How to Install a Compression Fitting on Plastic Tubing: 3 Steps


Compression fittings use a nut, a plastic compression ring, and a seal to create a tight seal on any pipe. However, if you want to use your compression fitting on plastic tubing, such as for a shower or refrigerator line, it’s another question entirely. 

Here, the answer is simple. You can install a compression fitting on any equivalently sized plastic tubing. However, you should use an insert to support the tubing. This will slightly narrow the tubing at that point. Otherwise, you shouldn’t have any issues. 

Compression Fitting Installation on Plastic Tubing Steps

Installing a compression fitting is a relatively simple job. Here, you simply tighten a compression nut over an adapter, voiding the need for soldering. 

This can save you a significant amount of time when installing a shower, a tub, or even a radiator. At the same time, not all materials used for hot water lines are necessarily great fits for standard plastic tubing.

Here, the largest barrier is that plastic tubing is soft. Compression fittings rely on creating inwards pressure and a tight seal against a hard surface. Therefore, if you’re installing compression fittings, you’ll want to add a fitting inside the tubing. 

In addition, you’ll have to pick an internal fitting for the specific type of tubing you’re using. For example, PEX tubing is sized differently than refrigerator lines. Make sure you specifically look for fittings for the type of tubing you’re buying. 

What you’ll need:

  • Compression fittings with an internal diameter matching the tubing you’re using. E.g., buy PEX fittings for PEX tubing. 
  • Internal fittings for the tubing you’re using 
  • Shears or cutting implements for your tubing 
  • Any sealant as needed by your inserts. Likely, none. 

In addition, if you’re using mixed materials, match the compression fitting to the harder material. E.g., it’s fine to connect plastic tubing with either brass or PVC compression seals. 

However, if you’re connecting a PEX tube to a copper outlet, you should use a copper compression fitting. If you’re connecting a PEX tube to a PVC water line, you’ll want a PVC compression fitting. 

That also holds true for the internal fittings. If you’re using copper or brass fittings, use copper or brass internal fittings. That’s because a brass fitting may not fully fit over a PVC.

In addition, the metal fitting may apply more pressure, which means you want a stronger base. 

1. Cut tubing 

Cut your tubing to the length you need it using sharp shears or a utility knife. Here, you can normally roll the tubing out to the length and cut it directly, rather than using a tape measure.

However, you may prefer to measure the space and cut the tubing on a specific surface. 

2. Disassemble the compression fittings 

Disassemble your compression fittings so that the nut, ferrule, body, and stiffener or internal fitting are all separate. You

3. Insert the internal fitting and compression fitting 

Insert your internal fittings into the tube. Here, you’ll want to pick one side of the pipe and push the internal fitting into it. In most cases, this requires some working back and forth. Make sure it’s tightly in place.

In addition, if your fitting type requires sealant or glue, apply it now. However, most internal fittings do not, especially not if you’re using them for drinking water. 

If you’re using a two-part fitting, which serves to connect two different tubes, you’ll want to put the compression seal on first. 

Start with the nut and put it on one side of the tubing. Then, press the sealing ring onto that. 

Sealing rings always have a tapered edge. That tapered edge is designed to allow you to press a pipe onto it. Make sure it faces the side the compression fitting is on, not the nut. 

Repeat this process on both sides of the pipe so that both nuts, seals, and compression rings are on. Then, screw the compression fitting onto that side of the tube. 

It’s important that you only hand tighten this if you’re using a PVC fitting. 

Here, the joint will never be under pressure. In addition, compression nuts use a tapered thread. The PVC threads gradually narrow as they get closer to the base. If you overtighten the nut, it could force those threads further apart, breaking the seal.

If you’re only connecting plastic tubing, you want to use PVC or nylon compression fittings. Why? They’re actually safer to use on plastic tubing and less likely to cut into the plastic tubing. 

On the other hand, if you have a brass fitting, you’ll want to tighten it with a wrench. Here, you’ll want to hand tighten the fitting first and then use a wrench to tighten it further. In most cases, 2-3 turns past a hand-tighten are good enough.

You may also want to use pipe dope or PTFE tape. However, that may not be necessary. 

Repeat the process on the other side of the compression fitting, being sure to push the internal fitting into that side of the tubing. 

Advantages of Compression Fittings

There are many reasons to install a compression fitting rather than a screw fitting or solder. 

No need to solder

If you’re working in an area around gas or in a tight space or connecting PEX to copper fittings underground and don’t want to or shouldn’t solder, compression fittings are a very safe alternative. While they do cost more, the increase in safety can be considerable. 

Ideal for mixed-material pipes 

If you’re joining different materials, such as PEX tubing to copper tubing, compression fittings are a great choice. After all, you can’t solder PEX and you can’t use glue or clamps on copper.

Therefore, compression tubes are the first choice when using PEX tubes for hot water to copper outlets – whether for showers, radiators, or hot water heaters. 

Easy to disassemble 

If you know you’ll have to eventually disassemble your tubing, it makes sense to use compression fittings. That’s because compression fittings simply unscrew and come off.

In most cases, you can even reuse your compression fitting. However, you may want to replace the rings if the fitting is old. 

Related Questions

Can I use compressing fittings on plastic pipe or tubing? 

Yes. However, it is important to do so carefully.

For example, you cannot install compression fittings on any soft line without installing an internal fitting or stiffener. This stiffener may be as simple as a similarly sized internal pipe. However, you often get internal fittings made for the purpose. 

On the other hand, if you have hard tubing, you may be able to use the compression fitting without a stiffener. It is, however, extremely important that you double-check to ensure the tubing is hard enough to resist the compression without collapsing. 

Should I use brass or PVC compression fittings? 

In most cases, it’s a good idea to match materials in mixed materials. E.g., connect to copper with brass and connect to plastic with PVC. 

Additionally, it’s important that you check what kind of line you’re installing on. Refrigerators and other coolant lines normally aren’t strong enough for brass compression tubing, even with an insert. Instead, the brass fitting may cut the line. Therefore, you want to use nylon or PVC. 

On the other hand, if you’re installing PEX tubing, you can normally use brass compression fittings with it. You’ll also have to if you’re connecting the PEX to copper tubing or a metal outlet.

Can I use a ¼ inch compression fitting on a water line? 

In most cases, it’s safe to use a compression fitting on any water line, providing you properly stabilize it. Here, you normally only need an internal fitting. This will slightly narrow the water passage. However, that should be fine for most purposes. 

Can I use compression fittings on gas lines? 

Compression fittings are not safe to use on gas lines. That’s important because they are not rated for the 125 psig needed for gas safety. Therefore, you should never use compression fittings on your gas lines.

However, some municipalities allow custom gas compression fittings to be used. This varies per region and is heavily controlled by local regulation. 

Conclusion 

Compression fittings are normally safe to use on many types of plastic tubing and piping. Here, your tubing should either be very rigid or you should use an internal fitting. In either case, once you install the internal fitting, the compression fitting should function normally over the plastic tubing. 

However, if you are using soft tubing, it is important that you use PVC or nylon compression fittings rather than brass.

Brandy Cross

Brandy is a freelance writer with a background and strong interest in DIY and home repair, stemming from her childhood and days spent helping her dad who worked as a handyman. Today, she's renovating an apartment she shares with 100+ houseplants.

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