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Continuous-flow gas hot-water heaters warm water only when a tap, shower or appliance calls for it. They remove the storage tank, but they do not remove design limits: the burner, heat exchanger, gas service, water flow, pipe layout and temperature controls must all suit the simultaneous demand. In Australia, select and install a system from the actual outlet flow, temperature rise, inlet-water conditions and building gas load, then have a licensed professional verify compliance with current state or territory requirements.
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What continuous-flow gas hot water is
A continuous-flow system—also called instantaneous gas hot water—has no tank of heated water waiting for use. When a hot outlet opens, a flow sensor starts the gas burner. Water passes through a heat exchanger, absorbs heat and travels to the fixture. When the outlet closes, the burner stops.
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The practical benefit is that the heater does not run out of stored hot water and avoids the standing heat losses associated with a tank. Its output is still finite. Flow rate, required temperature rise, incoming-water temperature, burner turndown and gas-supply capacity determine how many outlets can operate comfortably at once. A unit that works well for one shower may not maintain the same temperature when two showers and a kitchen tap open together.
How the heater delivers hot water
Flow sensing and burner modulation
The appliance senses water movement and modulates its burner to match demand. The heat exchanger must raise the incoming water to the selected delivery temperature without exceeding its operating range. If demand rises beyond the appliance’s rated output, the delivered temperature can fall or the flow may need to be reduced.
The short delivery delay
There is still water between the heater and the outlet. NSW guidance says that typically 1–2 litres pass through an instantaneous heater during the short delay before heated water arrives. Long pipe runs increase waiting time, water waste and heat loss, even though the heater itself has no storage tank.
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Outdoor and indoor locations
Gas units are commonly installed outdoors because combustion and flue requirements are simpler there. Indoor installation is possible only when the selected appliance, ventilation arrangement and installation instructions permit it. Confirm clearances, service access, drainage, weather exposure, flue termination and the exact location against the manufacturer’s instructions and local rules. Do not assume that an outdoor-rated model can be enclosed or relocated indoors.
How to size a continuous-flow gas hot-water system
Continuous-flow units are sized by the outlets likely to operate simultaneously, not simply by the number of people in the household. Victorian consumer guidance specifically uses outlet count, while broader Australian guidance notes that standard systems generally serve one or two points at a time and higher-performance models can serve several.
1. List realistic simultaneous uses
Write down the combinations that can occur in the busiest period: showers, baths, kitchen and laundry taps, a dishwasher and any other hot-water equipment. Use actual fixture flow rates where available. “Two showers at once” is not a complete design input; each shower’s litres per minute and the required temperature matter.
2. Establish the temperature rise
Temperature rise is the difference between incoming cold-water temperature and the desired hot-water delivery temperature. Cold inlet water in winter requires more burner output than warm inlet water in summer. Climate, mains temperature, a storage tank upstream and solar preheating can all change the inlet condition. Product litres-per-minute claims are meaningful only at the manufacturer’s stated temperature-rise condition.
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3. Match peak flow to current manufacturer data
Use the selected model’s current technical documentation for maximum flow at the relevant temperature rise, minimum operating flow, gas consumption, water-pressure range and approved installation configuration. Do not treat a nominal litres-per-minute number as universal capacity across climates or inlet temperatures.
4. Have a qualified person verify the result
A licensed gas fitter or appropriately qualified hot-water designer should check the demand calculation, supply pressure, pipe sizing, meter capacity, water pressure, controls and local installation rules. Product selection and connection details are project-specific decisions, not safe assumptions based on household size alone.
Can low-flow fixtures or solar preheat cause problems?
Every continuous-flow heater has a minimum flow at which it can ignite and regulate safely. The Australian Building Codes Board’s Handbook: Warm water systems (2020) states: “The minimum flow rate for most continuous flow gas water heaters is between 2 to 3 L/minute.” The handbook also warns that a booster may not ignite below that rate. The selected manufacturer’s minimum always takes precedence.
