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

Compression fittings are a practical choice for joining copper tube where soldering is unsuitable, restricted or simply unnecessary. They are quick to assemble, easy to inspect and can be disconnected later if the installation needs alteration. The important part is choosing the correct fitting, pipe size and material for the service—not treating every brass fitting as interchangeable.

This guide covers the everyday sizes and fitting types used by UK plumbers, including how the nut and olive make the seal, when DZR brass matters, and the checks that prevent leaks. If you need help matching a fitting to the pipe or application, call APM on 020 8702 8080 or visit us at 24 Western Avenue, Acton, W3 7TZ.

Quick Shop: Compression Fittings Available from APM

These are useful examples from APM’s current online range. Check the product page for the exact size and specification required for your job.

15mm Compact Compression Elbow

A standard 90-degree fitting for changing direction on 15mm copper pipe without soldering.

15mm × 1/2-inch Male Compression Coupler

Connects 15mm copper tube to a compatible 1/2-inch female threaded connection.

22mm Compact Compression Tee

Creates a three-way 22mm branch for suitable water and heating pipework.

22mm × 15mm Reducing Coupler

Provides a direct transition from 22mm tube to a 15mm branch or appliance supply.

22mm Compression Stop End

Caps a 22mm pipe end during alteration, testing or permanent termination where appropriate.

How a Compression Joint Seals

A standard compression fitting has three working parts: the fitting body, the compression nut and the olive. As the nut is tightened, the olive is compressed between the nut and the fitting body. The olive grips the outside of the tube and forms the seal.

The pipe end must be round, clean and fully inserted. A badly cut or distorted tube can prevent the olive seating evenly. Cut the tube square with an appropriate pipe cutter, remove internal and external burrs, clean away swarf, and check that the pipe surface is not deeply scratched where the olive will sit.

Slide the nut and olive onto the tube, push the pipe fully into the fitting, then bring the nut up by hand. Tighten with suitable spanners while supporting the fitting body so connected pipework is not twisted. Manufacturer instructions take priority over a generic turn count. Over-tightening can distort the olive or fitting; under-tightening may leave the joint weeping.

Choosing the Correct Size and Connection

For UK copper plumbing, 15mm and 22mm are common everyday sizes, while larger tube is used where the design flow requires it. Match the fitting to the actual outside diameter of the tube. Do not assume that a threaded size and a tube size are the same measurement: a 15mm × 1/2-inch male coupler, for example, joins 15mm tube to a 1/2-inch threaded connection.

Use elbows to change direction, tees to form branches, straight couplers to join equal sizes, reducing couplers to move between sizes, and male or female adaptors where tube meets threaded equipment. Confirm thread type and mating connection before installation. A fitting that screws together is not automatically the correct or safe combination.

DZR Brass: Where Material Selection Matters

DZR means dezincification-resistant. It is a brass formulation intended to resist selective loss of zinc in conditions where ordinary brass may be vulnerable. The need for DZR depends on the water conditions, component, application and relevant specification.

Do not assume every yellow-brass compression fitting is DZR. Check the manufacturer’s description, markings and approval information for the exact product. For drinking-water systems, incoming services, valves and other critical duties, confirm that the component is suitable for the intended service and installation environment. Gas, refrigeration and specialist systems require fittings specifically approved for that duty; a general water compression fitting must not be substituted.

Copper Olives, Brass Olives and Reusing Joints

The olive supplied or specified for the fitting should normally be used. Copper olives are softer and can conform readily around the tube. Brass olives are harder and are common in many compression products. The right choice depends on the fitting design and manufacturer’s instructions rather than a blanket rule.

A dismantled joint deserves close inspection. If an olive is damaged, heavily compressed or poorly positioned, replace the affected parts rather than forcing the old joint back together. Removing a fixed olive can mark the tube; if the sealing area is damaged, cut back to sound pipe where practical. Never stack olives or add a second olive to compensate for a failed seal.

Jointing Compound and PTFE Tape

A correctly assembled compression joint seals at the olive, not at the nut thread. PTFE tape should not be wrapped around the compression nut thread as a substitute for correct assembly. Where a manufacturer permits a suitable jointing compound, use only a small amount in the stated location and keep it out of the waterway. Threaded adaptor connections are different: the correct sealing method depends on the thread form and equipment instructions.

Final Checks Before Handover

• Confirm fitting material, pipe size, thread and service suitability.
• Make sure the pipe is fully inserted and the fitting is supported.
• Pressure-test in accordance with the system requirements and inspect every joint.
• If a joint weeps, isolate and depressurise before adjustment.
• Do not keep tightening indefinitely; remake a defective joint with sound components.
• Leave concealed joints accessible where the installation design and applicable requirements call for access.

Need the right fitting before you start? Browse the linked APM products above or call 020 8702 8080. You can also bring the old fitting or clear measurements to APM Plumbing & Electrical, 24 Western Avenue, Acton, W3 7TZ, and we will help identify the correct replacement from the available range.