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How does varying the service life of a component affect its life cycle cost?April 2012

In many cases the published life expectancy for a given type of equipment or machinery depends on which reference document you are looking at. For example, according to Building Life Plans the life of a fan is between 5 and 15 years, whereas according to CIBSE the life of an axial fan is 15 years and the life of a centrifugal fan is 20 years.

If we are using such wide-ranging life expectancy information to calculate life cycle costs, then the results can also vary widely and this can alter the recommendation being made.

As part of our work on the CILECCTA research project, being part-funded through the EU's 7th Framework Programme, BSRIA used low and high life expectancy figures to calculate the life cycle cost of a hypothetical ventilation system. The results are given in the figure to the right.

If the mechanical ventilation system is being compared with a natural ventilation solution, then choice of a long life or a short life for the fan can alter the conclusion of which system has the lowest life cycle cost (and is therefore the preferred solution). 

This simple example reinforces the need for great care to be taken by anyone carrying out life cycle costing analysis. It is important that the life expectancy information is appropriate and can be justified.

Download a full version of this article which includes a review of how the recommendations of ISO15686 can be used to develop more project-specific life expectancy information.

David Churcher is a Principal Research Consultant in BSRIA's Sustainable Buildings Group.

BSRIA provides life cycle costing cosnultancy, runs a life cycle costing theory and practice training course and has published a Whole life costing analysis guide.

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