Pipe Fitting Compatibility Reference: Copper to Plastic Transitions, DZR Brass, Dezincification Risk and Inserts
Quick Answer: Copper pipe uses compression fittings (Type A for copper, Type B for plastic) or solder fittings. Push-fit fittings require a stainless-steel insert (pipe stiffener) for plastic pipe (PEX, PB, MLCP) but not for copper. Never use standard brass compression fittings with hot water plastic pipes without DZR (dezincification-resistant) brass — ordinary brass fails over 5–10 years. MLCP (multilayer composite pipe) requires its own dedicated fittings.
Summary
Plumbing pipe compatibility is a frequent source of callbacks and installation failures. The UK has moved increasingly toward plastic pipework (Speedfit, Hep2O, polypipe, MLCP) for domestic plumbing, but the fittings rules are more complex than they appear.
The three most common compatibility failures are:
- Missing pipe insert in plastic tube push-fit or compression — the fitting crushes the tube causing leaks
- Standard brass used in hot plastic — dezincification eventually blocks or destroys the fitting
- MLCP fittings mixed with PE-X/PB — MLCP requires a pressed or dedicated push-fit fitting; standard push-fit collets designed for PB/PEX will not secure MLCP adequately
For tradespeople, knowing the compatibility matrix prevents costly callbacks, warranty issues, and water damage — all of which are disproportionately expensive relative to the cost of using the correct fitting.
Key Facts
- Pipe insert (stiffener sleeve) — Required in all plastic tube when using push-fit or Type B compression fittings. Not needed for copper. The insert prevents the O-ring from collapsing the tube wall.
- DZR brass (dezincification resistant) — Required for hot and cold water systems where water is soft or aggressive (pH <7 or hardness <100 mg/L CaCO3). Fittings marked "CR" (corrosion resistant) or with a triangle symbol. Check WRAS approval.
- Dezincification — Occurs when the zinc component of standard brass leaches out in aggressive water, leaving porous copper behind. Fitting fails structurally over 5–10 years. Common in Yorkshire, Scotland, south-west England where water is soft.
- Speedfit vs Hep2O interchangeability — Speedfit (JG) and Hep2O (Wavin) push-fit fittings use compatible O-ring/collet sizes for the same pipe OD. Fittings from one brand generally work with pipe from the other. However: mixing collets and seals is not recommended.
- MLCP (multilayer composite pipe) — Aluminium barrier layer bonded between inner and outer polyethylene. Cannot be used with standard push-fit collets (designed for single-layer plastic). Requires: MLCP-specific push-fit (some brands), press-fit (Viega, Sanha, Frabo), or MLCP-compatible compression fittings.
- Press-fit fittings — Use a hydraulic pressing tool to deform a stainless sleeve around the pipe. No insertion required for copper; different jaws required for each pipe material and OD. Jaw compatibility is brand-specific.
- Soldering copper — End-feed or solder-ring (Yorkshire) fittings. End-feed requires separate lead-free solder wire. Solder-ring has integral solder ring. Both require flux (non-aggressive plumbing flux or dental flux — not electrical acid flux).
- Pipe OD standards — 15mm, 22mm, 28mm (metric) for standard UK plumbing. Old Imperial: 1/2" OD = 15mm, 3/4" OD = 22mm. Copper and compatible plastic share same OD.
- WRAS approval — All fittings for UK potable water systems should be WRAS-approved (Water Regulations Advisory Scheme). Non-WRAS fittings may not comply with Water Supply Regulations.
Quick Reference Table: Fitting Selection
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Try squote free →| Pipe Type | Push-Fit | Compression | Press-Fit | Solder | Insert Required? |
|---|---|---|---|---|---|
| Copper | Yes (no insert) | Type A | Yes | Yes | No |
| PB (polybutylene) | Yes | Type B | No | No | Yes |
| PEX-a (crosslinked polyethylene) | Yes | Type B | No | No | Yes |
| PEX-b | Yes | Type B | No | No | Yes |
| MLCP (multilayer composite) | MLCP-specific only | MLCP-specific | Yes (MLCP jaws) | No | No (press-fit) / Yes (compression) |
| HDPE | No | Specialist only | Yes (specialist) | No | Specialist |
Compatibility Notes by Product
| Scenario | Compatible? | Notes |
|---|---|---|
| Speedfit collet with Hep2O pipe | Generally yes | Same nominal OD; O-ring seal governs — check body seal is intact |
| Copper in Speedfit fitting (no insert) | Yes | Copper doesn't need insert |
| Plastic in Speedfit fitting without insert | No — leaks | Insert mandatory |
| Speedfit collet with MLCP | No | MLCP requires dedicated collet/fitting |
| Standard brass compression on hot plastic | Avoid | Dezincification risk; use DZR brass |
| Lead-free solder on copper | Yes | Mandatory for potable water (BS EN 12502-3) |
| Press-fit jaws mixed between brands | No | Jaw geometry is brand-specific |
Detailed Guidance
When DZR Brass Is Required
Check your local water supply hardness using your water company's website. Postcode lookup tools are available:
- Soft water (<100 mg/L as CaCO3): DZR mandatory. Includes most of: Scotland, Wales, south-west England, northern England, Yorkshire.
