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A Complete Guide to Using Plumbing Fittings for Joining PEX Pipe, PVC and Copper

Eugene is a trained engineer and self-taught home improvement enthusiast with almost 40 years of professional and DIY experience.

A DIY Guide to Plumbing

Basic plumbing isn't rocket science. If you are reasonably adept at DIY, understand the basics, and take care when putting it all together, it's not too difficult. This is a comprehensive and easy-to-understand guide to pipe types, plumbing fittings, thread sizes, with instructions for how to use fittings to connect copper, PVC, and PEX pipes.

How to Use Plumbing Fittings for Joining PVC, PEX, and Copper Pipe.

How to Use Plumbing Fittings for Joining PVC, PEX, and Copper Pipe.

What's Covered in this Guide

  • Pipe and tube types and thread sizes
  • Compression fittings
  • Threaded fittings
  • Push fittings
  • Soldered capillary fittings
  • How to cut tube and pipe

What Material and Type of Plumbing Pipe Is Right for the Job?

Plumbing pipe is made from various materials, including copper, galvanized steel and iron, polyvinylchloride (PVC), chlorinated polyvinylchloride (CPVC), cross-linked high-density polyethylene (PEX), and polybutylene. Fittings are made from brass, plastic, copper, or malleable iron.

  • Copper. Copper is widely used and can withstand very high temperatures. It is always used for the final connections made to central heating boilers/furnaces where temperatures may exceed 100°C (212°F). The disadvantage of copper pipe (or "tube," as it is known in the plumbing industry) is that it is rigid and either must be bent by using special tools or by using separate lengths of pipe and 45° or 90° joints.
  • Stainless Steel. In the 70s during a copper shortage, stainless steel plumbing was popular. It is even more rigid than copper however, so any bending can be difficult. Stainless steel is supposed to be more corrosion-resistant than copper and is sometimes used for aesthetic reasons where plumbing must be exposed.
  • PVC and CPVC. Pipe made from these plastic polymers has several advantages over copper. Plastic pipe is ductile and can be easily bent, reducing the number of bend fittings required. If a really tight bend is required, 90° elbow joints can be used. It can also be easily routed through floor and wall spaces. Plastic is a better insulator than metal, so heat loss is reduced. Usually, plastic pipe has more "give" and will stretch more than copper when water freezes and expands inside the pipe, reducing the danger of bursting in sub-zero temperatures. PVC or polyvinyl chloride is the most basic type of plastic pipe used for both drinking water and waste pipe plumbing. Chlorinated PVC or CPVC has the same characteristics as standard PVC but has better corrosion resistance at higher water temperatures and is also significantly more bendable.
  • PEX. Plumbing tubing is also made from cross-linked high-density polyethylene (PEX). This is a tough, ductile material which can survive temperatures as low as -20°C (-4°F) without bursting. PEX is also less than half the price of copper and doesn't corrode when used in places where the water is acidic. Just like PVC, PEX is flexible and can be bent over a tight radius and easily routed to fittings. It does expand more than copper so it shouldn't be stretched to reach fittings and should be allowed to drop slightly between fixings to allow for contraction. On long runs, a loop of tubing can be included in a line to allow for a lot of contraction. When running tubing through holes in walls, make the holes sufficiently large so that the tube can slide freely as it expands and contracts and protect against abrasion from contact with rough surfaces. PEX is susceptible to UV from the sun and fluorescent lighting, and if used outdoors, it needs to be shielded (with insulation or otherwise) to prevent degradation.
    This link outlines other preventative measures to stop damage to PEX tubing.
  • Ductile (Malleable) Iron. Used for water, gas, compressed air, and as a rigid metal conduit (RMC) for housing cables in industrial and commercial applications, ductile iron is much stronger than copper or plastic pipe but not so common in domestic installations. Ductile iron or cast iron was often used for larger-diameter water mains before the invention of plastic.
  • Lead. No longer used for plumbing because of its toxicity, lead piping was phased out after WWII when copper became popular.

Note that local authorities who replace lead pipes may only do so up to the threshold of the property or the main stop valve/water meter and it is up to the property owner to change from consumers side of valve/water meter.

A note about plastic piping: Care needs to be taken when working near plastic piping or fittings to avoid heat damage from blow torches or other heat-producing tools. Piping should also be routed during installation so that it isn't subjected to high temperatures from heat sources such as flues.

Can Iron and Copper Pipe Be Mixed?

Iron and copper are dissimilar metals. Without the proper precautions the two metals in close contact can act like a battery resulting in galvanic action which can cause corrosion. So precautions need to be taken to avoid this and the details are outside the scope of this guide.

PEX Pipe

1/2 inch PEX tube can be bent to a min radius of 4 inches (depending on wall thickness).

1/2 inch PEX tube can be bent to a min radius of 4 inches (depending on wall thickness).

Steel Pipe

3/4 inch BSP ductile iron pipe and coupler.

3/4 inch BSP ductile iron pipe and coupler.

What Material and Type of Plumbing Fitting Is Right for the Job?

Fittings are made from various materials, including plastic, brass, copper, and iron. There is a huge variety of types with different functions, including 90° and 45° elbows, offsets, T-joints, Y-joints, cross joints, gate valves, ball valves, non-return (check valves), reduction fittings, couplers, and flange (bulkhead) fittings for making a connection to oil or water tanks.

