The system is fine. The joints are the argument
Plastic push-fit is a properly engineered, Kitemarked system. JG Speedfit fittings and their PEX and polybutylene barrier pipe are Kitemarked to BS 7291 Parts 1, 2 and 3 Class S, and Wavin Hep2O is listed in the Water Fittings and Materials Directory. There is nothing wrong with it as a material choice.
What there is, in London housing, is a lot of it installed quickly during flat conversions, under floors, behind boxing and in screed, by people working to a deadline. The failure modes below are all installation failures, and every one of them is documented by the manufacturers themselves in their own installation literature. That is worth saying plainly: when we cut out a failed push-fit joint, we are usually looking at something the manufacturer told the installer not to do.
Know what it is rated for
Before the failure modes, the numbers, because they get assumed rather than read. JG Speedfit publish working temperatures and pressures by application: cold water at 20°C maximum working temperature and 12 bar; central heating at 82°C usual, 105°C maximum, with short term malfunction at 114°C, and 3 bar; hot water at 65°C usual, 95°C maximum, and 6 bar. The system is explicitly not for gas, fuel oil or compressed air, and Wavin say the same of Hep2O, adding that it is not for underground supplies.
Those are not the same numbers as copper, and an uncontrolled heat source is specifically excluded. A push-fit tail into a solid fuel back boiler, or into a system with a failed thermostat, is outside the rating.
Failure one: the missing pipe insert
This is the most common thing we cut out. Both major manufacturers are unambiguous. Wavin: the SmartSleeve support sleeve is an integral part of the system and should never be omitted when using Hep2O pipe. JG Speedfit: pipe inserts must be used and fully inserted on all connections to MDPE, LDPE, PEX or PB pipe, and when using a compression fitting on plastic pipe a standard insert must be used to withstand the compressive pressure of the olive, with the olive located within the length of the insert.
Without the insert the pipe has nothing holding its circular section. Over months, under the grip of the collet teeth and the squeeze of the O-ring, it deforms. The joint does not fail on the day it was made, which is exactly why the installer's pressure test passed and why it starts weeping a year later.
Failure two: the cut, the burr and the score mark
JG Speedfit's instruction is to cut the pipe square with a pipe cutter, remove all burrs and sharp edges to prevent damage to the O-ring, and to reject scored pipe because score marks can cause leaks past the O-ring. The don'ts are equally specific: do not use hacksaws to cut the pipe or leave burrs on the end.
A burr shaves a line into the O-ring as the pipe goes in. That line is a channel. It leaks at a rate of a few drops an hour, which under a floor is a damp patch six months later and a pressure gauge that needs topping up every few weeks. It is one of the reasons a slow heating loss can go a long time without showing.
Failure three: gripped, but not sealed
This is the subtle one and it is worth understanding because it explains a leak that appears with no apparent cause. In a push-fit fitting the stainless steel teeth grip the pipe before the pipe reaches the O-ring. JG Speedfit put a warning in their own guide: the fitting may be gripped but not sealed if the pipe is not fully inserted, and if the pipe has been cut correctly the insertion mark will be level with the collet head.
So a joint can feel right, pass a pull test, and be wrong. Under working pressure it may hold for a long time and then start passing when the pipe creeps a millimetre or the system runs hot. Wavin solved the same problem differently, with a profiled sleeve end that gives a rumbling sensation when the pipe is fully home.
Failure four: side load and abrasion
JG Speedfit specify that pipe clips should not be fitted any closer than 60mm from the end of a fitting, and that pipe should be adequately supported to prevent undue stress on fittings. Wavin note that plastic pipe run adjacent to or through metalwork must not touch sharp edges, because thermal movement will abrade it, and they recommend grommets, edge profile or conduit.
Plastic moves. JG quote 1% expansion on length between 20 and 82°C. On a ten metre heating run that is real movement, and it has to go somewhere. Where it is restrained hard against a joist edge or a metal duct, the pipe wears. Where it is clipped tight against a fitting, the fitting takes a bending load it was not designed for.
Failure five: chemistry and light
Push-fit does not like certain things touching it. JG Speedfit list the ones to avoid: jointing compounds, cellulose-based paints, thinners or strippers, solder flux, acid-based descalers and aggressive cleaning products, including anything below pH 4, high in hypochlorite such as bleach, or containing hydrogen peroxide. Only water or oil based paints are acceptable.
That list is why a fitting that has been within splashing distance of a descaling job, or painted with the wrong thing during a refit, can craze and fail with no obvious mechanical cause. Ultraviolet is the other one: the products are stabilised to withstand limited exposure to sunlight but are not designed for permanent direct exposure, and where they are exposed they need painting or lagging.
Failure six: the joint that should not be there at all
This is the one that turns a small leak into a floor coming up. Schedule 2 paragraph 7 of the Water Supply (Water Fittings) Regulations 1999 says no water fitting shall be embedded in any wall or solid floor, and that no fitting which consists of a joint shall be a concealed water fitting. Water Regs UK guidance adds that concealed pipework in walls should be run in a chase, duct or void, should be accessible to enable inspection and replacement, and should not be embedded or come into contact with plaster or cement.
The manufacturers restate it. JG Speedfit: Speedfit pipe and fittings can be laid in concrete and masonry providing they are installed in conduit pipe with access boxes for the fittings, because as stated in Water Regulation Schedule 2.7 and BS 8000 Part 15, fittings and pipe should be removable for possible replacement. Wavin's answer is their Pipe-in-Pipe system, barrier pipe pre-sheathed in conduit, for installations into screed.
Most screed push-fit in London flats has none of that. When the joint fails, there is no duct and no access box, so the repair is a cut in the floor. That is why we quote reinstatement as part of the job rather than leaving you with an open hole.
Two separate problems compound here, and both are worth knowing before you book anyone.
The first is acoustic. Leak noise attenuates rapidly in plastic pipe and the signal that survives is low frequency and narrow band, which sits in the same part of the spectrum as a building's own background noise. The consequences and the alternatives are set out in the acoustic detection guide. In short: an acoustic-only firm can honestly report finding nothing on a plastic system that is definitely leaking.
The second gets overlooked. Plastic pipe is invisible to a cable and pipe detector. The NHBC Standards address this by requiring that where pipework is in or behind wall surfaces and would not otherwise be detected by a metal detector or similar equipment, a metallic tape should be applied to the pipework, and Wavin reproduce that requirement in their installer guide. In practice, in refurbishment work, the tape is very often not there. So you cannot scan the wall to find the run, which is the step that normally precedes cutting.
What works instead, in order: a pressure test to prove and size the loss, thermal imaging where the leaking circuit is hot, and tracer gas on the drained circuit, which does not care what the pipe is made of because the gas leaves through the same hole the water was leaving through.
When this is not our job
If a push-fit joint under your sink is weeping and you can see it, push the collet in, pull the pipe out, cut the end back square with a proper pipe cutter, fit a new insert and push it fully home until the insertion mark meets the collet. That is a ten minute job and you do not need us for it.
Where it becomes our job is when the joint is under a floor, in a wall, or in screed, and nobody knows where. Detection is £150 per hour, fixed before we attend, and if we do not find it you do not pay for the time we spent looking. See pipework repair and replacement, or the related post on how each pipe material fails.
