Can you smell it?


By Vollie Brink, Pr Eng
We often receive complaints from projects about unpleasant smells in the building. The three pillars of the National Building Regulations (NBR) are health, safety, and economy. SANS 10400-P (P2) describes how the drainage system must be designed and how it must perform.

Health is extremely important. The system must not endanger the health of the occupants inside the building, while the drainage system’s contents carry potentially dangerous human waste and foul air, which are high health risks.

The major challenge is to keep the foul air in the piping and that is done with water traps.

The piping must be tested to be both watertight and airtight and it is why we have ventilation pipes to extract the foul air from the piping and vent it into the atmosphere above the roof of the building.

The ventilation piping does not only allow the foul air to escape out of the drainage system, but it must also allow fresh air to enter into the piping to keep the pressure in the piping constant and equal to the air pressure outside the piping. This is why some people use a two-way vent valve instead of an open vent pipe.

The ideal ratio of air and effluent in a stack pipe is approximately 75% air and 25% effluent, and in horizontal piping it is 50% air and 50% effluent to ‘balance’ the pressure in the system.

As the effluent drops down in a stack pipe, it can cause positive and negative pressures, which can disturb the water slots in the fixtures and either ‘suck’ it in or ‘push’ it from the fixtures. This is why the ventilation in the piping is so important to keep the water slots intact and to keep the unhealthy air out of the building.

The appropriate ventilation of a sanitary drainage system is crucial to keep the building healthy and safe for occupants.

However, people often complain about foul smells in buildings and then you and I are called upon to solve the problem.

The question is how to go about finding the foul air leakage. Perhaps the following will help to seek the source of the leak:

  1. My experience is that in most cases, the source is a floor drain of which the ‘odour trap’ has been removed by the cleaner and not put back in position.
  1. We have often found that where there is a grease trap in a restaurant, they clean the grease trap, remove the odour trap from the floor trap, chuck the contents into the trap, and do not put the odour trap back. This is illegal. The contents of a grease trap must be removed from the site in an approved manner; it is illegal to discharge it on site and in the piping elsewhere from the grease trap. We have found that with smoke tests, the grease traps have serious leaks.
  1. Another culprit is the vent valve where a two-way valve is installed inside the building and occasionally discharges the foul air — sometimes even into the air-conditioning ducting, spreading it through the building. This is a widespread problem. A two-way valve shall not be installed inside a building or even outside under a window, which is also common and in contravention of the NBR.
  1. Many architects do not want vent pipes to protrude from the roof, and then the vent pipes inside the building are connected to a common horizontal collector vent pipe, of which the size is not big enough to allow enough air in the system. This can cause a problem.
  1. Manholes are not allowed inside a building, and an air leak from these manholes can cause serious health problems and odours. A sump pump inside a building is also a source of smells; even a lift sump pump causes it.
  1. A crack in the piping or a loose pipe joint can cause air leaks.
  1. The metal-type bottle trap on a urinal is a potential source of foul-air leaks. These metal bottle traps have a metal ‘skirting’ inside it to form the trap and the urine then corrodes it and it leaks. These metal ‘skirts’ must be replaced with non-metal ‘skirts’.
  1. The connection between an air-conditioning condensate pipe and a drain pipe is also in many cases the source of bad odours.
  1. Pipe joints can leak and crack in the piping.
  1. The position from where a vent pipe discharges is important: it must be above the highest window or opening and “above the roof level”, as per SANS 10400-P.
  1. The vent piping must not be near an air-conditioning intake.
  1. The negative and positive air pressures in the drainage piping can also cause the water traps in the fixtures to be ‘sucked in’ or ‘pushed out’.
  1. A vent pipe must always rise and not drop and run horizontally and then rise again.
  1. The depth of a water trap shall not be less than 75mm as per SANS 10400-P.
  1. The vent piping must be airtight to prevent leaks.
  1. When a smoke test is conducted, all the vent pipes must be blocked and all the traps must be full of water. It is better to test sections and not a large system.
  1. Always remember that the ‘bad smells’ are pathogens/germs and pose a health risk. An open vent pipe is preferable to a vent valve, as vent valves are never serviced and the spring-loaded valves are never replaced. In some cases, it is the source of bad smells.
  1. Floor drains that do not have a waste pipe discharging into it dries up and causes bad smells if it is not regularly filled with water.
  1. All covers to be properly closed and tight.

If you have experience in how to check for and locate the source of bad smells, we can add it to this list and share it with others in the plumbing business.