01.05.2012

Star Ratings Explained

Michael Plunkett explains the intention of the NatHERS scheme

What is NatHERS?

NatHERS – the Nationwide House Energy Rating Scheme – is a program administered by the Department of the Environment and Energy.

What is the intention of the scheme?

The aim of NatHERS is to provide a framework that enables a residential building’s theoretical thermal performance to be assessed against a set of rules (protocol) using an accredited software tool and that generates an indicator (star rating) that can be used by consumers to compare one house against another and to demonstrate compliance with minimum performance standards of the National Construction Code – Building Code of Australia (BCA).

The residential buildings are assessed by accredited assessors (not required in all States) that use one of three accredited software tools – BERS Pro, AccuRate and FirstRate5.

The software tools are interfaces to an underlying software engine – Chenath (also known as the Accurate Engine), which is owned by CSIRO, that performs the calculation of heating and cooling loads required for comfort.

Assessors enter data on all of the required characteristics of the building envelope and elements into their chosen software tool. Once the data entry is complete, the assessor simulates the performance of the building to generate the star rating required under the BCA.

During this simulation process, the software tool processes the entered data on the building and generates a file (scratch file) that is submitted to the underlying Chenath engine. The Chenath engine then calculates the heating and cooling loads for each zone at the end of each hour of the day for a complete year, ie 8,760 calculations for each zone. Calculations in some zones may be done more than once in accordance with the NatHERS protocols before the heating and cooling loads are determined (this is why, in some climate files, during the summer months, the Chenath engine appears to take a bit longer to calculate the loads).

Once the Chenath engine has generated the heating and cooling loads for the year, an output file is produced and read by the software tool. The software tool then displays this data, adjusted for the area of the house, as a star rating.

The three software tools display the star rating in slightly different ways. AccuRate and Firstrate5 display the star rating in a decimal format, 0.1 increments, while BERS Pro displays the star rating in half-star increments.

To understand the impact of these two different approaches, and to highlight that a higher rating generated in one software tool is not necessarily better, let’s have a look at the following example.

A house in Glen Waverley (Climate File 62 – Moorabbin) is assessed in the three software tools. The following summary shows the star ratings reported by the three software tools, and the total adjusted heating and cooling loads reported in each:

AccuRate – Total H & C = 81.2 MJ/m² 6.8 Stars
FirsRate5 – Total H & C = 86.7 MJ/m² 6.6 Stars
BERS Pro – Total H & C = 78.2 MJ/m² 6.5 Stars

It appears that AccuRate gives the highest star rating with a 6.8 stars. But if you examine the adjusted heating and cooling loads reported out of each tool, the lowest loads are generated using BERS Pro, ie in theory, it is the best performing house.

The star ratings are only an indicator used to demonstrate compliance with the 6 star minimum requirements of the BCA. Different houses rated using different tools will produce different ratings. A house that achieves a rating of 6.8 stars using AccuRate or FirstRate5 may not necessarily be a better performing house than another house that only achieves a 6.5 star rating using BERS Pro.

Both houses meet the minimum requirement of 6 stars which is all that is required under the BCA.

Care needs to be taken by assessors and their clients when using star ratings in advertising and the media, particularly those that exceed the minimum BCA requirements. Assessors are bound by their State’s Fair Trading legislation and must ensure any information is not misleading and is factual. Published material should contain notes and qualifications as to the claims made for a particular house to minimise the likelihood for future action.

Star ratings are not always what they appear to be.

Michael Plunkett is Principal of SmartRate and a Corporate Member of BDAV.