 90° rotation to create a strong seal. Pushing pipes and fittings together without rotation runs the risk of dirt leaving grooves all along the joint, potentially causing leaks.
Step 12 - Apply More Cement (If Necessary)
This being done, pre-assembly of the new section is complete. The cement must now be applied to its fitting(s) and to the end(s) of whichever of the severed pipe doesn’t require a flexible coupling. If a flexible coupling is to be used, disregard the next step and proceed with step 14.
Step 13 - Bring the Pieces Together
This step will be easier if one of the original pipe’s ends is held steady, either by an assistant or with something like a piece of plumber’s strapping as shown in Fig. 9. Firmly bring the new pre-assembled section together and slip the fitting onto it, then slip the other end of the severed pipe into the other fitting of the new section, pulling everything tightly together while rotating about 90° to creating a better seal.
Step 14 - Fasten the Flexible Coupling (If Necessary)
If you're going to use a flexible coupling, line up the original pipe’s severed end with the fitting while firmly bringing the new pre-assembled section together with it and slipping the fitting onto it. Finally, line the other end up with the other part of the severed pipe (shown in Fig. 10 at the top of the added section), and move the flexible coupling down to cover both pipes halfway. Tighten up the clamps with a flat-head screwdriver or a nut driver.