Legionella Compliance for Heat Pump Water Heaters

Legionella pneumophila is the bacterium responsible for Legionnaires’ disease, a severe and sometimes fatal pneumonia. Hot water systems are one of the most common sources of building-associated Legionella outbreaks. Warm, stagnant water in storage tanks and distribution pipes provides ideal growth conditions, and aerosols from showers or taps are the typical transmission route.

Heat pump water heaters are not inherently higher risk than gas or electric resistance systems, but they introduce specific compliance considerations. Many HPWHs operate most efficiently at storage temperatures below the 60°C threshold that Australian and New Zealand regulations use to control Legionella. Understanding how the applicable standards handle this, and what your product needs to do, is a prerequisite for any HPWH entering the market.

This guide covers the bacteriology behind the 60°C threshold, the regulatory frameworks in Australia and New Zealand, how refrigerant choice affects compliance options, and the difference in obligations between residential and commercial installations.

Why 60°C is the threshold

Legionella pneumophila grows fastest at 37°C and can survive up to 48–50°C. At 60°C, 90% of bacteria are killed within 32 minutes. At 70°C, kill is effectively instantaneous. Australian standards and New Zealand building codes anchor the storage requirement to this kill kinetic. The 60°C threshold is not a policy preference: it is derived from pasteurisation research.

At 55°C, Legionella can theoretically be killed, but only if every part of the tank holds 55°C continuously for many hours. This does not happen in practice. Tank stratification means cold water sits near the inlet in the 25–45°C growth zone. Dead legs in distribution pipework can maintain growth conditions regardless of tank temperature.

Australian requirements

AS 3498:2020 is the core product standard. It requires water heaters to store water at ≥60°C, or to comply with Clause 7.2. Clause 7.2 is the alternative pathway: 45% of tank volume must reach pasteurisation temperature daily, and 90% weekly. This is where thermistor placement matters. If the sensor sits at the three-quarter point, the controller reads 60°C when roughly 75% of tank volume is at that temperature, not 100%. Whether a specific thermistor position satisfies Clause 7.2 has not been addressed in any published regulatory ruling.

AS/NZS 3500.4 is the plumbing installation standard adopted by the National Construction Code. It sets the same ≥60°C storage minimum, plus a ≥55°C minimum on circulating return lines. This return-line requirement, formalised in the 2021 edition, means that circulating systems (common in apartment buildings and hotels) require separate treatment to prevent Legionella growth in distribution pipework regardless of tank temperature. Victoria mandated the 2025 edition from October 2025. Other states are following.

NCC 2022 Volume 3, Part B2 is where the gap becomes most significant for HPWHs. Performance Requirement B2P6 requires hot water to be stored and delivered under conditions that avoid Legionella growth. Deemed-to-Satisfy provision B2D8 refers to AS/NZS 3500.4. But NCC B2 explicitly states there are no Deemed-to-Satisfy Provisions for warm water systems. A HPWH operating below 60°C storage temperature in a commercial building cannot follow the DtS pathway. The designer must submit a Performance Solution and demonstrate compliance against B2P6. This increases engineering costs and project risk on commercial HPWH installations.

Delivery temperature at the tap is a separate layer. In general buildings, hot water must be delivered at ≤50°C through a thermostatic mixing valve. In aged care facilities, healthcare settings, early childhood centres, and schools, the delivery cap is ≤45°C.

New Zealand requirements

Building Code Clause G12 governs water supplies. Amendment 14 to the G12/AS1 Acceptable Solution, effective 2 November 2024, sets storage at ≥60°C with delivery at ≤50°C for general buildings and ≤45°C for high-risk facilities. The previous delivery maximum was 55°C. All new or replacement installations now require a tempering or mixing valve.

Health NZ’s Prevention of Legionellosis Guidelines, revised April 2025, flag constructed warm water systems as the primary source of L. pneumophila in New Zealand’s built environment. The guidelines apply to commercial buildings. G12 compliance is enforced at the installation point through building consent conditions.

New Zealand lacks the state-based warm water system registration and inspection regimes seen in South Australia and Victoria. For commercial buildings, compliance relies on building consent and the 2024 Health NZ guidelines. Ongoing operational enforcement is less formalised than in Australia.

Refrigerant capabilities

Refrigerant choice has direct consequences for Legionella compliance.

CO2 (R744): Residential CO2 HPWHs routinely deliver water at 63–70°C and can maintain ≥60°C storage natively without a supplementary element. Sanden’s GAU-A45HPC targets 63°C delivery with no immersion heater. Reclaim Energy’s controller targets 59°C at the thermistor, marginally below the AS 3498 threshold. Whether this satisfies Clause 7.2 has not been confirmed by regulators, and commercial or health-care operators should seek clarification before installing in those settings.

R290 (propane): Capable of output up to 75°C. Can maintain ≥60°C storage without supplementary heat. Hydrotherm’s R290 product targets 60°C at the three-quarter-point sensor and cites the Clause 7.2 volume rules.

R32 and R134a: Typically limited to 55–60°C output, falling further in cold ambient conditions. An electric immersion heater is required for reliable Legionella compliance across the year.

R410A: Often capped at 55–56°C in cold conditions. Reliable Legionella compliance requires a boost element.

For R32, R134a, and R410A products, most residential HPWH controllers schedule a weekly immersion boost cycle. A weekly cycle adds approximately $50–100 per year in electricity at Australian residential tariffs, and reduces the effective annual COP relative to any published rating. Anti-Legionella boost cycles are not currently captured in HPWH energy ratings under AS/NZS 4234 or AS/NZS 5125.1.

Residential and commercial obligations differ

In a residential single dwelling (Class 1 in Australia), AS 3498 and AS/NZS 3500.4 apply at installation. There is no ongoing registration or operational inspection requirement. The homeowner carries the obligation after handover, but routine enforcement does not occur.

In commercial buildings (Class 3–9 in Australia), AS/NZS 3666 applies mandatorily under NCC. State-specific requirements add further obligations. South Australia requires annual registration of all non-residential warm water systems with the Environmental Health Authority. Queensland mandates quarterly Legionella monitoring and thermal treatment protocols (70°C for 5–10 minutes at outlets) for hospitals and residential aged care. Victoria requires risk management plans for residential aged care, health service providers, and related facilities.

A HPWH replacing a gas boiler in a commercial building does not remove any of these obligations. The heat source changes. The public health requirement does not.

Product specification checklist

For residential installation, the practical minimum is a product with either native ≥60°C output capability or an electric immersion element with a weekly boost cycle. Check the storage temperature the product achieves under real conditions at the cold end of the target climate zone, not laboratory test conditions.

For commercial installation, four questions need to be resolved before product selection:

1. Can the heat pump maintain ≥60°C storage throughout the year at the installation’s ambient temperature range?
2. If not, is a supplementary electric element included and sized for full-tank pasteurisation?
3. Does the system design include a circulating return loop? If so, is that loop maintained at ≥55°C?
4. Is a Performance Solution required under NCC B2P6, and has an engineer confirmed that compliance can be demonstrated?

Commercial HPWH installations that do not address these questions carry legal liability under public health acts and workplace health and safety law.

How EnergyAE can help

EnergyAE works with HPWH manufacturers and importers on compliance across the full AU/NZ pathway, including Legionella compliance assessment as part of product registration and commercial project support. If you are unsure whether your product’s temperature architecture meets AS 3498 requirements, or need guidance on Performance Solution obligations for a commercial installation, get in touch.