HWU (Hot Water Unit)
HWU (hot water unit): types, NCC 2022 requirements for new Class 1 builds, Whole of Home energy scoring, and AS/NZS 3500.4 installation rules.
Ask Chalkline about this →HWU is the abbreviation for hot water unit, used on Australian construction drawings, hydraulic plans, and builder schedules to denote the water heater appliance and its associated pipework connection point. The term is interchangeable with “hot water system” (HWS) in documentation; HWU is common on hydraulic drawings, HWS appears more often in specifications and energy reports.
Six types of HWU appear in residential construction:
| Type | Heat source | Storage | Notes |
|---|---|---|---|
| Gas storage | Natural gas or LPG | Yes (50 to 400 L) | Reticulated gas sites; must meet 5-star GEMS rating (verified 2026-05-09, NCC 2022 Vol 3 Part B2) |
| Gas instantaneous (continuous flow) | Natural gas or LPG | No | On-demand; rated by flow rate (L/min) and star rating; restricted in new Class 1 builds unless 5-star rated under AS/NZS 5263.1.2 (verified 2026-05-09, NCC 2022 Vol 3 Part B2) |
| Electric storage | Resistive element | Yes (25 to 400 L) | Severely restricted in new Class 1 and Class 10 builds under NCC 2022: only permitted at 50 L or less under specific exemption conditions (verified 2026-05-09, NCC 2022 Vol 3 Part B2) |
| Electric instantaneous | Resistive element | Minimal (1 L) | Point-of-use units only; full-home electric instantaneous requires 3-phase supply and is rarely viable for Class 1 |
| Heat pump | Refrigerant cycle, ambient air | Yes (200 to 400 L) | Most energy-efficient electric option; 3 to 4 times more efficient than resistive; scores well in Whole of Home budget (verified 2026-05-09, energyrating.gov.au) |
| Solar (with booster) | Solar thermal collectors + gas or electric booster | Yes (roof-mounted or ground-level tank) | Must meet minimum Small-scale Technology Certificate (STC) thresholds per NCC 2022 Vol 3 Part B2 Table B2D2a (verified 2026-05-09, NCC 2022 Vol 3 Part B2) |
NCC 2022 requirements for new Class 1 and Class 10 buildings
Under NCC 2022 Volume Three Part B2, the choice of HWU in new residential construction is tightly regulated. Gas water heaters must be rated not less than 5 stars in accordance with AS/NZS 5263.1.2 (previously referenced as AS 4552) (verified 2026-05-09, NCC 2022 Vol 3 Part B2). Standard electric resistance storage heaters are effectively prohibited for whole-home use; solar and heat pump units must meet minimum STC thresholds defined in Tables B2D2a and B2D2b (verified 2026-05-09, NCC 2022 Vol 3 Part B2).
Whole of Home energy budget
The HWU is one of the fixed appliances scored in the Whole of Home (WoH) energy budget under NCC 2022 Part H6. The reference heater underpinning the WoH benchmark is a 5-star instantaneous gas water heater; the proposed building must achieve domestic services energy use at 70% or less of that baseline (verified 2026-05-09, NCC 2022 Vol 2 H6P2 and the ABCB Whole-of-Home Efficiency Factors standard). Heat pump HWUs typically score best in the WoH calculation. Resistive electric storage units score poorly and can cause a design to fail WoH without offsetting measures such as on-site PV.
Installation rules
Installation and commissioning of any HWU must comply with AS/NZS 3500.4:2025 (verified 2026-05-09, VBA plumbing standards update). Work is notifiable in most states; a licensed plumber issues a Certificate of Compliance (CoC) at completion. A tempering valve set to 50 degrees Celsius must be fitted at the HWU outlet to protect sanitary fixtures from scalding (AS/NZS 3500.4:2025). Storage units are kept at 60 degrees Celsius or above internally to suppress Legionella growth.
Also known as: hot water system (HWS), water heater, continuous flow (for instantaneous types), continuous flow hot water
Category: Plumbing services / energy compliance
Related
- TMV (Thermostatic Mixing Valve), the tempering valve fitted at the HWU outlet
- Whole of Home (WoH), the NCC 2022 energy budget that scores the HWU
- Plumbing Certificate of Compliance, the sign-off required after HWU installation
- Plumber (trade), the licensed trade that installs and certifies the HWU
- AS/NZS 3500 plumbing standard, the installation standard governing heated water services
See also
- NatHERS, the thermal star-rating scheme running alongside Whole of Home
- HVAC, the other fixed appliance group scored in the WoH budget
- Tempering valve, same as TMV, included here as a disambiguation cross-link
Last updated: 2026-05-09. Verified: 2026-05-09. Quarterly review for currency.