Control Settings for Heat Pump Water Heaters

HPWH control settings are one of the most consequential decisions a manufacturer makes before entering the Australian or New Zealand market. The combination of setpoint temperature, dead-band, and legionella strategy determines how the unit performs in the real world, how it behaves in AS/NZS 4234 modelling for SRES, VEU, and ESS scheme applications, and whether it satisfies the legionella requirements of AS 3498:2020 and the National Construction Code. Mistakes here affect certificate counts, installation manual compliance, and public health obligations.

What the Control Parameters Mean

The three parameters that define a basic HPWH controller are:

Setpoint temperature: the target storage temperature the heat pump runs toward. When the tank sensor reads below setpoint minus dead-band, the heat pump starts. When it reaches the setpoint, it stops.

Dead-band: the gap between the lower trigger temperature and the upper cutoff. A 60°C setpoint with a 10 K dead-band means the heat pump starts at 50°C and stops at 60°C.

Legionella control mode: a scheduled heating event designed to bring sufficient tank volume to a temperature that satisfies the pasteurisation requirement in AS 3498:2020 Clause 7.2.

Legionella Requirements Under AS 3498

AS 3498:2020 is the product standard for water heaters sold in Australia. Its Clause 7.2 provides two pasteurisation pathways applicable to HPWHs:

Daily method (Option (b)): at least 45% of tank volume must reach 60°C or above during every heating cycle. This requires the heat pump sensor (which triggers the cutoff) to be located more than 45% of the tank height from the bottom, so that when the sensor reads 60°C, at least 45% of the volume above the sensor has been heated. In modelling terms, this is typically expressed as a 60°C setpoint with a small dead-band (around 10 K), ensuring the heat pump runs to 60°C at least once per day.

Weekly method (Option (a)) with daily top-up: at least 90% of tank volume must reach 60°C or above at least once per week. This requires the sensor to be positioned such that 90% of the tank volume is above it. Because a single daily heating cycle at the standard setpoint may not reach the bottom sensor, this approach uses a higher weekly setpoint (typically 61°C or above) and relies on the weekly event to achieve full-tank pasteurisation. A daily top-up cycle (e.g. 55°C setpoint) maintains hot water availability on non-pasteurisation days.

The bacteriological basis for these thresholds is established: Legionella pneumophila is killed at 60°C in approximately 32 minutes and near-instantly at 70°C. At 55°C, effective kill requires continuous exposure across the entire tank volume for around 6 hours, a condition that installed systems do not achieve in practice due to thermal stratification, cold water inlets, and distribution pipework (AS 3498:2020; NCC 2022 Volume 3 B2V1).

The NCC Performance Requirement B2P6 applies to all HPWH installations, requiring heated water to be stored and delivered under conditions that prevent Legionella growth above 10 cfu/mL. There is no Deemed-to-Satisfy provision under NCC B2 for warm water systems operating below 60°C. Any HPWH design that stores water below 60°C in a commercial or multi-unit setting must be assessed via a Performance Solution. For residential Class 1 buildings, the AS 3498 Clause 7.2 pathways provide the practical compliance mechanism.

In New Zealand, Building Code Clause G12 requires hot water to be stored at 60°C at least daily for storage-type systems, consistent with these requirements.

Integral HPWHs: Typical Control Strategies

Integral systems (where the heat pump is mounted on or integrated with the tank) are the dominant form factor in the Australian residential market and the primary product type registered under SRES, VEU Activity 1D/3C, and ESS.

Daily 45% pasteurisation (Option (b)): the most common approach for integral R290 and R32 units. The controller runs to a 60°C setpoint each day, with a dead-band typically around 10 K. This satisfies AS 3498 Clause 7.2(b) provided the sensor is positioned above the 45% height mark, which most manufacturers already achieve in standard tank designs. The installation manual and tank drawing must confirm the sensor position and state that the default control mode achieves Clause 7.2(b) compliance.

Weekly 90% pasteurisation (Option (a)) with daily 55°C top-up: used where the manufacturer prefers a lower daily setpoint for efficiency reasons, or where the product is marketed as a more energy-efficient option. A typical implementation uses a 55°C setpoint with a 5 K dead-band for daily operation, plus a once-weekly scheduled cycle that raises the tank to 61°C or above until the lower sensor (positioned such that 90% of volume is above it) reads the target temperature. The weekly cycle must occur regardless of draw-off patterns; controllers must not skip it because the tank already feels “warm enough.”

The daily 45% approach is simpler to document and verify. The weekly approach requires careful sensor placement documentation, test data showing all five standard test sensors (T1 through T5) reaching 60°C by end of the heat-up cycle under all AS/NZS 5125.1 tested ambient conditions, and a controller that reliably executes the weekly event.

Stand-Alone (Separate) HPWHs: Control Differences

Stand-alone systems using CO2 refrigerant typically employ a sensor lower down in the tank, due to the highly stratified tank. The heat pump circulates heated water from the heat exchanger back into the upper section of the tank, and the controller reads a temperature sensor positioned low in the tank. When the lower sensor reaches the setpoint, the heat pump stops.

A typical CO2 control setting is a 60°C setpoint with a 20-30 K dead-band, meaning the heat pump starts when the lower sensor reads 40°C and stops when it reads 60°C. The wider dead-band reflects the longer thermal stratification gradient in a pumped-loop system: the upper portion of the tank reaches temperature well before the lower sensor triggers cutoff, giving a more gradual and energy-efficient heating profile.

Stand-alone systems are most commonly CO2 (R-744) units, single-pass designs where the refrigerant extracts heat from a large water temperature rise in a single pass through the gas cooler. This architecture suits CO2’s transcritical temperature glide and allows delivery temperatures of 63°C to 70°C.