Low flow can be an issue with water-saving showerheads, partly closed taps, thermostatic valves or a single small outlet. Warm inlet water creates another condition: the burner may be unable to turn down far enough without overheating the water, so some models shut down at low demand. This can occur in warm climates or where solar preheating raises the incoming temperature.
Check minimum and maximum flow, minimum temperature rise, burner turndown and any low-flow shut-off behaviour before pairing a heater with efficient fixtures or solar preheat. A design that works during a cold winter morning may behave differently when inlet water is already warm.
Will existing gas pipes support a continuous-flow heater?
Possibly, but it must be checked rather than assumed. High hot-water output can require a substantial gas flow. An existing installation may have been sized for a smaller storage heater, cooktop or space heater. Replacing the water heater can therefore require larger pipework, a different regulator or a meter upgrade.
The gas fitter should assess the entire building’s concurrent load: water heating plus cooking, space heating, fireplaces, boilers and any other gas appliances that may operate together. The check should include available supply pressure at the appliance, pipe length and diameter, regulator settings, meter capacity and applicable gas-installation rules. A heater that performs adequately in isolation can underperform if pressure drops when another appliance starts.
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Water-side layout and controls
Keep runs short
Place the heater as close as practical to the main hot-water outlets. Shorter runs reduce the 1–2 litre delivery delay, limit standing water in the pipe and reduce heat loss. In a large home or multi-residential building, the shortest route for one outlet may create a long route for another, so map the whole distribution layout before fixing the plant location.
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Insulate accessible hot-water pipes, particularly outside the conditioned envelope and in plant areas. Insulation reduces heat loss between the heater and fixture and helps maintain temperature during periods of intermittent use. Follow the applicable plumbing standard and the insulation requirements for the project.
Provide safety valves and discharge paths
YourHome identifies correctly installed safety valves and discharge pipes, leak-free connections, clear operating instructions and compliance documentation as installation expectations. The design must also account for pressure, expansion, drainage and safe discharge locations. These are licensed installation tasks, not DIY modifications.
Temperature safety and Australian compliance
YourHome states that hot-water systems must be designed and installed in accordance with Section 8 of AS/NZS 3500.4:2021. The same guidance discusses limiting delivered water temperature to 50°C to reduce scald risk. The exact requirement depends on state or territory rules, building classification, outlet type and use; health, aged-care, education and other settings can have additional provisions.
At handover, obtain the required compliance certificate and operating instructions. Confirm that temperature controls, valves, discharge pipes, isolation points and service clearances are accessible and labelled. State and territory requirements, product approvals and temperature limits can change, so verify them at design and installation time rather than relying on an old certificate or an online diagram.
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Central systems for apartments and other multi-residential buildings
For a multi-residential project, central gas instantaneous heating is one option among several central-plant arrangements. NSW BASIX lists gas instantaneous, gas-fired boiler, manifolded gas storage, electric storage, electric instantaneous and heat-pump-based systems.
A central design needs more than a larger heater. The hydraulic consultant should model peak demand and diversity, circulation flow, return temperatures, pipe insulation, plant-room ventilation, maintenance access, metering, redundancy and the consequences of a plant outage. NSW planning guidance strongly recommends a qualified hydraulic consultant for central hot-water design and notes that pipe insulation affects heat loss and energy use.
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Compare central plant with shared or in-unit equipment using the building’s actual occupancy, outlet pattern, available plant space, maintenance access, emissions assumptions and tariff structure. Circulation losses can materially change the result even when the central burner is efficient.
Energy use, emissions and alternatives
Continuous flow avoids tank standing losses, but it is not loss-free. Short draws still involve burner start-up and cool-down, and hot-water pipes lose heat between uses. Shorter runs, insulation and correctly sized controls improve the system’s real-world performance.
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1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteThe Australian Government reports that water heating accounts for 15%–30% of household energy use and can represent up to 25% of greenhouse-gas emissions for an average Australian home. These are broad national ranges, not a prediction for a particular dwelling.
Heat-pump water heaters are a different technology. Government guidance says they use 30% of the energy of a conventional electric hot-water system; that comparison is with conventional electric heating, not directly with gas instantaneous. Solar systems depend on solar resource, storage, backup heating and roof or plant-space conditions.
