Technical answers - car park ventilation, white rust and legionella outbreaksOctober 2012

BSRIA's John Sands provides answers to the latest technical questions posed by BSRIA members, including car park ventilation, white rust, and legionella outbreaks.

Car park ventilation

Q. What regulations apply to the design of ventilation systems for underground car parks, and what guidance is available?

A. The primary piece of legislation covering such systems is the Building Regulations, which will vary depending on the project's location. For instance, for England and Wales car park ventilation is covered by two documents.

Part B of the Building Regulations deals with fire safety. Section 11: 'Special provisions for car parks and shopping centres' of Approved Document B3, Internal fire spread (the structure), details the ventilation requirements for three common scenarios: open side, natural ventilation, and mechanical ventilation.

Part F of the Building Regulations covers ventilation, while Approved Document F1 New buildings other than dwellings also has requirements for car parks. Paragraph 6.18 details the ventilation provisions necessary for car park ventilation based on carbon monoxide levels. Paragraph 6.20 offers an alternative approach, with requirements detailed for both naturally ventilated and mechanically ventilated spaces, based on areas of openings and air change rates.

Both these documents are free to download at www.planningportal.gov.uk/buildingregulations/approveddocuments/downloads.

Further guidance can be found in Code of Practice for Ground Floor, Multi-storey and Underground Car Parks published by the Association for Petroleum and Explosives Administration (www.apea.org.uk); CIBSE Guide B 2005, Section 2.3.23.3; and Health and Safety Publication EH40: Occupational Exposure Limits for Limiting Concentration of Exhaust Pollutants.

Airtightness testing

Q. Should airtightness testing be carried out before or after ventilation system commissioning?

A. It doesn't make any difference whether airtightness testing is carried out before or after the commissioning of ventilation systems. Such systems should all be sealed temporarily for the duration of an airtightness test, so that any adjustments made during commissioning will not affect the building's air leakage characteristics.

The building envelope needs to be complete before the test is carried out. If there are any unsealed service penetrations, for example, this will increase the leakage.

The situation is different in rooms such as hospital isolation wards, which are pressurised or depressurised relative to their surroundings. Airtightness testing of these rooms (and any resulting remedial work) should be carried out prior to commissioning. However, the overall airtightness test for the building can be carried out any time after the building envelope has been completed.

White rust

Q. What is white rust? 

A. Galvanising is the process of coating steel components with a thin layer of zinc. Although the zinc is relatively reactive compared to other metals, the surface gradually oxidises in atmospheric air to form a passivation layer of zinc oxide to inhibit further oxidation.

If the underlying steel is exposed at a cut edge then the zinc will preferentially protect the exposed steel by sacrificial oxidation. Problems arise when the freshly galvanised surface is persistently wetted with soft water (condensation or rainwater). In this case the zinc reacts to form mixture of zinc oxide, hydroxide and carbonate - white rust. This can occur due to water ingress to galvanised products wrapped in plastic packaging or water ingress between layers of stacked galvanised material.

If the zinc surface is fully exposed to the atmosphere, and can immediately dry out after wetting, then visible damage is much less likely. White rust in services occasionally occurs due to weather ingress to ductwork systems and after humidifier sections in an air-handling unit.

The damage is usually worse than it looks as the volume of white rust formed is very much greater than the loss of underlying metal. However, where possible the white rust should be removed to inhibit further deterioration.

Mechanical removal (wire brushing) is normally adequate to remove the white rust. Over-painting with a zinc-rich paint can restore the appearance and level of corrosion protection.

Chemical removal of white rust is not usually recommended for installed services components due to the difficulty of rinsing the surface.

Legionella infections

Q. How frequently do outbreaks of Legionnaires disease occur in the UK?

A. The Health Protection Agency publishes statistics of various infections.

In 2010 there were 12 clusters or outbreaks of Legionnaires disease in the community (as opposed to hospital or travel-acquired infections). An outbreak is formally defined as "two or more cases where the onset of illness is closely linked in time (weeks rather than months) and where there is epidemiological evidence of a common source of infection".

There is a very good article on the statistics at www.hpa.org.uk/hpr/archives/Infections/2012/respiratory12.htm. 

For more information about the benefits of BSRIA Membership, including technical support from our expert engineers, contact: 

Tel: +44 (0) 1344 465532
or email michael.doig@bsria.co.uk