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, for 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The current version is AS/NZS 4234:2021, which superseded AS/NZS 4234:2008.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 2021 version added photovoltaic water heating, variable speed heat pumps, and commercial systems. It also updated reference system energy consumption to current MEPS requirements and refreshed the weather files used in annual performance assessment.

Heat pump water heater modelling approach

The standard defines how component test evidence is used in a whole-of-system annual simulation. For heat pump water heaters, this means AS/NZS 5125.1 test results are converted into performance correlations that can be used across a full year of changing operating conditions.

The modelling can include solar thermal collectors, pumps, air-source heat pumps, photovoltaic water heaters, storage tanks, and gas heaters. The relevant components depend on the product architecture being registered.

Performance is evaluated against climate data, daily and seasonal hot water loads, Legionella control requirements, and minimum delivery temperatures. The climate data includes hourly ambient conditions such as temperature, humidity, solar radiation, and wind speed.

The simulation method also needs to represent thermal stratification, mixing, pipework, heat pump flow-rate control, and any gas or electric boosting. For more complex products, the same modelling framework can also cover PV water heating, solar-boosted heat pump water heaters, and 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.

A split integral system has a condenser integral to the tank, with a separate outdoor unit connected by refrigerant piping. An all-in-one integral system keeps all components in one unit, with the condenser still integral to the tank.

A split stand-alone system has the condenser in the outdoor unit and connects to the storage tank using water piping and a pump. An all-in-one stand-alone system keeps those components inside one unit, but the condenser remains separate from the tank and is connected by water piping.

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 to 9 C.

AS/NZS 4234 reports

AS/NZS 4234 sets requirements for what must be included in a modelling report. The report needs to describe the product modelled, show the schematic, list the product parameters and test results, explain the control logic, identify the simulation software used, and report the purchased energy use.

The report also calculates annual energy savings relative to the reference water heater. Incentive schemes use this output to check the certificate calculations and understand how the product was represented in the model.

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 fine-tune designs to improve energy efficiency and thermal performance.