Gas hot water systems: what decides between storage and continuous flow
Two showers at once is the question that separates the two types, and the answer is a number the brochure quotes optimistically. Here is what that number means, and what changes if you are on bottled LPG.
A gas hot water system heats water by burning natural gas or LPG, and it comes in two forms that behave completely differently in daily life: storage, which keeps a tank of hot water ready and can run out, and continuous flow (also sold as instantaneous), which heats only while a tap is open and never runs out but delivers a fixed flow rate. Almost every complaint about a gas system traces back to that one distinction being glossed over at the quote stage. Gas also brings two constraints the electric options do not: it has to be sited and flued correctly, and everything on the gas side of the appliance is licensed gasfitting.
Storage or continuous flow?
This is the axis people skip, and it changes daily life more than the brand does.
| Gas storage | Gas continuous flow | |
|---|---|---|
| Runs out? | Yes — then recovers, faster than electric | No — but the flow rate is capped |
| Two showers at once | Fine, until the tank is spent | Each gets roughly half the rated flow |
| Space | A tank-sized footprint outdoors or flued | A wall box; very little space |
| Standing loss | Yes — it keeps a tank hot all day | None — it is cold between uses |
| Needs a power point? | Not always | Yes, for ignition and controls |
That last row surprises people every year. A continuous flow unit is a gas appliance with an electric brain: no power, no hot water, even though the gas is flowing normally. After a blackout or a tripped circuit, that is the first thing to check.
Flow rate: the number that decides everything
A continuous flow unit is rated in litres per minute, and that rating is quoted at a stated temperature rise — how many degrees it lifts the incoming water. Two things make the headline number optimistic:
- Winter mains water is colder, so the same unit lifting water to the same tap temperature delivers fewer litres per minute in July than in January.
- Simultaneous outlets split the flow. A shower plus the kitchen tap, or two bathrooms in the morning, share one capped rate. An undersized unit does not run cold so much as feel weak — and that is the single most common continuous-flow complaint.
So ask for the litres-per-minute figure at a realistic winter temperature rise, not the headline. And size on how many hot outlets run in the same twenty minutes, not on how many people live in the house.
Two related quirks worth knowing. A burner needs a minimum flow to fire at all, which is why a barely-open tap can run cold on a continuous flow system. And the cold water sandwich — a slug of cold arriving mid-shower — happens when the unit stops and restarts between short draw-offs; it is characteristic behaviour, not necessarily a fault.
Natural gas or LPG: what actually changes
A unit is built and configured for one gas type. Natural gas arrives by pipe and does not run out; LPG comes in bottles, which do, and that is worth knowing before you diagnose a fault. If the hot water died with no warning and no other symptom, check the bottles first — an empty bottle, or a changeover valve still pointed at an empty one, produces exactly the same symptom as a dead appliance.
LPG units also run at a different supply pressure and have different injectors. Converting an appliance between gas types is gasfitting work, not a setting.
Gas or electric?
If you already have gas at the meter and limited outdoor space, continuous flow gas is the path of least resistance: small, never runs out, no standing loss. If you have no gas connection, do not add one for hot water alone — the connection and the appliance together make it a poor trade against a heat pump, which needs only a circuit and some air. And a gas storage tank is the weakest of the three on standing loss, because it keeps a tank hot around the clock whether you use it or not. The full comparison across all four types goes through the sizing and siting trade-offs in detail.
Siting, flues and clearances
Gas units are normally installed outdoors in Australia. Indoors they need a proper flue, and either way there are minimum clearances from windows, doors, vents and the boundary, because the appliance vents combustion products. These distances are set by the plumbing and gas codes and are the installer's responsibility — not something to work around because the old unit was there. If a quote proposes a spot near an openable window or a sub-floor vent, ask for the clearance figures in writing.
What can you check, and where does it stop?
Yours: is the isolation tap to the unit open; has a bottle run empty or the changeover not switched; has the power point or circuit for a continuous flow unit tripped; is the controller showing a fault code (write it down — it is the single most useful thing you can hand a technician); and is the lack of hot water at one tap only, which points at a mixer or tempering valve rather than the system.
This far, and no further. Installing, replacing or repairing a gas hot water system needs a licensed plumber holding the relevant gasfitting authorisation, and the rules and the compliance paperwork are set by each state and territory. Check the requirements with the plumbing regulator where you live, ask for the licence number, and keep the compliance certificate with the property records.
Frequently asked questions
Is gas or electric hot water better?
If you already have gas at the meter and limited outdoor space, gas continuous flow is usually the easiest choice: compact, no standing loss, and it never runs out. If you have no gas connection, adding one for hot water alone rarely pays against a heat pump, which needs only a circuit and clear airflow. A gas storage tank is the weakest of the options on standing loss, because it keeps a tank hot around the clock.
Why does my gas hot water run cold when someone else turns on a tap?
A continuous flow system delivers a fixed flow rate in litres per minute, and simultaneous outlets share it. Two showers at once means each gets roughly half. It usually feels like weak, lukewarm water rather than genuinely cold water, and it means the unit is undersized for how the household actually uses it.
Does a gas hot water system need electricity?
A continuous flow unit does — it needs a power point for ignition and its controls, so no power means no hot water even when the gas supply is fine. After a blackout or a tripped circuit, check the power point before assuming the appliance has failed. Gas storage units do not always need one.
My LPG hot water stopped working. What should I check first?
The bottles. An empty bottle, or a changeover valve still pointed at one that is empty, produces exactly the same symptom as a failed appliance and is by far the most common cause. Beyond checking the bottles and the isolation tap, anything on the gas side of the unit is licensed gasfitting work.
Can I relight the pilot on my gas hot water system?
No — treat it as licensed gasfitting work. Relighting a pilot, opening the burner compartment or adjusting the gas valve is not householder work in Australia, and the failure modes here are carbon monoxide and fire. Note any fault code on the controller and call a licensed plumber with gasfitting authorisation.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)