Working in detail

Professor Max Fordham stated in his Presidential address: "The challenge of climate change puts services in a pole position of importance for buildings."

As engineers concerned with creating systems to move energy from one place to another, we should all be highly conscious of the need to do so as effectively as possible. The ideal in energy terms would be naturally ventilated, neutral energy buildings making use of suitable recycled materials and renewable energy, with the occupants walking or cycling to work. The practical reality however is that city centre air conditioned commercial buildings will be designed for some time to come.

The future will hopefully see better integration of the services with the structures and better understanding of the need for buildings to be airtight to an objective and measurable degree. The new Part L Building Regulations will contribute significantly to more airtight structures. Building services designs will have to undergo a rethink when this takes place because currently we make huge and often inappropriate assumptions about the amount of uncontrolled ventilation that takes place.

Almost traditionally we add margins all along the design path¹. They may be for:

• uncertainties in initial assumptions – the brief is unclear
• actual uncertainties in structural element performance – actual construction affecting real U-values and infiltration due to poor detailing
• uncertainties in calculation methods – steady state compromises versus complex dynamic models
• uncertainties in equipment performance – performance at stated catalogue limits which are often inappropriate for UK conditions and often subject to tolerance, what you can achieve under ideal test conditions can rarely be repeated in a site situation
• uncertainties in system performance – interaction of components and installation quality/limitations
• allowing performance to deteriorate over time as systems and equipment become dirty – assumptions over maintenance and service work quality
• allowing intermittent operation – for a boost at the start of the day
• allowing a degree of future proofing – often requested by the client
• satisfaction of custom and habit – often stated in company design manuals with no justification or cautionary notes on use
• protection of professional indemnity – producing systems that just cannot under-perform

Some of the above are valid and necessary, most are not. The last item is interesting in that there are many experiences of systems that fail to deliver. The engineering is often called into question as either not producing enough heating or cooling effect whereas in many instances it is the building structure that is failing to perform, often being too leaky. There are almost no cases of engineers being taken to task (or court) because systems have too much capacity.

Oversized plant and systems will generally be inefficient and consume more energy than is necessary. Engineers must think carefully about the assumptions they make and margins they add so that in the detailed design stages, the plant is the optimum size for the job. A better understanding of the use of margins can be found in the CIBSE research report Engineering design calculations and the use of margins.

Margins are one design aspect that must be considered and used with extreme care, the other aspects are the choices made during the design process. For example designing for 'just sufficient' performance. If Professor Fordham's suggestion is followed "we can contribute to the form if we enter into the spirit of the team".

Margins are one design aspect that must be considered and used with care.

Services designers must try much harder to influence the form and the construction as this has so much influence on the energy use of the finished building. Figure 1 shows for just one simple building model, the very significant impact that altering the amount of glazing or infiltration has on heating or cooling loads. Will these sorts of decisions affect the functionality of the building?

If there was the time (and fees), it would be possible to better analyse and explain the various influences on the loads that designs have to ensure that clients understand the significance of their decisions. Not only must plant not be oversized through the application of margins, it must be capable of responding correctly to the varying loads. This places huge demands on the controllability of systems, something that must be built into the detailed design, not just left to the successful controls supplier to deal with.

Figure 2 shows the effect of varying the flow rate of chilled water through a cooling coil and the effect on coil duty. A better understanding of how flow rate affects coil performance can be found in BSRIA's Commissioning of Pipework systems.

A final thought direct from the Presidential address is that we must "design building services correctly and get them installed to work". Understanding the consequences of the decisions we make will be of increasing importance, especially dealing with the looming skills shortages in design and installation work and the inevitable use of new entrants to the construction industry with limited applicable knowledge. BSRIA is trying to help with this through a research project entitled Technical design quality control which will be published later this year and is effectively distilling years of experience on the detail of building services.