Understanding Standards: AS/NZS 4234

AS/NZS 4234 outlines the modelling methodology for annual performance assessment of solar water heaters (SWHs) and heat pump water heaters (HPWHs).

The results of AS/NZS 4234 modelling are the annual electricity consumption of the water heater and the percentage energy savings compared to a conventional electric storage water heater. These metrics are used to calculate the number of certificates (STC, VEEC, ESC) that the product is eligible for under Australian federal and state incentive schemes (SRES, VEU, ESS, REPS).

The standard applies to a range of energy efficient water heating systems, applicable to both residential and commercial sites across Australia and New Zealand.

The AS/NZS 4234:2021 Version Update

The current version is AS/NZS 4234:2021, which superseded AS/NZS 4234:2008. The key updates were:

Additional technologies added, including PV water heating, variable speed heat pumps, and commercial systems
Reference system energy consumption updated to current MEPS requirements
Weather files updated

Heat Pump Water Heater Modelling Approach

The standard specifies how to treat:

Component testing for:

Solar thermal collectors
Pumps
Air-source heat pumps
Photovoltaic water heaters
Storage tanks
Gas heaters

Performance evaluation including:

Climate data (ambient air temperature, humidity, solar radiation, wind speed — hourly data provided)
Thermal energy loads — daily and seasonal load patterns
Legionella control
Minimum delivery temperatures

Modelling methodologies for:

Thermal stratification and mixing
Piping configurations
Heat pump flow rate control
Gas water heating
PV water heaters
Solar-boosted heat pump water heaters
Commercial systems

Modelling Heat Pump Water Heaters

AS/NZS 4234:2021 section 4 specifies how to model HPWHs, including the treatment of water flow rates for water-cycling systems, stand-by power, thermal stratification, and low ambient temperature operation.

The main category separation for water heaters is whether the condenser is integral to the tank (wrap-around coil, microchannel heat exchanger, or immersed coil), or stand-alone. Whether a system is “integral” or “stand-alone” has a major impact on the AS/NZS 4234 assessment.

There are four main HPWH types:

Split integral: condenser integral to tank with a separate outdoor unit, connected by refrigerant piping
All-in-one integral: all components together in a single unit, condenser integral to tank
Split stand-alone: outdoor unit contains the condenser, connected to storage tank with water piping and pump
All-in-one stand-alone: all components together in a single unit, condenser separate to tank, connected with water piping and pump within the single unit

Component testing is completed to AS/NZS 5125.1:2014, which provides correlation equations for COP and power input used in the HPWH model to determine thermal capacity at each timestep of the simulation.

For Class A HPWHs capable of operating in low ambient conditions, AS/NZS 5125.1 test condition 5 (low ambient) data is used to determine the low ambient temperature operation penalty, otherwise known as the “frosting penalty”. This penalty de-rates the COP at ambient temperatures below the initial frosting temperature (typically 6–9 °C).

AS/NZS 4234 Reports

AS/NZS 4234 sets requirements on what details must be included in a modelling report. These include:

Description and schematic of the product modelled
Product parameters and test results
Control logic description
Simulation software used
Purchased energy use
Calculation of annual energy savings relative to reference water heaters

All energy efficiency incentive schemes require a copy of this AS/NZS 4234 modelling report to assist auditors with verification of the modelling procedure.

How EnergyAE Can Help

EnergyAE has significant experience with TRNSYS and the AS/NZS 4234 standard. We produce accurate, compliant reports and help you get the best performance out of your system. Our knowledge of how these models work means we can help you to fine-tune designs to improve energy efficiency and thermal performance.