Summary

Compression fittings are the most widely used mechanical jointing method for copper pipe in UK domestic plumbing and heating. They work by trapping a soft brass or copper olive between the fitting body and a threaded compression nut; as the nut is tightened, the olive deforms against the pipe to create a watertight seal. Available in sizes from 6 mm to 108 mm, the vast majority of domestic work uses 15 mm and 22 mm fittings. They are suitable for both hot and cold water supply (up to 85-99 degrees C depending on manufacturer) and central heating, with a rated working pressure of up to 25 bar. Unlike soldered capillary fittings, compression joints can be assembled and disassembled without heat, making them essential for repair work, connections near timber, and situations where pipes cannot be fully drained.

Key Facts

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  • Standard domestic sizes: 8 mm, 10 mm, 15 mm, 22 mm, 28 mm — with 15 mm and 22 mm accounting for the majority of domestic work
  • Rated to BS EN 1254-2 (copper tube) or BS EN 1254-3 (plastic/multilayer pipe)
  • Maximum working pressure typically 10-25 bar depending on size and manufacturer
  • Maximum operating temperature 85-99 degrees C (check manufacturer data sheet)
  • Fittings for concealed or underground use must be DZR (dezincification resistant) brass, marked "CR"
  • The Water Supply (Water Fittings) Regulations 1999 prohibit compression fittings in concealed locations unless accessible via an inspection hatch
  • Compression fittings are classified as Type A (non-manipulative, with olive) or Type B (manipulative, without olive) — Type A is standard in UK domestic work
  • Olives should never be reused — always fit a new olive when remaking a joint

Detailed Guidance

What types of compression fitting are available?

Fitting Type Description Common Use
Straight coupling Joins two pipes in line, same diameter Extending pipe runs, repairs
Reducing coupling Joins two pipes in line, different diameters Stepping down from 22 mm to 15 mm
90-degree elbow Changes direction by 90 degrees Corners, pipe drops
45-degree elbow Changes direction by 45 degrees Gradual direction changes
Equal tee Three-way junction, all same size Branch connections
Reducing tee Three-way junction with reduced branch 22 mm flow with 15 mm branch-off
Stop end Caps off a pipe end Temporary or permanent termination
Tap connector Compression to BSP thread Connecting to taps, valves, cylinders
Tank connector Through-wall fitting with backnut Cold water storage tanks, cisterns
Flexible connector Compression ends with flexible body Awkward connections, vibration isolation

Most manufacturers also offer male and female iron adaptors for connecting compression pipework to BSP-threaded components such as boilers, pumps, and zone valves.

How do compression fittings compare with push-fit and soldered joints?

Factor Compression Push-Fit Soldered (Capillary)
Skill level Moderate — needs correct technique Low — cut, insert, done High — requires soldering competence
Tools required Two adjustable spanners Pipe cutter, deburrer only Blowtorch, flux, solder, heat mat
Speed Moderate Fast Slow (including cool-down)
Demountable Yes — can be undone and remade Yes — with release tool No — must be cut out
Cost per joint (15 mm elbow) Approx. 1.00-1.80 Approx. 2.00-3.50 Approx. 0.30-0.60
Concealed/buried use No (unless accessible) No (O-ring degradation risk) Yes — preferred method
Max temperature 85-99 degrees C 65-82 degrees C (varies by brand) 110+ degrees C
Max pressure 10-25 bar 6-10 bar 10+ bar
Lifespan 50+ years 25-50 years (O-ring dependent) 50+ years
Use on wet pipes Yes Yes No — must be dry
Fire risk None None Yes — blowtorch required
BS EN standard BS EN 1254-2 BS EN 1254-4 / manufacturer BS EN 1254-1

When should you use compression fittings?

Compression is the right choice when:

  • The joint must be accessible and demountable — service valves, isolation valves, appliance connections
  • Working near combustible materials (timber joists, insulation) where a blowtorch would require a fire permit or heat mat
  • Pipes are wet or cannot be fully drained — compression works on damp pipes, solder does not
  • Making repair joints on existing pipework where cutting out and resoldering is impractical
  • Connecting to threaded components — boiler unions, cylinder connections, pump valves
  • In confined spaces where there is insufficient clearance for a blowtorch
  • On unvented hot water cylinder connections (typically 22 mm compression, supplied with gland nuts and olives)

Compression is NOT suitable when:

  • The joint will be concealed within a wall, floor, or ceiling without an access panel — use solder or press-fit
  • The fitting will be buried underground without access — use solder; if compression must be used, it must be DZR/CR brass
  • On discharge pipework from T&P relief valves on unvented cylinders — Building Regulations Part G requires metallic pipework, and these joints should be soldered
  • Within 1 metre of a boiler casing — residual heat can exceed fitting ratings; use copper with soldered joints

What sizes are used in domestic plumbing?

