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Technical questions - prisons designs, false fire alarms and boiler condensateNovember 2008

'Ventilation and airflow in buildings' is available to BSRIA members from the library. It explains why ventilation in buildings should be measured and covers all the important technical information for ventilation measurements.

BSRIA's technical librarian Stephen Loyd shares the answers to the some interesting technical enquiries from BSRIA's consulting and contracting membership. This month members want to know about prison design, de-commissioning gas mains, and problems with false alarms.

Regulations for prison designs

What regulations and specifications will apply to the prison building programme?

The construction of prisons is subject to the same regulations as other building construction that includes Building Regulations where appropriate, health and safety and fire regulations, and various regulations relating to the provision of utility services.

Government policies relating to energy and sustainable development also apply, including the requirement to achieve the highest BREEAM rating of excellent for energy efficiency for each building.

Design, security and technical specifications and standards produced by the National Offender Management Service (NOMS) also apply to prison design. NOMS produce a wide range of specifications and standards covering every aspect of building design, including lighting and ventilation systems.

Chemicals in boiler condensate

What is the chemical analysis of condensate from condensing boilers?

The condensate generated by condensing boilers is of a low-level acidic discharge with a pH in the range 3.6 - 4.3. This will depend upon the nature of the combustion system and the temperature within the boiler.

The acidity is derived mainly from nitrogen in the combustion air and is nitric or nitrous. Obviously the condensate will require suitable drainage.

A typical chemical analysis of condensate is as follows:

  • pH: 3.7
  • Fluoride: 0.7 ppm
  • Chloride: 1.9 ppm
  • Nitrite: 19.5 ppm
  • Nitrate: 115 ppm
  • Sulphate: 26 ppm
  • Aluminium: 20 ppm
  • Copper: 24 ppm
  • Iron: 0 (precipitated)

Preventing false alarms

How can I prevent false fire alarms?

The number of false alarms recorded by local authority fire services has increased over recent years. Fire authorities are reviewing the way in which they respond to automatic fire alarm actuations. In some cases evidence of a fire will need to be ascertained before firefighters attend the scene.

False alarms from fire detection alarm systems can arise from various different causes such as:

  • pollutants in the air (dust, aerosols, insects) setting off smoke detectors
  • high temperatures setting off heat detectors (such as from activities involving hot work)
  • vandalism or malicious acts
  • mistakes occurring in the use of the system
  • faulty equipment or poor system maintenance
  • fire detectors or red break-glass boxes sited in the wrong place and being accidentally set off

BS 5838-1:2002 Fire detection and alarm systems for buildings, Code of practice for system design, installation, commissioning and maintenance gives a lot of information to help minimise false alarms. The Standard suggests that in well managed environments a rate of one false alarm per 100 detectors should be achievable, whereas in more industrial premises a rate of 1 per 75 detectors is more realistic.

See also A guide to reducing the number of false alarms from fire-detection and fire-alarm systems from the CLG website.

Gas-main de-commissioning

What are the procedures for de-commissioning a gas main?

When a pipeline is to be taken out of service for a long period it must be de-commissioned, disconnected from the gas supply and the ends sealed.

In all circumstances gas venting to atmosphere must be minimised and carried out under controlled conditions. After venting, a test should be undertaken to ensure that the pressure does not increase due to an unknown back-feed.

BS EN 12327 Gas supply systems. Pressure testing, commissioning and decommissioning procedures gives the functional requirements for de-commissioning. It also gives a method of purging, and suitable and approved methods of isolating the pipeline section from the gas supply.

Floor void airtightness values

I am considering underfloor air conditioning, what are the airtightness requirements for floor voids?

Where floor voids are used for ventilation plenums, BSRIA recommends an airtightness criterion of 1.0 litre/s per square metre of floor area at a test pressure of 50 Pa, excluding the air leakage to the occupied zone.

It is important that the conditioned air in the floor void supplies air to the occupied area. The system can be severely compromised if the conditioned air leaks into cavities and risers, or other areas of a building.

For further information, visit BSRIA's airtightness pages where you can also download in depth guidance, including BG 4/2006 Airtightness testing - The essential guide to Part L2 of the 2006 Building Regulations.


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