Compression Fittings: A Trade Guide to DZR Brass, Olives, and Sizing

Compression fittings are one of the most versatile joints in the trade toolkit. No heat, no flux, no torch — just a spanner and a straight pull. But get the wrong fitting for the job — wrong size, wrong brass spec, or the olive seated badly — and you’re back the next day with a drip. This guide covers everything you need to know to spec and fit compression correctly.

How a Compression Fitting Works

A Type A (non-manipulative) compression fitting has three components: the fitting body, the olive (also called a ferrule), and the compression nut. When you tighten the nut, it drives the olive into a tapered seat in the body. The olive deforms slightly and bites into the outside of the copper tube, creating a watertight mechanical seal.

No solder, no glue, no pipe prep beyond a clean square cut. This makes compression the go-to for:

• Repairs where you can’t use a torch (near joists, in ceiling voids, near gas)
• Connections to existing pipework you don’t want to disturb
• Chrome-exposed pipework under sinks and basins
• Connections to appliances and flexible hoses
• Any situation where the joint may need to be remade later

Type B (manipulative) compression is less common in UK domestic plumbing — it requires the pipe end to be flared or beaded before fitting and is mainly used in refrigeration and some industrial applications. For heating and plumbing, you’ll almost always be using Type A.

Sizing: 10mm to 42mm

Compression fittings follow the same nominal sizes as copper tube to BS EN 1057:

• 10mm — microbore, used for underfloor heating and some older radiator circuits
• 15mm — the standard domestic size. Hot and cold supplies, radiator connections, basin tails
• 22mm — mains-fed cold supply, boiler flow and return, primary circuits
• 28mm — gravity-fed primaries, some commercial heating
• 35mm and 42mm — commercial and industrial heating, mains water connections over 25m runs

Don’t confuse nominal bore with outside diameter. 15mm copper tube has an OD of 15mm — the size refers to the tube outside diameter, which is what the compression fitting grips. This differs from older imperial pipework: 1/2" nominal BSP copper has a 15.9mm OD, so a modern 15mm compression fitting won’t seal on it. Always check what you’re connecting to.

DZR Brass: When You Need It

Standard brass is a copper-zinc alloy. In aggressive water conditions — soft water, acidic water, high chloride areas — zinc can leach out of the alloy through a process called dezincification, leaving behind a porous copper sponge that looks intact but has no strength. The fitting fails without warning.

DZR (Dezincification Resistant) brass has a modified alloy composition — typically with added arsenic — that prevents this. It’s identifiable by a red dot or ring on the fitting body. Water suppliers in soft-water areas (much of Scotland, Wales, South West England, and parts of the Midlands) may require DZR fittings. WRAS (Water Regulations Advisory Scheme) approval covers both standard and DZR grades — check the fitting’s marking.

For heating-only applications (closed systems, no potable water contact) standard brass is fine. For cold water mains, hot water service, and particularly outdoor or underground service connections, DZR is the safe spec. Embrass Peerless manufacture their compression range in both standard and DZR — their DZR fittings are clearly marked and WRAS approved for potable water.

Standard vs Compact Compression Fittings

Standard (or "heavy pattern") compression fittings have a longer body and larger nut — they’re easier to grip with a spanner and give a more robust fixing in accessible locations. Compact compression fittings achieve the same watertight seal in a shorter body profile. They’re designed for tight spaces: inside meter boxes, under-sink installations, inside consumer unit compartments, or anywhere that a full-length fitting won’t swing in.

The olive and sealing principle is identical. Compact fittings use a smaller spanner flat — typically needing a 22mm spanner for a 15mm compact fitting versus 20mm for heavy pattern — so carry both sizes if you’re doing a mixed job.