R32 and R290 also appear in stand-alone configurations, but these typically use a multi-pass (recirculating) heat exchanger arrangement rather than the single-pass approach, and their control logic differs accordingly: without the large temperature lift of a single-pass CO2 system, multi-pass R32 and R290 units rely more on pump flow rate control and may use narrower dead-bands to maintain adequate delivered temperature. Higher flow rates typically result in less degrees of stratification than in CO2 systems.

For stand-alone systems, the legionella control logic must confirm that the pump continues running until the lower sensor reaches the setpoint, so that the heat that has been delivered to the upper tank is distributed sufficiently through the volume. If the pump stops early (e.g. on a timed basis rather than a temperature basis), the lower portion of the tank may remain in the Legionella growth zone.

The Efficiency and Availability Trade-Off

Raising the setpoint or narrowing the dead-band makes hot water more available: the tank triggers more frequently, and starts from a higher temperature floor. Lowering the setpoint or widening the dead-band improves heat pump efficiency: each heating cycle starts from cooler water, giving the compressor a lower condensing temperature and a better COP. This trade-off is real, but the direction of the efficiency gain does not mean lower setpoints are always the right choice.

A wider dead-band means longer intervals between heating events. If a draw-off event happens to fall within a long gap, the available volume at adequate temperature may be insufficient. This is especially likely in the morning or evening when hot water demand peaks. A very wide dead-band also increases the risk that the daily legionella cycle does not occur: if the tank is drawn down late in the day but the heat pump does not complete a full heat-up cycle before the next morning, the Clause 7.2 daily pasteurisation requirement may not be met.

For most integral residential HPWHs, a maximum dead-band of 12 K is a practical limit when paired with a 60°C setpoint. Above this, availability complaints and legionella cycle failures become more likely, particularly in smaller tank sizes and households with variable draw-off patterns.

A 55°C setpoint is sometimes proposed to improve COP. The efficiency trade-off is not typically worth it. At 55°C, Legionella management becomes more difficult to guarantee without daily pasteurisation cycling, hot water at the tap is cooler (and thermostatic mixing valves deliver proportionally less mixed volume), and the system has reduced margin for cold ambient conditions where heat pump output drops. Products operating at 55°C as their default also face harder questions from scheme administrators about legionella compliance documentation. The COP improvement from dropping from 60°C to 55°C is real but modest, typically 5 to 15% depending on refrigerant and ambient conditions. The reduction in system usefulness and compliance simplicity is not worth it.

AS/NZS 4234 Modelling and Scheme Compliance

AS/NZS 4234 models the annual energy performance of a HPWH for scheme applications including SRES (CER), VEU (ESC), and ESS (IPART). The modelled control settings must reflect the typical factory default mode of the product.

This matters because the control settings used in modelling drive the certificate count, and the installation manual must match the modelled behaviour. A product modelled at 55°C/10 K must be shipped and installed with those settings as the factory default. If the manual allows easy user adjustment to less efficient settings (i.e. HP 60°C/5K, or “Hybrid” (HP + Element) mode), the scheme administrator may challenge whether the modelled performance reflects real-world operation.

The factory default must also be suitable for the majority of users without requiring them to rely on electric element or hybrid modes for adequate hot water. A well-sized HPWH system should be able to serve the vast majority of daily hot water demand primarily through heat pump operation. The electric element, where fitted, should function as a boost for two specific scenarios: very low ambient temperatures where heat pump output drops significantly, and periods of unusually high peak demand such as a large household gathering or missed heating cycle. If a product’s factory default settings require element assistance on a routine basis simply to maintain adequate hot water volume, the sizing assumptions or control strategy need review before scheme application.

Products that ship with setpoints or dead-bands optimised for maximum certificate counts but not for real-world performance create compliance risk: if the default settings produce poor hot water availability or fail legionella requirements in normal use, the product fails its own compliance basis. Setting sensible defaults that work for the majority of users is the right foundation, not an obstacle to maximising scheme outcomes.

Summary of Practical Recommendations

For integral systems targeting the Australian residential SRES, VEU, or ESS market:

Use a 60°C setpoint as the factory default. This satisfies AS 3498, simplifies legionella compliance documentation, and maintains adequate hot water availability in all but the most adverse ambient conditions.
Keep dead-band at 12 K or less. Wider dead-bands improve efficiency on paper but increase legionella cycle failure risk and reduce hot water availability during peak demand.
Choose Option (b) (daily 45% pasteurisation) or Option (a)/daily top-up depending on sensor position and test data, and confirm the approach in the installation manual with explicit reference to AS 3498 Clause 7.2.
Do not use 55°C as the factory default setpoint. The efficiency gain does not compensate for the reduction in hot water availability, the legionella compliance complexity, and the operational margin lost in cold conditions.
Ensure the AS/NZS 4234 modelled control parameters match the factory default shipped in the product. The certificate count flows from these parameters; so does the legal compliance basis.

For stand-alone systems, confirm that the low sensor setpoint and dead-band are validated against the legionella compliance pathway, that the pump control logic ensures adequate tank volume reaches pasteurisation temperature on the required schedule, and that the heat exchanger arrangement (single-pass CO2 or multi-pass R32/R290) is correctly reflected in the modelling approach.

Standards cited: AS 3498:2020 (Safety and Public Health Requirements for Plumbing Products: Water Heaters and Hot-Water Storage Tanks); NCC 2022 Volume 3, Performance Requirement B2P6 and Verification Method B2V1; AS/NZS 3500.4:2021/2025; NZ Building Code Clause G12 (G12/AS1 amendment 14, 2024); AS/NZS 4234:2008 (SRES/CER), AS/NZS 4234:2021 (VEU/ESS).