Compare gas, heat pump and solar against the same load and location. Include:
- Peak simultaneous flow and required temperature rise.
- Seasonal inlet-water temperature and local climate.
- Minimum stable flow and compatibility with low-flow fixtures or solar preheat.
- Gas, electrical and water-service capacity.
- Ventilation, clearances, plant space and pipe-run length.
- Purchase, installation, maintenance and expected service-life costs.
- Energy prices, emissions intensity and any current rebates or planning conditions.
Gas is a fossil fuel, while electricity emissions depend on the supply mix and operating pattern. No technology is universally the cheapest or lowest-emissions option without defining the site, tariff, climate and demand assumptions.
Design and installation workflow
- Define the brief: record the building type, occupants, fixtures, likely simultaneous uses, desired delivery temperature and any special users or hygiene requirements.
- Measure the services: document incoming-water pressure and temperature, existing gas meter and pipe sizes, available electrical supply, drainage and possible heater locations.
- Calculate peak demand: use actual fixture flows, simultaneous-use scenarios and the required temperature rise rather than a household-size rule.
- Screen candidate appliances: check current manufacturer data for output at the project temperature rise, minimum flow, modulation range, gas input, water-pressure limits and approved mounting conditions.
- Check the whole gas load: have a licensed gas fitter verify pressure, pipework, regulator and meter capacity with all concurrent appliances included.
- Lay out distribution: keep runs short, insulate exposed pipe, provide service access and plan safe drainage and discharge paths.
- Confirm compliance: verify AS/NZS 3500.4:2021 Section 8 provisions, state or territory rules, ventilation and clearances, temperature controls and product approvals.
- Commission and document: test ignition, stable operation, outlet temperature, safety devices and simultaneous demand; then obtain compliance documents and give occupants operating and maintenance instructions.
Common design mistakes and how to avoid them
Sizing from bedrooms or occupant count alone
People count does not reveal whether two showers, a bath filler and a dishwasher overlap. Base the calculation on outlets, flow and temperature rise.
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Using the headline flow rate without its test condition
A litres-per-minute figure can be quoted at a particular temperature rise and inlet temperature. Check the full performance table for the conditions that match the site.
Ignoring warm-weather or preheated inlet water
A heater that is stable in winter may cycle or shut down at low demand when incoming water is warm. Confirm burner turndown and minimum-flow behaviour.
Assuming the existing gas service is adequate
Have the complete concurrent gas load assessed before ordering. A meter or pipe upgrade can affect cost, programme and appliance choice.
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The resulting water pipe may be long, delaying hot water and increasing heat loss. Coordinate gas, water, ventilation, drainage and maintenance access as one layout.
Treating compliance as a final paperwork step
Temperature limits, valve discharge, ventilation and clearances must shape the design from the beginning. Obtain the certificate and handover information after commissioning.
Key figures and their qualifications
| Figure | What it describes | Qualification |
|---|---|---|
| 15%–30% | Share of household energy use attributed to water heating | Broad Australian Government range for households |
| Up to 25% | Share of greenhouse-gas emissions attributed to water heating in an average Australian home | Government-reported national estimate, not a site-specific forecast |
| 30% of the energy | Energy used by a heat-pump water heater compared with conventional electric hot-water heating | Government comparison; it is not a direct gas-versus-heat-pump result |
| 1–2 litres | Water that typically passes through an instantaneous heater during the short delivery delay | NSW guidance; actual volume depends on pipe length and layout |
| 2–3 L/minute | Minimum flow for most continuous-flow gas water heaters | ABCB Handbook: Warm water systems (2020); the selected manufacturer’s requirement prevails |
Bottom line
A continuous-flow gas hot-water system is a compact, tank-free solution when its peak flow, temperature rise, minimum-flow behaviour and gas demand match the building. Treat sizing, gas-service verification, pipe layout, temperature control and compliance as one design task. For an Australian project, have a qualified professional confirm the current appliance data and state or territory requirements before purchase or installation; guidance pages accessed in October 2026 may not reflect later product, code or incentive changes.
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