Pipe Size Typical Application
8 mm Micro-bore heating circuits, some gas meter connections
10 mm Micro-bore heating circuits
15 mm Cold and hot water distribution, radiator tails, most domestic pipework
22 mm Hot water cylinder connections, boiler flow/return, mains cold feed, bath taps
28 mm Larger heating circuits, commercial, multiple-bathroom properties
35 mm Light commercial, larger distribution
42 mm Commercial heating mains
54 mm+ Commercial and industrial — less common in compression, usually press-fit or flanged

How do you install a compression fitting correctly?

  1. Cut the pipe square using a pipe slice or tube cutter — not a hacksaw. An angled cut prevents the olive from seating evenly.
  2. Deburr inside and out using the reamer on your tube cutter or a dedicated deburring tool. Burrs damage the olive and cause turbulence.
  3. Slide the nut onto the pipe (thread facing the fitting), then the olive.
  4. Insert the pipe fully into the fitting body until it hits the internal pipe stop. Mark the pipe at the fitting edge with a pen so you can confirm it hasn't pulled out during tightening.
  5. Hand-tighten the nut until finger-tight.
  6. Tighten with two spanners — one holding the fitting body, one turning the nut. This is critical: using one spanner risks twisting the pipe and breaking adjacent joints.
  7. Turn approximately 1 to 1.25 turns past hand-tight. Over-tightening crushes the olive and can split the pipe. Under-tightening leaks.
  8. Pressure test before concealing any pipework.

Pro tips:

  • Apply a thin smear of jointing compound (e.g. Fernox LS-X or Boss White) to the olive before assembly. This lubricates the olive for a better seal and makes future disassembly easier. PTFE tape on the olive is also acceptable but compound is preferred.
  • If remaking a joint, always replace the olive. A used olive has already deformed and will not re-seal reliably. If the olive is stuck, use an olive puller or carefully split it with a junior hacksaw.
  • For plastic pipe in compression fittings, always use a pipe insert (stiffener) to prevent the pipe collapsing when the olive compresses.
  • On 22 mm and 28 mm fittings, consider using a pipe wrench for better grip than an adjustable spanner.

What is DZR brass and when is it required?

DZR (Dezincification Resistant) brass contains small amounts of arsenic (up to 0.15%) that inhibit the chemical leaching of zinc from the alloy. Standard brass fittings can suffer dezincification over time — particularly in aggressive water conditions — leaving a weak, porous copper shell that eventually fails.

DZR fittings are legally required when:

  • The fitting will be a concealed water fitting (Water Supply Regulations 1999, Regulation 4, Schedule 2)
  • The fitting is installed underground
  • The fitting is in contact with aggressive water (low pH, high chloride)

DZR fittings are marked with "CR" (Corrosion Resistant) stamped on the body. If in doubt, use DZR — the cost premium is minimal and the failure mode of standard brass is catastrophic.

Frequently Asked Questions

Can I use compression fittings on plastic pipe?

Yes, but you must use a pipe insert (metal stiffener sleeve) inside the plastic pipe to prevent it collapsing under the olive's compression. Some manufacturers produce compression fittings specifically designed for plastic pipe to BS EN 1254-3, with longer bodies and deeper pipe stops. Check the fitting is rated for the pipe type (PB, PEX, or multilayer) — they are not all interchangeable.

How many times can I redo a compression joint?

You can redo the joint as many times as needed, but you must fit a new olive each time. The fitting body and nut can be reused indefinitely provided they are not damaged. If the pipe end is scored or damaged from the previous olive, cut back to clean copper and start fresh.

Why is my compression joint leaking?

The five most common causes are:

  1. Pipe not fully inserted to the internal stop — the olive is not seated in the correct position
  2. Pipe not cut square — the olive cannot compress evenly around the circumference
  3. Burrs left on the pipe — preventing the olive from seating flush
  4. Under-tightened — needs another quarter to half turn
  5. Over-tightened — the olive has been crushed past its effective sealing range, or the pipe has been distorted. Replace the olive and try again.

If a joint weeps after initial pressurisation, try a quarter turn more with two spanners. If that does not solve it, strip the joint, inspect the olive and pipe end, and reassemble with a new olive.

Are compression fittings legal on gas?

Compression fittings are used on gas installations, but only fittings specifically approved for gas use. Gas compression fittings are manufactured to BS EN 1254-2 and must carry the appropriate certification. All gas work must be carried out by a Gas Safe registered engineer. The jointing compound used must also be gas-rated.

Regulations & Standards