Trust us: it's going to work

There has been much comment in the technical press about the Simplified Building Energy Model, the software tool used for calculating compliance with the new Part L. Building reported on 13 April that "architects and building service engineers have rejected the SBEM software and have resorted to a longhand methodology to ensure reliable results". However, many users have commented that once you get used to it, entering or modifying details for a building is straightforward, and not as time-consuming as they feared.

It is important to point out that SBEM is actually composed of two main elements. The core of the tool, but the part no user actually sees, is the calculation engine that is the "real" SBEM. This is a set of processes and algorithms that takes the input from the interface and performs calculations in accordance with the procedures set out in a series of draft CEN standards.

The part you do see when you use the tool is the interface - iSBEM. This is the part that interacts with the user, collecting information about the building and feeding back the answers.

A number of the issues raised relate to the interface rather than SBEM itself. A key factor here is training: although the user guide is comprehensive, it is no substitute for proper training in the use of the tool, which often clears up misunderstandings about what information to enter and how. All users are encouraged to take advantage of one of the courses advertised on the ncm.bre.co.uk website.

This interface was developed in Microsoft Access as a pragmatic solution to getting something ready on time. It was always intended to be the default, rather than the only, option. One common misunderstanding is that users have to buy Access to use iSBEM. This is not true. It's possible to receive a limited version of Access (a "run-time" version), free of charge on a CD-ROM. You can get this from the web-site. In the meantime, several suppliers of building software are working with the government and BRE to incorporate the core SBEM calculation tool, as well as the Part L compliance checking routines, in their own software. These tools should be accredited and approved by the Department for Communities and Local Government very shortly.

A number of users are finding that SBEM gives unexpected answers when they enter a building's details. Experience from analysing feedback shows that there are generally three explanations for this:

Data has been entered incorrectly: As explained above, there is no substitute for proper training in the use of the tool.

SBEM ‘allows’ the design of fully glazed buildings because of the way Part L is intended to operate. With high-performance facades and highly efficient plant, it will sometimes be possible to meet the CO2 target

The answer is surprising but correct: What SBEM does is to implement a completely new way of calculating the integrated energy performance of a building, which it expresses in terms of carbon dioxide emissions. This is a fundamental principal behind the new Part L: the calculation exposes the complex relationship between the building's form and fabric and its service systems more transparently than before.

This can be illustrated with a couple of examples. The first is glazing, and the realisation that SBEM "allows" the design of fully glazed buildings. This is not because of anything SBEM does or doesn't do, but because of the way the new Part L is intended to operate. With high-performance glazed facades and highly efficient plant, it will sometimes be possible to meet the CO2 target. The other example relates to particular heating, ventilation and air-conditioning systems, which SBEM "won't allow". Again, this is not usually the fault of SBEM. It is a fact of life that some heating and cooling systems are inherently less energy-efficient than others. To use such systems in a building, designers will have to compensate by making improvements to other elements, such as the building fabric or lighting system.

The calculation algorithms or underlying data may be faulty: Despite intensive testing, this might be true in some cases. If users believe that either SBEM itself, or the underlying data, is flawed they should submit their comments, with full details, through the ncm website. The development team has been responding to feedback from the industry both before and after the issue of the beta test versions in 2005 - and will continue to do so into the future.

All feedback is analysed to assess what has caused the problem. Definite bugs are cleared up as soon as practicable. Suggestions for refinements or enhancements are prioritised for possible future action.

The latest version - v1.1.a, released last week - incorporates a number of improvements such as in the number of weather locations, the handling of thermal bridges, easier ways to input some parameters and corrections to some of the supporting databases.

Really urgent queries on the use of iSBEM can be handled on the telephone helpline, 0870-460 8141. It is worth checking the FAQ page of the website first, though - it's possible that other people have raised the same query before.