Gas vs Electric Hot Water for Commercial Buildings

What You Need to Know

Commercial hot water systems fall into two camps: gas-fired and electric. Gas boilers have dominated Australian commercial buildings for decades, but heat pump technology and tightening energy regulations under NCC 2025 Section J are shifting the equation. The right choice depends on your building type, peak demand profile, available space, and how much weight you give to long-term operating cost versus upfront capital.

This memo breaks down the technical differences, code requirements under AS/NZS 3500.4 and AS 3814, cost comparisons at current Sydney energy prices, and the practical design decisions that determine which system suits your project.

• Hot water must be stored at 60°C or above to prevent Legionella bacteria growth, regardless of heat source (AS/NZS 3500.4, Clause 1.9)
• Delivery temperature at taps must not exceed 50°C generally, or 45°C in healthcare, aged care, childcare, and schools (AS/NZS 3500.4, Part 2)
• Gas installations must comply with AS 3814 for equipment above 500 MJ/h input, covering flue design, ventilation, and combustion air supply (AS 3814:2018)
• Gas appliances below 500 MJ/h must comply with AS/NZS 5601.1 for installation requirements including clearances and flue terminals (AS/NZS 5601.1:2013)
• NCC 2025 Section J sets energy efficiency requirements that strongly favour high-efficiency systems including heat pumps, with minimum performance thresholds for water heating (NCC 2025, Section J)
• Hot water plant must be sized to the peak hourly demand using fixture loading units and recovery rates, not average daily consumption (AS/NZS 3500.4, Section 5)
• Tempering valves or thermostatic mixing valves (TMVs) are required between the heat source and all personal hygiene outlets (AS/NZS 3500.4)

Gas Boiler Systems

Gas boilers for commercial hot water come in two configurations: storage and instantaneous. Storage systems heat a large volume of water (typically 300 to 2,000 litres) and hold it at 60°C or above, ready for peak demand. Instantaneous gas boilers heat water on demand as it flows through the unit, eliminating the need for large storage tanks but requiring higher gas input rates.

Modern commercial gas boilers achieve thermal efficiencies of 85% to 95%, with condensing models at the top of that range. Condensing boilers recover latent heat from flue gases by cooling them below the dew point, which requires a condensate drain connection and corrosion-resistant flue materials (typically stainless steel or polypropylene).

Every gas boiler needs a flue. The flue must terminate externally and comply with separation distances from openable windows, air intakes, and boundaries under AS 3814 or AS/NZS 5601.1. For rooftop plant rooms this is straightforward. For basement or ground-floor plant rooms, the flue must run vertically through the building or horizontally through an external wall, which adds cost and coordination complexity.

Gas supply sizing is critical. A 500 kW gas boiler consumes approximately 55 cubic metres of natural gas per hour. The gas meter, regulator, and supply pipe must be sized accordingly. Jemena (the Sydney gas distributor) requires a formal application for commercial gas loads, and lead times for new or upgraded connections can be 8 to 12 weeks.

Heat Pump Hot Water

Heat pumps extract thermal energy from ambient air and transfer it to water using a refrigeration cycle. The key metric is COP (Coefficient of Performance). A heat pump with a COP of 4.0 produces 4 kW of heat for every 1 kW of electricity consumed. Commercial heat pumps typically achieve COPs of 3.0 to 5.0, depending on ambient temperature and water delivery temperature.

COP drops as ambient temperature falls. At 7°C ambient (a cold Sydney winter morning), a heat pump that delivers COP 4.5 at 20°C may drop to COP 2.5 to 3.0. In Sydney's climate, this seasonal variation is manageable. In colder climates (Canberra, Hobart), oversizing or hybrid configurations may be necessary.

Heat pumps produce noise. Outdoor condensing units for a commercial system typically generate 55 to 65 dB(A) at 1 metre. Acoustic treatment, setback distances, and barrier walls may be required to meet EPA noise criteria, particularly on sites adjacent to residential buildings. This needs to be addressed at design stage, not after installation.

