OPINION
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This month: BIM and the building services engineer, zero carbon logic behind the Code, and the risks of nuclear delay
The reality of BIM Thanks for a usefully demystifying collection of articles on BIM (CIBSE Journal, June 2013). It is vital that less-technical project directors and managers do not see BIM as some kind of magic silver bullet. To complete the exercise, it’s worth remembering how and why it all happened and noting the limitations imposed – on all aspects of procurement too – by necessary evils such as tendering procedures. Firstly, the origins of BIM were
a desire for a method of ensuring designs were fully coordinated and for recording what had been included in the design. The big advance came with CAD and BIM really took off when 3D representation became cheap enough to allow its use within the fee structures of ordinary projects. But, as with layered drawings, the
primary function was – and probably is – visualisation of elements in a particular volume in order to avoid clashes and late on-site revisions. Trying to go further than this leads to Thomas Taggart’s point that the
16 CIBSE Journal August 2013
data used in BIM ‘drawings’ will not necessarily be suitable as the calculation input, be it spreadsheet or full program, or for a measured quantities program. The problem with cost control – of
which tendering is only a part – is that the information for items cannot be input as fi rm data before tender. The more freedom tenderers have to offer – or insist – on alternatives, the less chance there is of a fully-fi rmed cost-component of a BIM model at that stage. It follows implicitly that the accuracy
of any plant sizing and characteristics before it is tendered is, at best, a good guess, and that trade-offs made by a tenderer – such as offsetting a less- effi cient, but much cheaper, fan by increasing the dimensions of certain ducts – become much harder to entertain. Behind this is the lack of a ‘trade- approved’ all-singing, all-dancing calculation program that allows, for instance, inputs of fan and pump characteristics at both variable speeds and shifting operating points as
throttling controls act. Most do not really allow input of non-standard ductwork shapes and dimensions. Until we have facilities such as these, the differences between energy consumption fi gures as calculated and as measured (as discussed elsewhere in the June Journal) will not be removed by implementing BIM. All we can do about this particular problem is to insist that our energy consumption fi gures apply only to (a) the occupancy and (b) the ‘small’ power, both as briefed by the client before tender. This alone might, after appropriate adjustments to the arithmetic, reduce the differences to those acceptable as ballpark fi gures. But, a full program, into which observed occupancy and measured small power can be input at any time to revise the energy consumption, is the only way of achieving better approximation. John Moss MCIBSE, consultant, Arup Advanced Technology and Research
It is vital that less-technical project directors and managers do not see BIM as some kind of magic silver bullet
ZERO OPPORTUNITY?
Under the Code for Sustainable Homes, it is easier to design ‘zero carbon’ mansions than one-bed flats, says Dr Ralph Evins, who blames calculation methods for the bias
26 CIBSE Journal June 2013
www.cibsejournal.com H
In order to protect what’s left of Britain’s countryside, planners now prefer high- density urban housing to suburban sprawl and developers are happy to oblige, as it enables them to sell more homes on the same-sized plots. As well as losing out on living space,
those living in ‘hobbit homes’ will also find it more expensive to upgrade their property to ‘zero carbon’ status than those living in more spacious abodes. Because of the way ‘zero carbon’ is calculated in the Code, smaller dwellings have a harder renewable energy target than bigger dwellings. It means that, to build or upgrade to ‘zero carbon’, as defined under the Code, is disproportionately more expensive for smaller houses. The standard for low-carbon dwellings
in England and Wales is the Code for Sustainable Homes2
aspects, of which energy and CO2
. This area covers nine
Logical thinking I enjoyed Dr Evins’ article on the Code for Sustainable Homes, and the injustice of a more challenging energy target for Code Level 6 for small than for larger homes due to their presumed higher occupancy per unit area. While this may seem unjust to those living in small homes, there is a certain logic to it. It’s a reasonable assumption that a family in a big home should not use more than six times as much energy on internal appliances as a small one. The general idea of ‘zero carbon homes’ is to offset the energy usage of the home, and so the metrics in the code – as presented – seem quite logical. On a visit to Vienna, I went round a university hall of residence (think rabbit hutch density), which met – not Code Level 6 – but Passivhaus standard – routine for new buildings there. There seem to be
CODE FOR SUSTAINABLE HOMES ‘ZERO CARBON’ BIAS
omes are getting smaller. The average new home in England is now 76m2
compared1 to the average for all homes of 85m2 .
emissions are the most heavily-weighted. To be classed as ‘zero carbon’, for stamp duty purposes a dwelling must achieve a CodeLevel 6, which includes a mandatory CO2
emissions requirement. This corresponds to net zero emissions
when energy for internal uses – like appliances and cooking – is offset by renewable energy generation. This requirement is assessed using the Standard Assessment Procedure (SAP), the same as for Part L compliance. An additional calculation (SAP Appendix L2 and L3) is used to find the predicted energy use for appliances and cooking, which is not included in normal Part L compliance calculations. Appliance energy use is based on floor area and occupancy; cooking energy use is based solely on occupancy. The occupancy assumptions in SAP are
based on floor area, but are highly non- linear, as shown in Figure 1. For example, 60 m2 occupancy
gives an
The larger house uses six times more energy for internal appliances in total, but only a third as much if measured per floor area
www.cibsejournal.com June 2013 CIBSE Journal 27
www.cibsejournal.com
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