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NABERS Energy Guide to Building Energy Estimation
The design loadings must not be used as these are intended to be maximum loads rather than realistic
operational loads.
2.6.5 Equipment Hours of Operation
It is common for offices to leave over 50% of their equipment operating overnight. This represents a
major problem for the achievement of a good rating in practice. If tenants are already known, it is
worthwhile investigating what level of equipment turn-off they currently achieve, as this will probably be
carried over to the new location. The default value of 50% overnight load is to be used unless
demonstrable evidence to the contrary can be provided. Even in such a case, higher percentages of
overnight equipment operation is to be considered as a potential energy performance risk for the building.
2.6.6 Occupancy
The occupancy shall be modelled in a manner that reflects a realistic projection of the operating patterns of the site.
This can viably be based on the operating patterns of the tenant at their previous site is this data is available.
Occupant density can be based on the tenant fit-out if known; allowance should be made for less than one person
per desk actually in attendance, as this is normal. Design occupant densities should not be used as these are
normally overestimated, and are intended to be maximum loads rather than realistic operational loads.
The building should be modelled without vacant areas. If it is expected that the building may be only partially
occupied at the time of the performance rating, then this should be investigated as an off-axis scenario.
If the tenant is unknown, then the default should be used.
2.6.7 Tenant Supplementary Air Conditioning
Tenant condenser water loops can be significant energy uses within base buildings and supplementary airconditioning, particular for 24 hour loads such as computer room air-conditioning, can be very substantial energy
uses within tenancies and indeed may create significant loads for base buildings in terms of pumping and cooling
tower energy.
The electrical input into the supplementary units should be determined on the basis of realistic loads, bearing in
mind that computer rooms in particular tend to be considerably oversized relative to the actual cooling requirement.
If metered data from a previous tenancy is available, this should be used to determine probably loads. If such data
is not available, the default methodology should be used.
The impact on base building energy use should be assessed on the basis of the estimated incremental cooling tower
energy arising from the calculated tenant loads plus the electrical input to the tenant supplementary conditioners
plus the mechanical energy delivered to the condenser water by the pumps. Incremental cooling tower energy may
be estimated on the basis of the average cooling tower energy use per unit heat rejection. In addition, the pumping
energy must be counted.
2.7 Simulation of HVAC
The simulation of HVAC energy use is an area with a high probability of producing misleading or
inaccurate results. This likelihood will be reduced if the following factors are taken into account.
 System choice. The system modelled is to be a good representation of the system being installed in
each part of the building.
System design. The plant size, number of systems, airflows and zoning is to be as per the design.
Default efficiency curves for boilers and chillers are to be replaced by the actual known efficiency
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Version 2011-June