Measures to succeed

6 April 2006 saw the introduction of the latest revisions to Part L2 of the Building Regulations. The basic premise of the new law is to ensure that "buildings can be operated and maintained in such a manner as to use no more energy than is reasonable." One of the ways this will be achieved is the introduction of compulsory sub-metering on any new commercial building over 1000 m2.

The Regulations state that appropriate metering must be included at the design stage so that building operators can monitor where energy is consumed. The meters and sub-meters must account for at least 95% of the estimated energy consumption in a building. General Information Leaflet 65, or GIL65 - Metering guide, helps designers develop an appropriate metering strategy to meet Part L2.

The method suggested allows you to compare any metering proposals with the predicted energy use of the building. If this proposal covers less than 95% of the expected energy consumption then it must be revised.

Before you can develop a metering strategy you need a clear picture of how energy from each fuel source will be used. A detailed assessment of the building and all energy end uses will provide the most accurate picture of total energy use. CIBSE's Technical Memorandum TM22 provides a methodology for this.

You should estimate the energy use of all equipment in as much detail as possible. This data can be entered into a tree diagram (see figure 1, over), which provides a useful overview of the building.

Multiplying the two values in each box gives the overall value. Simply adding together the main end uses then gives an estimated total consumption.

Developing the strategy

Having gathered together all the details about a building's energy needs you can begin to develop a metering strategy. GIL 65 provides worksheets to help this process; the guide suggests a six-step method.

The first step is to enter the data on incoming energy use. The estimated total for each fuel type can be taken from the upper level of the tree diagram. Water is treated as energy, so the 95% rule applies here also.

Step two involves entering the energy type, use and estimated consumption of the main end uses. Here it is useful to identify three or four of the largest end uses that are easily metered, for example lighting, computer room and air cooling. This is shown in the second level of the tree diagram. The consumption data for different fuels should be presented separately.

Each of the end uses that are easily measured should then be broken down into subdivisions by area, system, circuit or tenancy. Where it is difficult to meter main end uses it may be easier to meter the subdivisions and provide more detail for the operator. Here you should take account of distribution needs, layout and physical location.

Meters and sub-meters must account for at least 95% of the estimated energy consumption in a building

Having identified the main end uses and subdivisions you must then decide how to meter the main end uses. The Building Regulations allow you to use various estimation methods where direct metering is impractical. These are illustrated in figure 2.

Now you need to estimate the annual consumption through the proposed meters. Step six involves adding together the consumptions for each energy type and dividing this by the estimated total incoming fuel needs. If the resulting figure is under 95% then more metering needs to be included.

Assuming that the proposal meets the 95% requirement you must draw up a schedule that includes meter codes and locations grouped by end use. A diagram of the strategy should also be prepared to show how the scheme fits together. The schedule and diagram must be added to the building logbook.

Automatic systems

Metering on its own does not save energy. It is the actions taken as a result of installing and monitoring meters that will achieve savings. Establishing a useable strategy helps building occupiers understand and monitor patterns of energy use. It can allow operators to compare actual consumption with targets, spot things that go wrong and analyse trends.

If the data is not collected and acted upon then even the best metering strategy will have failed. For this reason it is almost always cost-effective to use meters that allow connection to a building energy management (bems) or automatic metering system (ams).

Regular manual meter reading may become too onerous for busy building operators, so the frequency of reading may reduce. As such, wastage that occurs at specific times of day may be missed. Automatic systems should reduce the manpower needed for monitoring and may provide results more rapidly.

I favour using an automatic metering system because this keeps things simple for the operator. Consumption data is presented as simple profiles and reports. A dedicated ams generally provides a more tailored solution than a BEMS; for example the manager can use exception reporting so that they only get a report when something needs rectifying.

You must ensure that the final schedule of meters and metering strategy, including the estimates of energy use made, are included in the logbook. A simple summary form of this information will help the operator monitor the building's energy performance.