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Energy software compliance toolsOctober 2010

The Energy Performance Requirements of the incoming Building Regulations will demand greater accuracy from software compliance tools. David Bleicher explains.

One of the requirements of the 2002 European Energy Performance of Buildings Directive was that each member state had to devise a method of calculating energy performance. The response in the UK, for non-dwellings at least, was the National Calculation Methodology, or NCM for short. This is now used to assess compliance with Part L of the Building Regulations and also to produce the Energy Performance Certificates that are required on the construction, sale or rent of buildings.

Various NCM software tools are available, starting with the Simplified Building Energy Model (or SBEM) and moving up to dynamic simulation models. There are various SBEM interfaces, including a free to download interface called iSBEM. There are also three different simulation models.


Figure 1: NCM 2006 works out the emissions from a notional building which met the 2002 Building Regulations (click image to zoom)
While the NCM has stayed static since it was introduced in 2006, SBEM has undergone various incremental improvements. With the changes to Part L that come into effect from October 2010, a new version of the NCM has been released in addition to further improvements to SBEM.

How does NCM work?

To achieve compliance with Part L, the building carbon dioxide emissions rate (BER) must be less than the target carbon dioxide emissions rate (TER). The biggest change to NCM is how this TER is calculated. NCM 2006 works out the emissions from a notional building which met the 2002 Building Regulations. It then reduces these emissions by an improvement factor of around 25 per cent to obtain the TER (see Figure 1).

NCM 2010 uses a completely new definition of the notional building, and doesn't apply any improvement factors to obtain the TER (see Figure 2). The specifications of the 2010 notional building have been set so that the building stock built under 2010 Part L will, on aggregate, achieve 25 per cent lower emissions than the building stock built under the 2006 edition of Part L.


Figure 2: NCM 2010 uses a completely new definition of the notional building, and doesn't apply any improvement factors to obtain the TER (click image to zoom)
The additional cost of compliance is similar across all buildings. When modelled under NCM 2010, some buildings will get a TER more than 25 per cent lower than a similar building modelled under NCM 2006. Others will end up with a TER less than 25 per cent lower.

Why does the target keep moving?

In both the 2006 and 2010 versions of the NCM, a notional building takes on the same size, shape, use profiles and heating, cooling, ventilation system type as an actual building. This means that changing from, say, VAV to VRV air-conditioning will affect both the BER and the TER; whereas changing, say, the boiler efficiency will only affect the BER.

What about electric heating?

It's always been the case that buildings with electric heating are given a higher target than buildings with gas heating. When electric heating is used, the heating energy use in the notional building is assigned higher emissions than if gas heating were used. In NCM 2006, it's 37 per cent higher, whereas in NCM 2010 it's 50 per cent higher.

On the surface this sounds like a huge concession to the electric heating industry. Bear in mind that all other things being equal, the emissions from heating in the actual building will be much higher if electricity is used - about 140 per cent higher using the 2006 emissions factors, or 190 per cent higher using the 2010 emissions factors. So, regardless of whether a building falls under 2006 or 2010 Building Regulations, it will always be difficult to get it to pass with electric heating, and it will only pass if lots of compensating energy efficiency measures are incorporated.

The effect on heat pumps

Under NCM 2006, if a heat pump is used for heating, the heating energy use in the notional building is assigned the same emissions factor as if gas heating were used. So, other things being equal, the TER stays the same. That means using a heat pump will help with compliance if it has a coefficient of performance higher than about 2 - very easy to achieve with contemporary technology.

Under NCM 2010, if a heat pump is used for heating, the heating energy use in the notional building is assigned emissions 15 per cent lower than if gas heating were used. That means the heat pump will need a coefficient of performance of at least 2.76 to help with compliance. This is still easy to achieve, but it means that heat pumps don't have quite the competitive edge that they used to have.

David Bleicher is a building services engineer and BSRIA's expert in Building Regulations. He runs many of BSRIA's training courses.

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