ALF help

Results Help

On the Results pages you see the results of the ALF calculations for the designs, which you have entered in the Design pages.

BPI Calculation

BPI calculation

The New Zealand Building Code sets a performance level for the thermal efficiency of residential buildings in Clause H1 - Energy Efficiency. The Approved Document of Clause H1 2007 sets a BPI target of 1.55 kWh/DM m2 for all locations in New Zealand. Because the BPI is calculated by dividing by degree months it is largely independent of the climate, i.e. a particular building has ideally the same BPI in any climate.

BPI calculation forumla

A number of standard conditions apply to maintain the building at all times within a year at a constant internal temperature for the heating energy calculation when for BPI calculations. These are:

  • A continuous temperature of 20oC throughout the building
  • An air change rate of 1 change per hour or the actual air leakage rate, whichever is the greater
  • A heat emission contribution arising from internal heat sources of 1000 kW hours for the first 50 m2 of floor area, and 10 kW hours for every additional square metre of floor area
  • No allowance for:
    • carpets or
    • blinds, curtains, or drapes on windows
  • Windows to have a shading coefficient of 0.6 (made up of 0.8 for windows and recesses and 0.75 for site shading)
  • Occupant numbers are linked to the total floor areas:
    • <120m2: 3 occupants
    • >=120m2 but <180m2: 4 occupants
    • >=180m2: 5 occupants

The ALF algorithms apply all these conditions automatically when calculating the BPI.

The BPI calculation as set out above will apply to the following climate zones on and after the following dates:

  • Climate zone 3 – 31st October 2007
  • Climate zone 2 – 30 June 2008
  • Climate zone 1 – 30th September 2008

(The climate locations for BPI calculations are set in the definition section of the NZBC.)

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Old Measure BPI calculation

The Old measure BPI calculation as set out by New Zealand Building Council Compliance Document Clause H1 Energy Efficiency – 2nd edition will cease to apply to the following climate zones on and after the following dates:

  • 31st October 2007 – climate zone 3
  • 30 June 2008 – climate zone 2
  • 30th September 2008 – climate zone 1

The Old Measure BPI is defined as the heating energy used during four winter months (May to August) using an indoor heating level of 20°C for 24 hours each day divided by the number of degree days (base 15°C) and the total floor area: BPI = E/(DD x Afloor). A number of defaults apply for the heating energy calculation when it is used for BPI calculations, these can be found in the relevant compliance document. Because the BPI is calculated by dividing by degree days it is largely independent of the climate, i.e. a particular building has ideally the same BPI in any climate. Clause H1 - 2nd edition of the New Zealand Building Code has a BPI target value for code compliance which requires a BPI of 0.13 kWh/DD m2 or smaller for the warm locations and 0.12 kWh/DD m2 or smaller for the cool locations. The climate locations for BPI calculations are set in the definition section of the New Zealand Building Code. Cool locations are areas within New Zealand where the degree day total (base 15°C) for the period 1st May to 31st August equals or exceeds 920. Locations with less than 920 degree days are defined as warm locations.

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Contact us at Branz for further information about the ALF 3.2: Annual Loss Factor.