Heat pumps need outdoor space for the air-source evaporator unit. A 100 kW heat pump system may require 4 to 6 square metres of outdoor area with adequate airflow clearance on all sides. Rooftop installation is common but adds structural loading considerations.

Electric Storage and Instantaneous

Direct electric resistance hot water (storage or instantaneous) converts electricity to heat at a 1:1 ratio, meaning a COP of 1.0. This makes it the most expensive option to run. Electric storage heaters are simple to install, have no flue requirements, and take up minimal space. However, the high operating cost and poor energy efficiency make them difficult to justify under NCC 2025 Section J for anything beyond small supplementary systems.

Instantaneous electric units can handle point-of-use applications (a single basin in a remote location, for example) but draw very high electrical current. A single 24 kW instantaneous unit draws over 100 amps at 240V, which has significant implications for switchboard and cable sizing.

Capital and Operating Cost Comparison

For a mid-sized commercial building (50 to 100 fixtures), indicative installed costs in Sydney as of early 2026:

Gas boiler system (storage): $40,000 to $80,000 installed, including storage tanks, flue, gas connection, and controls. Lower upfront cost but ongoing gas supply charges.

Heat pump system: $70,000 to $130,000 installed, including heat pump units, storage tanks, controls, and acoustic treatment. Higher upfront cost but significantly lower operating cost.

Direct electric storage: $25,000 to $50,000 installed. Lowest upfront cost but highest operating cost by a wide margin.

At current Sydney energy prices (gas at approximately $0.04 to $0.05 per MJ, electricity at approximately $0.28 to $0.35 per kWh), a heat pump with COP 4.0 costs roughly 40% to 60% less to operate than a gas boiler per unit of hot water delivered. The payback period on the additional capital cost of a heat pump versus gas is typically 4 to 7 years, depending on demand volume.

NCC 2025 Section J and the Shift to Electrification

NCC 2025 Section J sets stringent energy efficiency requirements for water heating in commercial buildings. Heat pumps are strongly favoured because their high COP translates to lower greenhouse gas intensity per litre of hot water. Gas boilers face a steeper compliance path, often requiring additional energy efficiency measures elsewhere in the building to offset the lower efficiency of gas combustion.

The broader trajectory in NSW is toward electrification. The NSW Electricity Infrastructure Roadmap, combined with falling renewable electricity costs and potential future restrictions on new gas connections, makes heat pumps the lower-risk long-term choice for new buildings. Gas boilers remain viable for existing buildings with established gas infrastructure, but specifying new gas connections on a greenfield site carries increasing regulatory and commercial risk.

Carbon Emissions

Using the current NSW grid emission factor of approximately 0.68 kg CO2-e per kWh, a heat pump with COP 4.0 produces roughly 0.17 kg CO2-e per kWh of heat delivered. A gas boiler at 90% efficiency produces approximately 0.23 kg CO2-e per kWh of heat delivered (based on natural gas emission factor of 51.53 kg CO2-e per GJ). As the NSW grid decarbonises with more renewables, the heat pump emissions figure will continue to fall while gas remains fixed.

Legionella Compliance

Both gas and electric systems must store water at 60°C minimum per AS/NZS 3500.4. Heat pumps can deliver water at 60°C to 65°C, though COP drops at higher delivery temperatures. Some commercial heat pumps use a booster element to achieve storage temperature when ambient conditions are unfavourable. Gas boilers handle the 60°C requirement without difficulty. Regardless of heat source, circulating loops must maintain return water above 55°C, and dead legs must be minimised to reduce Legionella risk.

Where Each System Excels

Gas boilers suit: Hotels and hospitality with high simultaneous peak demand (hundreds of showers in a short window), buildings with existing gas infrastructure and limited electrical capacity, and projects where rapid recovery rate is critical.

Heat pumps suit: Offices, retail, and commercial buildings with moderate and steady hot water demand, new builds where NCC 2025 Section J compliance is a priority, and projects where the client values long-term operating cost reduction and future-proofing against gas phase-out.

Direct electric suits: Small supplementary applications only. A remote basin or kitchenette where running hot water pipework from the central plant is impractical. Not appropriate as a primary system for any commercial building.