AS/NZS 4234 Amendment 1: A New Condenser Model for Integral Heat Pump Water Heaters

Standards Australia and Standards New Zealand, through committee CS-028, have published AS/NZS 4234:2021 Amendment 1:2026 to Heated water systems: Calculation of energy consumption. AS/NZS 4234 is the simulation standard behind Australia and New Zealand’s heat pump water heater (HPWH) incentive schemes, though which edition a scheme calls up, and therefore whether Amendment 1 applies, depends on the scheme.

Which schemes this affects

The VEU program and the Energy Savings Scheme (ESS) both call on AS/NZS 4234:2021, and both already require the TYPE 104 modelling approach described below for integral HPWH. Amendment 1 is directly relevant to VEEC and ESC modelling.

The Small-scale Renewable Energy Scheme (SRES) is not affected. The Clean Energy Regulator’s Renewable Energy Determination 2016 calls up AS/NZS 4234:2008, not the 2021 edition, so Amendment 1 has no bearing on STC calculations. The CER has also not yet approved use of TYPE 104 for SRES modelling, though that may change in future.

The condenser modelling change

The largest technical change in Amendment 1 is a full rewrite of Clause 4.8.5.1, which governs how integral HPWHs are modelled. An integral HPWH is one where the refrigerant condenser couples directly to the tank, rather than heating water through a separate water loop to a standalone heat pump.

How the condenser was modelled before Amendment 1

Under the outgoing Clause 4.8.5.1, an integral HPWH’s condenser was represented as a water-to-water heat exchanger. The refrigerant side was replaced by a fictitious high-flow water stream, and the heat added to the tank was set from the coefficient of performance and power coefficients in the AS/NZS 5125.1 regression.

The part that needed calibration was the water-side inlet temperature, expressed as the mean tank temperature plus an offset commonly called ΔT_hx, or “DThx” among modellers. DThx had no independent physical basis. It was found by simulating a single heat-up cycle under AS/NZS 5125.1 condition 2 and adjusting DThx until the simulated average tank temperature matched the tested tank temperature within 0.5 K, reported against Appendix C’s Table C.13.

How the condenser is modelled under Amendment 1

Amendment 1 removes the calibration step. Heat is still added to the tank using the coefficient of performance and power from the same AS/NZS 5125.1 regression, but the condensing temperature is now derived directly from the thermal resistance of the actual heat transfer path between the refrigerant and the tank water, added to the local tank temperature. That resistance is built up layer by layer from the condenser’s real construction: wall thickness and conductivity, the bond layer or air gap between condenser and tank, the tank wall itself, and the glass lining where present. The resulting heat is then distributed across the tank nodes the condenser physically covers, weighted by the vertical position and temperature of each node.

Appendix C’s Table C.6 has been rewritten to match, and now asks for the physical construction of the condenser under three configurations Amendment 1 explicitly recognises:

• Microchannel extrusion, where flat aluminium multi-port strips are bonded to the tank wall.
• Wrap-around coil, where round or D-shaped refrigerant tube is bonded or wound to the tank wall.
• Coil submerged in the tank, where a twin-wall refrigerant tube sits directly in the tank water.

Each configuration needs its own set of dimensions, wall thicknesses, and thermal conductivities through the full heat path. Table C.13, the DThx test-condition-2 comparison, has been deleted along with its accompanying note, since there’s no longer a calibration result to report.

For readers who build these models directly: this mirrors a shift already reflected in TRNAUS, the Australian extension package for TRNSYS 15 (TYPE 104 and TYPE 138 are TRNAUS components, not part of the native TRNSYS 15 library). TRNAUS’s TYPE 104 component, used for integral HPWH tanks with separate modes for microchannel, wrap-around coil, and submerged coil condensers, replaces the older TYPE 138 stratified tank component’s calibrated heat exchanger representation. It doesn’t change the reference system or the annual load profiles, only how the heat pump’s condenser transfers heat into the tank.

Other changes in Amendment 1

A handful of smaller changes accompany the condenser model rewrite. Products installed in Australia now need the simulation to track and report legionella control conformity across the full standardised annual operation, for both the selected load and the very small load, in every selected zone.

Minimum delivery temperature reporting is also more detailed: the simulation must report the minimum and maximum delivery temperatures achieved, not just whether the 45°C requirement was met. EnergyAE’s AS/NZS 4234 reports already report the minimum delivery temperature; future reports will add the maximum delivery temperature now required alongside it.

Two new clauses address numerical stability. The standard timestep of 0.02 h can only be reduced if the simulation can’t converge at that step, and a new clause requires the number of tank nodes to be reduced if the modelled flow rate would move more fluid through a node each timestep than the node holds.

The low temperature (frosting) operation penalty has been restructured rather than changed in substance. The old clause is deleted and replaced by a new one that makes explicit that the penalty applies to products claiming Class A low-temperature capability under AS/NZS 5125.1:2014 Section 5. The reference system values in Table 3.1 for an integral heat pump with in-tank electric boost have also been updated.

What this means for manufacturers and importers

For VEU and ESS work, Amendment 1 formalises an approach that’s already established rather than introducing a new one. EnergyAE’s AS/NZS 4234 modelling for integral HPWHs has followed the condenser method Amendment 1 now writes into the standard since late 2024, so products modelled by EnergyAE for VEU or ESS over the last two years are already built on this method.

For SRES, Amendment 1 doesn’t apply. The scheme’s regulator, the CER, calls up AS/NZS 4234:2008 under the Renewable Energy Determination 2016 and hasn’t approved TYPE 104 for that edition.

Where Amendment 1 creates work is for integral HPWH products intended for VEU or ESS that haven’t yet been modelled under this condenser method, whether because a product predates EnergyAE’s 2024 change, was modelled by another provider, or hasn’t been assessed since. The method needs physical condenser detail that DThx calibration never asked for: condenser dimensions, wall and bond thicknesses, and thermal conductivities for every layer between the refrigerant and the tank water. If that detail isn’t already documented in your AS/NZS 5125.1 test report or technical file, it will need to be sourced from your condenser design or measured before the product can be modelled to Amendment 1.

How EnergyAE can help

EnergyAE’s AS/NZS 4234 modelling for integral HPWHs has run on this condenser approach since late 2024, ahead of Amendment 1 formalising it in the standard. We can review what condenser construction data Table C.6 requires for your product, check whether your existing AS/NZS 5125.1 test report and technical file already cover it, and confirm which scheme, and which edition of AS/NZS 4234, your product needs to be modelled against.

Get in touch with us if you’d like to discuss how Amendment 1 affects your product range.