  • Brass Threaded and Compression Fittings have two or more ports or entry points for connection to pipes or fixtures. They may be compression-only, compression-and-screwed, or screwed-only. The compression port is used for connecting a pipe, and the screwed port (if included) connects to a fixture or device such as a radiator, WC, furnace, spigot, pressure gauge, or water tank. The screwed section of the fitting may have male or female threads. Fittings with compression ports only are used to join two or more pipes together and are simple to use by the average DIYer once pipes are prepared correctly, and the fitting is assembled properly. Brass threaded and compression fittings make use of a copper or brass olive ring (also known as a "ferrule" or compression ring) which is slid onto the end of the pipe before insertion. This is then bent and squeezed tightly against the tapered surface of the fitting as a nut on the pipe is tightened. This seals the fitting and prevents water from leaking out.
  • Compression fittings can be used for connecting plastic or copper pipe and, although easy to use, are relatively bulky and more expensive than other joining methods. Another disadvantage is that a pipe can possibly turn within the fitting (e.g., a coiled copper pipe behind an ice-making refrigerator can turn as the fridge is pushed in and out repeatedly), and over time, this can cause leaks. Compression fittings can be easily disassembled. Plumbers often use PTFE tape or jointing compound (which is like soft putty) on compression fittings to be totally sure they don't leak. However, manufacturers don't recommend this because they reckon it can prevent the olive from sealing properly. Brass fittings are not recommended for use underground.
  • Capillary Fittings are made from copper and require a soldered joint to connect pipes together. The pipes-to-be-joined are inserted into the fitting, which is heated. Solder fills the small gap between pipe and fitting, sealing and making a strong joint which is resistant to pulling and rotation of the pipes. This type of fitting either comes with a ring of solder included (Yorkshire fitting) or solder wire must be melted and fed into the joint as it is heated. Capillary fittings are slim and neat, inexpensive, and form a strong joint between pipes. They are probably the most reliable too for use in walls, under floors and other inaccessible locations as the joint is more secure and less likely that pipes will separate or O-rings deteriorate, resulting in leaks. However, they are more difficult to use, and since the fitting must be heated using a blowtorch, there is always the danger of fire. Also, preparation of pipes and fitting is essential, and these must be cleaned with wire wool so that solder flows and coats all the surfaces being joined.
  • Ductile (Malleable) Iron Screwed Fittings can be used with water and compressed air and are relatively inexpensive. These fittings are often encountered on the inlet/outlets of central heating boilers/furnaces, on pumps, and on-air compressors. Because iron is less expensive and stronger than brass, iron fittings are advantageous, especially when large sizes are required.
    Galvanized versions of these fittings are available which are resistant to external corrosion.
  • Plastic and Copper Alloy Push (Stab-In) Fittings are available for joining plastic and copper pipe. To use them, you just have to square-cut the end of a pipe and push it into the fitting; an internal O-ring compresses tightly against the pipe when inserted. The fittings also incorporate some form of collet/toothed ring arrangement which digs into the pipe and prevents it from being pulled out again. They are very easy and convenient to use, especially in tight spaces, enabling pipes to be jointed quickly. Disassembly is by hand or by using a special key. In either case, the teeth retract, allowing the pipe to be removed. Fittings which can be disassembled by hand, without a key, sometimes have a twist-lock feature. This locks the collet, preventing the inadvertent release of a pipe from a fitting if something accidentally pushes against the collet. Sharkbite and Speedfit are two well-known brands of this type of fitting. A major advantage of push fittings is that the fitting can be rotated on the pipe after installation.

Push fittings can be used with copper, CPVC, and PEX tube, but aren't suitable for PVC. See Connecting PEX to PVC for more details.

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Read More From Dengarden

For large projects, the cost of push or compression fittings on PEX can mount up. A cheaper alternative is crimp fittings, which are slimmer but must be installed using special tools. See How to Install PEX Tubing—Making a Crimp Connection.

Examples of Plumbing Fittings

Clockwise from top left: compression elbow, iron reduction bushing with BSP threads, copper capillary elbow, and compression elbow with 1/2 inch threaded BSP port.

Clockwise from top left: compression elbow, iron reduction bushing with BSP threads, copper capillary elbow, and compression elbow with 1/2 inch threaded BSP port.

Male and female threads.

Male and female threads.

What Is a Union Fitting and How Do I Use It?

A union fitting enables two parts of the fitting to be attached to each other without having to turn the whole fitting or attached pipes.

  • These are often encountered on the plumbing to radiators or on the end of gas or water hoses.
  • A nut on one part of the fitting is tightened, pulling the two halves of the fitting together.
  • A curved or cone-shaped profile on one part presses into a similar-shaped socket on the other half to make a watertight seal.
  • The two halves are easily separated by undoing the nut, e.g., when removing a hose from an outside tap or replacing a radiator.
Wall plate and spigot, bib, or outside tap. The wall plate has 1/2 inch female BSP threads for connection to the spigot and a compression input port for attaching to copper or plastic pipe. A 3/4 inch BSP union is included for attaching to a hose.

Wall plate and spigot, bib, or outside tap. The wall plate has 1/2 inch female BSP threads for connection to the spigot and a compression input port for attaching to copper or plastic pipe. A 3/4 inch BSP union is included for attaching to a hose.

This 1/2 inch, malleable-iron, 90° elbow, used on the output of an air compressor, has female BSP threads.

This 1/2 inch, malleable-iron, 90° elbow, used on the output of an air compressor, has female BSP threads.