What Is a Compression Fitting?

A compression fitting is a mechanical plumbing connector used to join pipe or tubing to a valve, adapter, or another section of tubing. It seals by squeezing a ring, called a ferrule (also known as an olive), between the nut and the fitting body. This compresses the ferrule around the tube and against a tapered seat, forming a tight, leak-resistant joint without heat, glue, or special tools.

How It Works

A standard compression fitting has three parts:

• Body: The main fitting with a tapered seat and a stop for the tubing.
• Ferrule (olive): A soft ring (brass, copper, or plastic/Delrin) that deforms to create the seal.
• Compression nut: Tightened to drive the ferrule into the seat and onto the tubing.

As you tighten the nut, the ferrule grips the outside of the tube and presses into the body’s seat, sealing on two surfaces. The seal is not from thread engagement, so thread tape or pipe dope is not used on the compression threads. If the fitting includes a separate NPT or BSPT threaded side (for example, a compression-to-NPT adapter), only that tapered pipe thread may need thread sealant.

Common Uses in DIY and Home Improvement

• Under-sink shutoff valves (angle stops/straight stops) on copper, CPVC, or PEX (with inserts)
• Faucet and toilet supply lines (3/8 in. OD compression typical in North America)
• Dishwasher, refrigerator, and ice maker hookups
• Quick repairs on copper lines using compression couplings or unions
• Baseboard heating and hydronic connections where allowed and accessible
• Water filtration and RO systems using small-diameter tubing

Many codes require compression joints to remain accessible (not buried in walls or underground), because they can need retightening or service.

Types and Variations

• By shape:
  • Straight couplings/unions: Connect two lengths of tubing
  • Elbows and tees: Change direction or branch lines
  • Stop valves: Shutoff valves with a compression outlet to the fixture
  • Bulkhead fittings: Pass through a tank or panel
• By material:
  • Brass: Most common for potable water; look for lead-free and DZR (dezincification-resistant) brass in aggressive water
  • Stainless steel: For corrosion resistance or higher pressure
  • Plastic/acetyl ferrules: Often used on plastic tubing to avoid cutting into the tube
• By tubing type:
  • Copper: Rigid or soft copper; no insert required
  • PEX/CPVC/plastic: Requires a tube stiffener/insert and often a plastic ferrule
• By standard/size:
  • North America: 1/4 in., 3/8 in., 1/2 in. outer diameter (OD) common for supplies
  • UK/EU: 10 mm, 15 mm, 22 mm common
• Single- vs double-ferrule:
  • Single-ferrule is typical in household plumbing
  • Double-ferrule (instrumentation style, e.g., Swagelok) offers higher performance but is uncommon in typical home projects

Choosing the Right Compression Fitting

• Match the tubing OD: Compression fittings are sized by the tube’s outer diameter (e.g., 3/8 in. OD). Don’t mix metric and imperial parts.
• Material compatibility: Use brass with copper; plastic ferrules and tube inserts for PEX or plastic. For aggressive water or coastal areas, consider DZR brass or stainless.
• Code compliance: Keep compression joints accessible. For gas lines, choose approved flare fittings—compression fittings are commonly not allowed.
• Valve style: Quarter-turn ball-type stops last longer than multi-turn compression stops.
• Pressure and temperature rating: Check the manufacturer’s specs for heating systems or hot water applications.

How to Install a Compression Fitting (Step by Step)

1. Shut off water and relieve pressure. Open a nearby faucet to drain residual water.
2. Cut the tubing square. Use a tubing cutter for copper. For plastic, use a sharp, square cut. Avoid hacksaw burrs.
3. Deburr and clean. Lightly ream the inside and smooth the outside edge. Wipe away grit; debris can cause leaks.
4. Slide on the nut, then the ferrule. Orientation matters: the nut threads toward the fitting body; the ferrule’s tapered side matches the fitting’s seat.
5. Seat the tube fully. Push the tube into the fitting body until it stops. Mark the tube to confirm it stays fully seated while tightening.
6. Hand-tighten the nut. Then use two wrenches: one to hold the fitting body (backup wrench), one to tighten the nut.
7. Tighten to spec. Typical guidance is hand-tight plus about 1/2 to 3/4 turn. Follow the manufacturer’s instructions.
8. Pressurize and check for leaks. If you see a drip, tighten in small increments (1/8 turn) until it seals. If a leak persists, disassemble, inspect for scratches or debris, and replace the ferrule if needed.

Notes:
- Do not wrap the compression threads with PTFE tape; it can prevent proper ferrule seating. If you have a compression-to-NPT adapter, apply sealant only on the NPT side.
- For PEX or plastic tubing, always use the correct tube insert/stiffener so the ferrule can grip without crushing the tube.

Maintenance and Reuse

• Periodic checks: After installation, check once after a day and again after a week for any weeping. Retighten a small amount if needed.
• Reusing parts: You can often reuse the fitting body and nut, but the ferrule typically stays stuck on the tube. Replace the ferrule, or cut the tube behind it and start fresh. Ferrule pullers can help remove a stuck ferrule without cutting.
• Accessibility: Keep joints where you can reach them. Avoid burying in walls, ceilings, or soil.

Common Mistakes to Avoid

• Overtightening: Crushed tubing or a split ferrule can cause leaks. Tighten gradually.
• Under-tightening: The ferrule needs enough compression to seal. If it weeps, add small turns.
• Using thread tape on compression threads: The seal is at the ferrule/seat, not the threads.
• Skipping deburring: Burrs can cut ferrules or prevent proper seating.
• Poor alignment: Misaligned tubing won’t seat fully and may leak.
• Mixing sizes or standards: Don’t combine metric ferrules with imperial fittings.
• Reusing a deformed ferrule: Once compressed, a ferrule rarely reseals if moved.
• No insert on plastic/PEX: The tube can collapse and leak.
• Hiding the joint: Keep it accessible for future service.

Related Terms

• Flare fitting: Uses a flared tube end and a flare nut; common for gas and refrigeration.
• Sweat/solder fitting: Joined with solder and a torch on copper pipe.
• Push-fit fitting: O-ring sealed, tool-free connectors (e.g., SharkBite) for many pipe types.
• NPT/BSPT threads: Tapered pipe threads that seal with tape or paste (not the same as compression threads).
• Slip-joint (drain) fittings: Compression-style nuts and washers in sink drains; different sealing method and materials than water supply compression fittings.

Practical Examples

• Adding a new faucet stop valve: Cut the copper stub-out square, deburr, slide on nut and ferrule, push the valve body on fully, then tighten. Connect a 3/8 in. OD compression supply line to the faucet.
• Repairing a pinhole in copper: Cut out the damaged section, clean ends, and install a compression coupling with two ferrules and nuts. Tighten, test, and leave the joint accessible.
• Installing an ice maker line: Run 1/4 in. OD tubing from a nearby cold shutoff valve (with a 1/4 in. compression outlet), using compression nuts and ferrules at each end. For plastic tubing, use plastic ferrules and tube inserts.

By understanding the parts, choosing the right materials, and following careful tightening steps, you can make reliable, leak-free connections without soldering or special equipment.