APM stocks the full Embrass Peerless compact compression range:

• Embrass Peerless 15mm Compact Compression Coupler — straight joint for in-line repairs and extensions
• Embrass Peerless 22mm Compact Compression Elbow — 90° direction change in tight spaces
• Embrass Peerless 22mm Compact Compression Tee — branch connections on 22mm mains-fed circuits
• Embrass Peerless 15mm Compact Compression Stopend — capping off dead legs

Copper Olives vs Brass Olives

Copper olives are softer and deform more easily — they’re the standard choice and the one most experienced plumbers default to. They work well on copper tube and grip consistently when tightened correctly.

Brass olives are harder and more resistant to creep over time. They’re preferred on stainless steel tube (which copper olives don’t grip as effectively), and some plumbers use them on boiler connections where thermal cycling is aggressive. Brass olives are also less likely to be accidentally over-tightened to the point of failure.

Keep a stock of both. Embrass Peerless 22mm Copper Olives (Pack of 10) cover most heating work. For the odd stainless or high-temperature application, brass olives are worth having on the van.

Fitting Types: What’s in the Range

Straight coupler — equal diameter join, the most common repair fitting. Comes in equal and reducing versions (e.g. 22mm x 15mm) for pipe size changes.

Elbow — 90° bend. Equal elbows (15mm, 22mm) for direction changes. Reducing elbows for size transitions at bends.

Tee — branch connection. Equal tees for same-size branches; reducing tees for smaller branch off a larger main.

Wallplate elbow — 90° elbow with a threaded female BSP backplate for fixing to a wall or joist, used for basin tails, bath tap tails, shower inlets. The Embrass Peerless 15mm x 1/2" HP Compression Wallplate Elbow is the standard fitting for this application.

Male/female adapters (couplers) — one compression end, one BSP threaded end. Used to connect copper pipework to appliances, valves, or fittings with BSP threads. Female couplers take a male threaded appliance connection; male couplers connect into female-threaded valves and bodies.

Stop end (plug) — caps off a pipe end. Use to terminate dead legs when decommissioning a branch or temporarily capping during a phased installation.

Air vent — the Embrass Peerless 15mm Compression Air Vent screws into a compression tee to provide a manual bleed point on a heating circuit. Useful when you can’t position an automatic air valve.

Installation: Getting It Right First Time

Cut the tube square — a pipe slice is faster than a hacksaw and leaves no burr. If you use a hacksaw, deburr the inside and outside of the cut. Any burr sitting under the olive will cause a slow leak that doesn’t show up on initial fill.

Push the tube fully home into the fitting body before tightening. You should feel it seat against the internal stop. If the tube is short by even 2-3mm, the olive won’t engage properly and the fitting will leak at pressure.

Hand-tighten the nut fully, then add one full turn with a spanner — no more. On 15mm, that’s typically 1 to 1¼ turns past hand-tight. On 22mm and above, 1 full turn is usually sufficient. Over-tightening splits the olive and distorts the fitting seat; the joint will weep and you’ll need to cut it out and start again.

For re-use: if you undo a compression fitting to reposition a pipe, replace the olive. A deformed olive won’t reseal reliably on a new tube end. Keep spare olives on the van — they’re cheap insurance.

When Not to Use Compression

Compression fittings rely on a correctly-formed tube end. They don’t work on plastic pipe (Hep2O, barrier pipe, CPVC) without an insert stiffener — and even then, you should be using the manufacturer’s own push-fit or compression system. Using a brass compression fitting directly on plastic pipe without a stiffener will deform the tube wall and leak.

For fully concealed joints in floor screeds, wall chases, or inaccessible ceiling voids, end-feed solder or press-fit is the better long-term choice — compression fittings can loosen over time with thermal movement when they can’t be inspected.

Stock It, Pick It Up Today

APM Electricals in Acton stocks the full Embrass Peerless compression fittings range and the complete compact compression fittings range — 10mm through 42mm, standard and DZR, copper and brass olives. Trade counter open Monday to Friday. Call ahead on 020 8702 8080 to check stock, or come in to 24 Western Avenue, Acton W3 7TZ. Same-day collection, no minimum order.