When compression, soldered, push-fit and press joints suit a job, and the strengths of each.
Choosing the right joint is about matching the method to the pipe material, the location and whether the joint may need to come apart later. Here are the common options and where each shines.
Compression joints use a nut that squeezes an olive onto the pipe to seal it. They need no heat, can be undone and remade, and suit awkward positions or jobs where a flame is not wanted, though they are bulkier and rely on correct tightening.
Soldered, or capillary, joints draw molten solder into the gap between a copper fitting and pipe by capillary action. They give a neat, strong, permanent connection that is ideal for runs of copper, but they need heat and clean, properly prepared surfaces.
Pressure push-fit fittings generally use a seal and a retaining ring. Cut and prepare compatible pipe correctly, fit the specified support insert where required, and push to the full insertion depth before checking retention. Waste ring-seal sockets are a different design and must not be assumed to use the same components.
Press (press-fit) joints use a special tool to crimp a fitting permanently onto the pipe with a seal inside. They are fast and flame-free, which suits sites where hot work is restricted, but they need the right tool and jaws. Lawton's procedure for its copper press fittings: check the O-ring is present and seated, cut the tube square, deburr it, push it in with a twisting motion and mark the tube with a permanent pen to show it is fully inserted. With every tube in, check the depth marks again, then place the jaw around the fitting's bead and press. Afterwards the depth marks should still be in the same place. Its fittings up to and including 54 mm have a leak-before-press O-ring, so a joint that was assembled but never pressed leaks when the system is filled or tested, and is found straight away.
Match the fitting to the pipe, fluid, temperature, pressure and location. Use clean, properly prepared pipe and drinking-water-suitable materials where the service requires them. Being quick to assemble does not remove the need to inspect and test the connection.
Solvent-weld and threaded joints
Two more jointing methods complete the picture alongside compression, soldered, push-fit and press.
Solvent-weld sockets use a compatible cement to join suitable plastic surfaces, such as specified PVCu or ABS waste systems. Other waste systems use ring seals or purpose-made compression connectors. Match the pipe dimensions, fitting and jointing materials: the same nominal waste size does not prove that parts from different systems will fit.
Low-carbon steel pipe traditionally uses screwed fittings, with pipe threads cut using a stock and dies. For a connection designed to seal on its threads, apply suitable PTFE tape or jointing compound as specified; wind tape in the tightening direction. A threaded adaptor or union may instead seal on a washer, O-ring or mating face. Identify the actual sealing surface before choosing the jointing material.
Unions and parallel threads
The Copper Board's guide compares the two kinds of union. A flat-faced union, like a flange or a tap connector, seals by compressing an approved fibre or rubber washer, which can be damaged or displaced if it is not aligned and tightened with care. A taper union seals like a wedge forced into a tight gap, with no washer or jointing compound. Circulator pumps are connected with flat-faced unions, so the pump lifts out without the pipework being sprung apart, as taper unions would need.
Examine the threads on equipment before choosing a fitting: a female thread is probably parallel, while a male one may be tapered or parallel. Two parallel threads do not tighten as they are assembled, so a flanged connector or a flat-faced union with a fibre washer makes the better joint. Where a tapered male thread goes into a parallel female one, approved jointing material such as PTFE tape on the male thread is compressed as the taper tightens.
Matching pipe material to the job
Choose a pipe material for the duty it must do: the temperature and pressure it will see, its location, and the water or fluid it carries. Cost, ease of use and what is in stock only matter once the material suits that duty.
At boilers and hot-water stores, follow both appliance and pipe-system instructions for the permitted connection. A specified copper section protects against conditions outside the plastic system’s temperature or installation limits. Some manufacturers permit direct plastic connections under defined conditions, so neither a universal copper-tail length nor a blanket ban is a substitute for checking. Wavin's Hep2O guide is one example: it allows a direct connection only to a boiler with a high-limit cut-out thermostat and pump overrun, whose connections are outside the casing 350 mm from the heat source. Otherwise a run of at least 1 m of copper goes between the boiler and the start of the plastic, and a boiler maker's own instructions always take precedence.
Dissimilar metals in a conducting water environment can corrode galvanically. Choose a compatible system and a transition arrangement that addresses the risk; a generic metal adaptor does not necessarily provide electrical separation. On a drinking-water service, copper-bearing water reaching galvanised steel downstream can also cause attack. Consider materials, water conditions and flow direction, rather than assuming every mixed-metal system is forbidden or every adaptor solves the problem.
Plastic pipe run outdoors needs a UV-stabilised grade or protection from direct sunlight, since ultraviolet light degrades some plastics over time, and it still needs frost protection like any exposed pipe.