A compression fitting is the plumbing equivalent of wearing a snap-on necktie... unless someone yanks on it, no one is the wiser! Compression fittings have similar characteristics... they go together easily, but are prone to some "stress-related" problems if you don't choose their applications wisely!

What is a compression fitting?

A compression fitting is a type of coupling used to connect two pipes or a pipe to a fixture or valve. It consists of three parts... the compression nut, the compression ring, and the compression seat. As you can see in the diagram at the left, the nut is slid onto the pipe, followed by the compression ring.

The pipe is slid into the fitting (in this case a toilet shutoff valve) and the nut is tightened down. As the nut is tightened, the compression ring is pressed into the seat, causing it to compress against the pipe and the compression nut, providing a watertight connection. Usually, that is.

What are the advantages and disadvantages of compression fittings, and how about listing some basic guidelines for their common sense installation and use?

Though the primary advantage of compression fittings may seem to be in their ease of assembly, their real advantage is in their ease of disassembly! If you look throughout your home, you will see that the use of compression fittings is generally limited to appliances and fixtures that will... given enough time, normal use, and wild beatings with a hairbrush... wear out and need to be replaced. Look under your bathroom vanities, behind your toilets, under your dishwasher or behind your refrigerator... and you will see compression fittings in flagrante delicto!

In fact, the now ubiquitous use of compression fittings in homes conveniently corresponds with the advent of do-it-yourself , user-friendly fixtures and appliances. After all, if you want to sell faucets to folks with limited skills, you want to make their installation as painless as possible. Ergo, compression fittings!

But compression fittings have a dark side, and may leak if not used properly. Here are my rules for the use of compression fittings. If you don't follow them, you may regret it!

Compression fittings are to be used on stationary connections only...

If you look critically at compression fittings, one thing is apparent... they are designed to inhibit the movement of the pipe outward from the fitting. However, they do a poor job preventing the pipe from turning within the fitting. To visualize this, take the example of the connection for a refrigerator icemaker.

Typically, a compression fitting is used to connect copper tubing to the icemaker. To prevent the thin 1/4" tubing from kinking or bending when the refrigerator is pushed back to the wall, an excess of tubing is normally "coiled" behind the refrigerator. This acts like a spring, expanding when the refrigerator is pulled out, and compressing when the fridge is pushed back.

A problem arises if the tubing is not solidly clamped to the rear of the refrigerator. Without this clamping, the tubing creates a rotational stress on the compression fitting as the fridge is moved, and can over time cause the tubing to rotate within the fitting, or even loosen the compression nut, causing a "stealth" leak behind the refrigerator. This can be a particularly nasty leak, because it can soak the floor with not evidence until the damage is severe... or until friendly, family-minded carpenter ants find the nice, damp wood!

Double Compression Fittings for lengthening pipe...

Another use for compression fittings that is a qualified NO is lengthening of pipes using a straight or angled double compression fitting (left). About the only time this is acceptable is under a sink, where there is no movement or stress. If there is even a chance that the compression connection will be subjected to rotational movement or impact of any kind, it is better to use a solder, or "sweat", connection instead.

Compression fittings are designed to be used once, but you might get lucky...

Each compression fitting is a little different in the way it orients itself while tightening... yet sometimes a fitting can be reused. Refrigerator icemaker connections and dishwasher connections are two that come to mind. If you decide to try to make a connection using the existing permanently attached ring and nut, be sure to lubricate the ring and the threads on the compression nut. You will improve your chance of a successful seal..

You can not remove a compression ring from tubing once it has been used. If you can't get a leakproof connection, the tubing behind the ring will have to be cut, and a new ring installed. Though the compression nuts can often be reused, I would recommend against it because the old nut may be slightly deformed and lessen the quality of the new connection.

Always lubricate the compression nut threads before installation...

No, I'm not being redundant. It is just good practice to lubricate new compression fittings too, so I thought I would mention it... for the record!

Do not overtighten the fitting, especially when using plastic pipe...

There is a point beyond which tightening a compression nut will not longer yield any gain. Once you encounter resistance in turning, tighten no more than an additional half turn. Test the connection by turning on the water slightly. If there is leakage, turn the water back off and tighten the compression nut by no more than a quarter turn at a time until all leakage stops. This is one of those "touchy-feely" skills that you develop over time, so don't expect a perfect, leak-free connection on the first try. Even us wizzened old pros have to sometimes tweak the connections to get them right!

If an old compression fitting starts to leak, you may be able to save it!

Simply loosen the compression nut slightly, then retighten it a little beyond the original position. Loosening first is an old pipefitter's trick... it breaks the resistance of time and corrosion on the compression nut threads, allowing you greater tightening ease. If this does not work, or if the compression nut will not tighten, loosen it and apply some plumber's grease to the threads and try again.

If you are connecting plastic tubing with a compression fitting, do not use a metal compression ring... use a plastic one!

Just trust me... don't do it or you will be courting disaster! If you didn't get one with the tubing, you can purchase one at the hardware store.

Be sure that the pipe is pushed deeply into the fixture before tightening the compression nut...

This is a common cause of leaky compression fittings. Most shutoffs have a limiting "lip" to prevent you from pushing the tubing in too far. If you are jointing two pieces of tubing with a double compression fitting, put a pencil mark or a piece of tape on the tubing to indicate the proper depth... a little less than half of the length of the fitting. At the least, the pipe should extend 1/8" beyond the compression ring.

Compression fittings will not work on bent tubing!!

Why? Because 90% of the time compression rings will not slide onto bent tubing. But there is a warning hidden here... when you bend your tubing, always leave a the last few inches of tubing straight or you will have trouble making your connection... a very sad thing!