- Moderately hard water (100–200 mg/L): DZR still recommended for hot water systems. Normal brass may be acceptable for cold.
- Hard water (>200 mg/L): Standard brass generally acceptable. DZR is still best practice.
Standard brass: Cu58-Zn42 alloy. Susceptible to dezincification. Visual sign: pinkish copper-coloured deposits around fitting; green/white mineral deposits; fitting becomes soft or crumbly.
DZR brass: marked with triangle symbol on fitting, or "DR" or "CR" in product code. BS EN 12164 Alloy CW602N is the standard DZR grade.
Always use DZR for:
- All fittings in hot water systems (above 60°C)
- All fittings in central heating (above 60°C, especially with inhibitor)
- Any system in soft water areas
Pipe Inserts: Mandatory for Plastic
When connecting plastic pipe to any compression or push-fit fitting, always insert the stiffener sleeve first:
- Cut pipe with specialist rotary cutter (not hacksaw — produces swarf inside pipe)
- Deburr inside of cut end with reamer or deburring tool
- Insert the pipe stiffener (insert) flush with the pipe end
- Assemble fitting over pipe as normal
Copper does not need an insert — it is rigid enough to resist deformation by the fitting.
Without insert: The olive (compression) or O-ring/collet (push-fit) grips the soft plastic and deforms the pipe wall. The fitting may appear to hold under pressure test but will weep or fail over time, or fail immediately under repeated thermal cycling.
MLCP Specifics
MLCP (multilayer composite pipe, also called barrier pipe) has a sandwich construction: inner polyethylene, aluminium layer, outer polyethylene. This makes it semi-rigid — it can be bent to shape and retains form.
Approved fitting systems for MLCP:
- Press-fit (preferred) — Viega Raxofix/Raxpress, Sanha, Frabo. Press with appropriate MLCP jaws. Leak-before-press feature on quality fittings.
- Push-fit (MLCP-specific) — Some manufacturers (Wavin) offer push-fit specifically tested for MLCP. These have modified collets.
- Compression (MLCP-specific) — Available; insert is required. Not as widely specified.
Never use standard push-fit collets (designed for PB/PEX) with MLCP — the collet mechanism is designed for the elasticity of single-layer plastic. MLCP is more rigid; the collet does not grip adequately and the fitting can pull off under pressure.
Soldering Copper
Key rules:
- Always use lead-free solder (BS EN 29453 grade S-Sn97Cu3 or similar). Lead solder is prohibited for potable water.
- Use non-aggressive plumbing flux. Electrical acid flux is corrosive and must not be used.
- Clean the pipe end and fitting socket with scotchbrite or emery cloth before fluxing.
- Heat the fitting body, not the pipe — this draws solder into the joint.
- On end-feed fittings: heat until the solder flows smoothly all the way round the joint with no gaps.
- Wipe away excess solder and flux immediately with damp cloth while still hot.
- Allow to cool before moving the joint.
Frequently Asked Questions
Can I use the same push-fit fittings for copper and plastic?
Yes — most Speedfit and Hep2O push-fit fittings accept both copper and plastic at the same nominal size. The difference is that plastic requires a pipe insert (stiffener), copper does not. Some fittings come with inserts pre-loaded for plastic use.
How do I remove a push-fit fitting from plastic pipe?
Insert the proprietary disconnect tool (available from Speedfit and Hep2O — these are small plastic rings that depress the collet). Slide the disconnect tool over the pipe to the fitting collar; push in and simultaneously pull the pipe. Without the correct tool, you will damage the fitting or the pipe.
My compression fitting is leaking at the olive — what has happened?
Common causes: (1) The olive has been over-compressed (too many turns) — the olive deforms and loses sealing capacity; replace with new olive. (2) Plastic pipe used without insert — deformation under the olive; re-do with insert. (3) Wrong olive for the pipe material — ensure the olive matches the pipe type (annealed copper olive for copper; specifically harder olive for MLCP).
Regulations & Standards
Water Supply (Water Fittings) Regulations 1999 — WRAS approval required for potable water fittings
BS EN 12164:2016 — Copper and copper alloys: dezincification resistant alloys
BS EN 1254-2 — Copper and copper alloys: compression fittings for use with copper tubes
BS EN 1254-3 — Compression fittings for use with plastic pipes
BS EN 12502-3 — Protection of metallic materials against corrosion: guidance for soldering
copper soldering — Detailed soldering technique
push fit fittings — Push-fit overview
pipe materials — Pipe material comparison
water regulations — WRAS and Water Supply Regulations