How To Use Compression Fittings On Copper Pipes

If you enjoy DIY plumbing, you’ll no doubt run into the need to use compression fittings on your copper pipes. When will that be?

You’ll want to use compression fittings in a situation where the run of pipe needs to be taken apart or when an area of the pipe will be difficult to solder. One common location in which this occurs is on supply lines to a bathroom, laundry, or kitchen sink because they have compression fittings at both the stop valve and the faucet inlet.

Here’s What You’ll Need

You won’t need much to complete this simple compression fitting project.

  1. Time

    About 15 minutes to make a simple connection.

  2. Skills

    Here’s the part you’re really going to love. You need no particular skills for this project.

  3. Tools

    Two crescent wrenches and some pipe joint compound.

How Does a Compression Fitting Work?

Firstly, time to get into the nitty gritty of compression fittings. But more so, we need to explain how a compression fitting works. While reading this, take a look at the diagram below. It will help you visualize how everything’s put together.

The compression nut forms a seal by squeezing the ferrule against the copper pipe. A ferrule is a metal band that forms or strengthens a joint.

Because copper is a soft metal, the seal can be extremely tight. Use pipe joint compound (or Teflon plumber’s tape) to make sure the seal is watertight. You should also anchor or support the tubing within two feet of either side of the fitting.

How does a compression fitting work - diagram
A side-view showing how the compression nut forms a seal by squeezing the ferrule to the pipe.

How to Make a Compression Joint

Here are the four basic steps in making a compression fitting joint.

1. Shut Off the Water

As always, when working on a plumbing project related to your water pipes and compression fittings, start by turning off the water shut-off valve.

Always begin by turning off the main water shutoff valve. Usually it’s located in your basement on the street-side of your house.

2. Position the Parts

Bend the tubing into position and slip on the nut and the ferrule. The ferrule (a metal band used to strengthen a joint) will not go on if the tubing end is bent or less than perfectly round. You may have to sand it with emery cloth to get it to slide on. Smear pipe joint compound (like Pro Dope) on the ferrule and the male threads of the fitting.

How to make a compression joint - position the parts

3. If the Tubing Kinks

If you don’t bend the pipe in a gradual, sweeping arc, you’ll be surprised how fast it will kink. And a kink in the pipe is nearly impossible to get back to the right shape. You’ll need to throw the piece away.

A good way to do this safely is to use a coil-spring tubing bender, a device that is inexpensive and will allow you to bend either soft copper or aluminum.

Compression Fittings - coil spring tubing bender
This is an example of a coil spring tubing bender. You simply insert the tube (copper pipe) into the correct-sized bender and use your hands to bend. This method is inexpensive and prevents the copper from being kinked (and therefore useless).

4. Tighten the Nut

Tighten the compression nut with a wrench, forcing the ferrule down into the tubing to secure and seal the connection. If the joint leaks when the water is turned on, tighten the nut a quarter turn at a time until the leak stops. Don’t over-tighten the joint. Too much pressure can crush the tubing or crack the nut.

tighten the nut
Place your crescent wrench here to stabilize the fitting while using your other wrench to tighten the nut.

How to Join a Compression Union

If you don’t need to create a compression joint, but rather need to join a compression union, then follow these steps.

1. Shut Off the Water

I know, it gets annoying having to read this same instruction over and over. But really, it’s that important. Too many water damage disasters have happened because this simple step wasn’t followed. We’ll spare you by not showing you the picture again.

2. Position the Parts and Join Them

Bend the pieces into position and slip a nut and a ferrule onto each piece of tubing. Smear pipe joint compound on the ferrules and on the male threads of the union. Slide the pieces together and hand-tighten the nuts until they are snug.

How to join a compression union - position the parts
Pretty simple, right? Just position the parts correctly and smear pipe joint compound onto the ferrule and male threads before joining and tightening.

3. Tighten Each Side

Place one crescent wrench on the union. Use your second crescent wrench to tighten each side (the first wrench is used to keep the pipe stable while you’re tightening with the second wrench).

Once snug, tighten about one more half turn. Turn the water back on, and if there’s a leak, gently tighten a bit more.

How to join a compression union - tighten each side the parts
After you have hand-tightened, you will now fully tighten using your two crescent wrenches.

Frequently Asked Questions

How reliable are compression fittings?

Compression fittings are more reliable than threaded fittings. They are easier and faster to install than soldering. And they are safer because no torch is needed. However, compression fittings can be prone to issues from vibration and repeated bending.

Why are my compression fittings leaking?

In a DYI situation, the most common cause is over-tightening. Follow the instructions above for how far you can tighten.

Can I remove my compression fitting?

Probably not; you’ll need to cut it off.

Final Thoughts on Compression Fittings

Compression fittings are a great solution for do-it-yourselfers who want a safe way to join (or connect) their copper pipes without a torch. Although not without their potential issues, they are commonly used to make watertight connections quickly and easily.

Call 1-Tom-Plumber

We can help you repair, remove, or install any type of pipes and fittings, including compression fittings and other solutions. Don’t hesitate to contact or call us at 1-Tom-Plumber (1-866-758-6237). We will immediately handle any emergency plumbingdrain, and water damage problem, including excavation of underground water lines and sewer main lines.

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