Extreme testing at minimum grade

New Zealand’s building code differs from that of Australia. A recently updated New Zealand drainage compliance document (Acceptable Solution G13/AS2) permits gradients on a DN 100 drain (nominal 100mm diameter, i.e. 4 inches diameter) to be significantly flatter than in most other parts of the world, down to a minimum gradient of 1:120 (0.83%).

This minimum gradient was based on evidence from several local authority regions where flat gradient drains had been permitted in order to overcome difficulties connecting to relatively shallow sewers in areas of flat topography.

The compliance document requires that at all times the drain be laid at the ‘maximum practical gradient’ (Grade of Drains: clause 3.5.2). In addition, the document also requires that where the grade of 1:120 is employed the relevant levels must be determined by ‘verifiable levelling devices’ with ‘Laser or Dumpy Levels being recommended’ (Construction: clause 5.2.2). It should be noted that at this gradient the maximum drain discharge unit loading is limited to 104 fixture units.

Caroma considered that a field trial under such conditions would provide an important further test of its SmartFlush product as part of a complete plumbing system, so a number of range models using the 4.5/3L technology were selected and installed.

Brent Mallinson, a plumbing consultant based in Christchurch on New Zealand’s South Island, who runs Plumbing Design and Consultancy Ltd, was selected to manage and monitor the field trial. He chose eight new home sites that met the criteria, and with the assistance of builders and homeowners, arranged the installation of some 20 toilets.

Each site was CCTV-recorded before the homeowners took up residence and then on a monthly basis over the next year. The owners also completed user-satisfaction questionnaires before and after the trial.

The video inspection results produced some interesting observations. A number of the uPVC drain line installations were found to be poorly installed with rough and burred joints, use of excessive solvent cement, construction debris left in the pipe, and sections of pipe with negative grade.

These installation issues surprised the test team as New Zealand operates an extensive plumbing installer apprenticeship program and plumbing is a regulated industry.

Over the period of the inspections, any occasional waste build-up noticed on the CCTV reports was directly attributed to rough joints, builder’s debris that had entered the drain during construction, or negative gradient sections. Importantly though, no drain blockage was reported by the occupants over the period of the trial, in conditions far more severe than was originally intended.

However, one thing that the trial did emphasize was that an important task that couldn’t be neglected by any plumbing engineer/specifier was to ensure that the installation standards of contractors and their staff were regularly checked, as drain lines couldn’t be expected to be as forgiving when attached to low-flow fixtures.

Regrettably, these problems have also occurred in Australia and elsewhere in the world. As societies come to grips with the ever-decreasing availability of water as a valuable commodity, it is the responsibility of everyone involved to realize that as the trade and industry ‘pushes the boundaries’ there is a need to apply good and safe practices at all times if we are to achieve the goal of reducing our water consumption.

From the toilet cistern to the authorities’ sewer main, each component of the installation requires a ‘matched performance’ to ensure adequate and continuous energy exists to provide an efficient and water-conserving solution to the increasing problem of water consumption.