The Hydraulic Tower was devised by Geberit as part of a major commitment to training, to show exactly how plumbing operates in real situations – which may be somewhat different to expectations.

A similar tower will be an integral feature of Geberit’s UK training centre at the UK headquarters in Warwick. This latter version shows what can happen in typical UK situations, including when an installation is carried out in compliance with all relevant standards, and how to overcome the problems. These can range from cross-flow contamination to noisy installations – all rather undesirable and certainly best avoided.

A frequent requirement for plumbers is to construct soil pipe manifolds for up to seven WC’s. This may involve back-to-back configurations which can lead to problems with cross-flow if the pipework from opposing WC’s is not properly constructed.

Recommended practice for gravity drainage designs – BSEN12056 part 2 – is that connections should be staggered if a common horizontal drainage pipe is utilised to prevent this cross-flow.

An alternative is to use two separate manifolds which combine at the drain point with a corner branch. However this may require more space between the opposing ranges and, with clients desire to minimise duct space, this may not be a viable option.

The Tower, by using clear pipework throughout, clearly shows just how easily cross-flow occurs when soil pipes are fitted directly opposite each other and how offsetting pipework overcomes the problem.

Cross-flow can also be a problem when installing a WC and a number of other appliances to a vertical stack. Connections to the stack must comply with certain dimensional criteria to prevent this cross-flow.

With regard to WC connections via a swept branch and any other appliances, the connection must be either at the same level or, if below, at right angles to the WC connection. The zone of no connection extends downwards for 200mm and is measured only around the back 180 degrees of the branch/pipe opposite the branch entry.

Moulded branches generally have connections pre-moulded in the areas of permissible connections. Some fittings have been designed to allow waste connections immediately below the WC branch connection but above the 200mm dimension. This is acceptable because they are not in the non-permitted zone on opposite sides of the branch. It provides a very useful above floor connection for low level appliances such as baths and showers (illustrations).

With today’s emphasis on keeping costs to a minimum and because of the limited space generally available for pipework, there is often a temptation to combine both bath and washbasin wastes. Due to the quantity of water discharged when a bath is emptied, full bore flow conditions occur in the waste pipe resulting in negative pressure being generated which in turn will siphon any other appliance that is connected to the same waste pipe, combining the washbasin waste pipe into the bath waste pipe will always result in noisy plumbing as air is pulled through the trap seal when the bath is emptied, increasing the risk of loss of the water seal in the washbasin trap.

If there is no alternative to combining wastes it is important to
a) use a self resealing trap on the washbasin although it will still not overcome the noise problem
b) b) increase the pipe diameter to 50mm at the point where they combine
c) c) add a relief vent to the system.

It is common plumbing practice to use a 50mm waste pipe to connect the wastes from several washbasins together. However there are limits to how many washbasins can be connected together before induced siphonage of the appliance traps occurs, creating potential hygiene problems as well as noisy plumbing.

Design code of practice BSEN12056 part 2 identifies a maximum of five washbasins to be connected to a common waste. Even so, all the appliances must be fitted with 75mm deep seal traps to ensure a water seal is maintained.

Additional protection to the water seals can be provided by:-
a) replacing standard traps with self resealing traps, thereby allowing air to pass through them when negative pressure occurs.
b) fitting an automatic air admittance valve to the end of the pipe run.
c) running a loop vent from the end of the run back to the stack, bearing in mind that connections must be made above the highest connection on the stack (illustrations).

BSEN12056 part 2 also imposes design constraints for long basin waste pipework. Waste pipework normally used for washbasins, which has a bore of only 32mm must not exceed 1.7m in length. Any longer pipework will result in self siphonage of the water seal in the trap because of the negative pressures generated by the full bore discharge which will occur with such small diameter pipe. If long pipe runs are unavoidable, solutions are to
a) increase the pipe diameter
b) fit a seal resealing trap
c) fit an air admittance valve to the end of the wastepipe.

The Hydraulic Tower visibly demonstrates the cross-flow problems with their potential for unhygienic situations as well as the noises created by poor plumbing practice. As in any walk of life, there are grey areas and it is always advisable to err on the side of caution. In plumbing in particular, attempting to overcome these difficulties after installation is a major task with considerable cost implications.

Using best practice and proven products for every installation will earn you a reputation for quality and efficiency in both the domestic and commercial market sectors. Geberit invest heavily in research and development of technologically advanced products to help installers create better plumbing installations. We all know how neighbours talk to each other and if Mrs Jones’s plumbing makes noises and Mrs Smith’s doesn’t, Mrs Smith will be one very happy customer who will recommend your installer to all and sundry. That makes good business sense.

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