Most people only think about their hot water system when it dies mid-shower on a cold morning. By then you’re making a rushed decision under pressure, often replacing like-for-like without checking whether the old system was ever the right size. A bit of planning now saves you from that scramble and from paying for a unit that can’t keep up with your household.
This guide walks through the two main gas options, how to size them properly, and what actually happens when it’s time to install.
TL;DR
- Gas hot water comes in two forms: storage (a heated tank) and continuous flow (heats on demand, no tank).
- Continuous flow suits smaller homes and tight spaces and delivers effectively unlimited hot water; storage suits larger households with heavy overlapping use.
- Size storage by litres (roughly 90-135L for 1-2 people, 135-170L for 3-4, 170-250L for 5+). Size continuous flow by litres per minute (16-20 for one bathroom, 20-26 for two, 26+ for three or heavy overlap).
- Bathroom count and morning overlap matter more than the number of people alone.
- Gas hot water installation and any gasfitting is licensed work in NSW. It’s not a DIY job.
First, confirm gas is actually the right call for your place
Before comparing models, check a few basics that decide whether gas even makes sense:
- Do you have natural gas connected? Natural gas rates don’t change across the day, so there’s no off-peak juggling. If you’re on LPG bottles instead, expect higher running costs.
- Where can the unit go? Gas units are usually installed outdoors because of venting. Indoor installation is possible but needs proper flueing to the outside, which adds cost and complexity.
- How far is the unit from your main taps? A long pipe run means longer waits for hot water and wasted energy. Shorter runs, or at least insulated pipes, help.
If gas isn’t available at your address, that’s worth knowing early rather than after you’ve picked a model.
Storage vs continuous flow: the core decision
Both heat water with gas. The difference is when.
Gas storage (tank)
A storage system heats water and holds it in an insulated tank until you need it. Gas tanks reheat faster than electric ones, which helps during back-to-back use. The trade-off: they take up more space, they must sit outdoors, and there’s some standing heat loss because you’re keeping a tank warm around the clock.
Storage tends to win when a busy household draws a lot of hot water in a short window, for example several showers plus a load of washing in the same hour.
Gas continuous flow (instantaneous)
A continuous flow unit heats water only as it passes through, so there’s no tank and effectively no running out. These units are compact and mount on an external wall, which is handy when space is tight. They’re often cheaper to buy than a tank system.
One thing to set expectations on: “instantaneous” doesn’t mean the hot water hits the tap the instant you turn it on. There’s still a short delay while heated water travels down the pipe, and that delay grows with distance from the unit.
Quick comparison
| Factor | Gas storage | Continuous flow |
|---|---|---|
| Hot water supply | Limited by tank size, then needs to reheat | Effectively unlimited |
| Space needed | Larger footprint, outdoors | Compact wall-mounted unit |
| Handles heavy overlap | Good, if sized right | Good, if flow rate is high enough |
| Standing heat loss | Yes, keeps a tank warm | Minimal, heats on demand |
| Best suited to | Larger households, simultaneous demand | Smaller homes, tight spaces, steady use |
Sizing it properly (the part most people get wrong)
Getting the size right matters more than the brand. Undersize it and you get cold showers at the worst time; oversize it and you pay for capacity you never use.
Storage: think in litres
As a general starting guide for gas storage:
| Household | Common tank size |
|---|---|
| 1-2 people | 90-135L |
| 3-4 people | 135-170L |
| 5+ people | 170-250L |
Roughly, one person uses about 50L of hot water a day, more if you take long showers or run hot wash cycles.
Continuous flow: think in litres per minute
Continuous flow is rated by flow rate, so bathroom count and overlap drive the choice:
| Usage pattern | Common flow rate |
|---|---|
| 1 bathroom, one shower at a time | 16-20 L/min |
| 2 bathrooms, occasional overlap | 20-26 L/min |
| 3 bathrooms or regular overlap | 26+ L/min |
The real question: what runs at the same time?
Two homes with the same number of people can need very different systems. What matters is your busiest 60 to 90 minutes. A standard shower uses roughly 8-12 L/min, a rainfall shower 12-16 L/min, and a kitchen tap 6-8 L/min. If two showers can run at once in your home, your peak demand climbs fast. Count the fixtures that realistically run together during the morning rush, and size for that, not the average day.
Bathroom count is often the tie-breaker. One bathroom usually means one shower at a time, so the baseline sizing holds. Two or three bathrooms with regular overlap pushes you to the upper end of the range or a higher flow rate.
Common mistakes worth avoiding
- Replacing like-for-like without checking. Your old unit may have been undersized for years, or your household may have grown or shrunk since it went in.
- Buying on upfront price alone. Running costs, recovery speed, and how it copes at peak times all matter over the life of the unit.
- Ignoring the pipe run. A unit far from the bathrooms wastes water and time while you wait for hot water to arrive.
- Forgetting placement rules. Gas units need proper venting and clearances. Where it can legally and safely go can change which model fits.
Installation is licensed work, not a weekend project
This is the part to be clear on. In NSW, installing or replacing a gas hot water system involves gasfitting and regulated plumbing work that must be carried out by a licensed professional. It’s not a job to attempt yourself, and doing so can be dangerous and unlawful.
What you can safely do is the planning: work out your household’s peak demand, check whether you have natural gas or LPG, note where the current unit sits, and think about whether it kept up with your needs. Leave the sizing sign-off, gas connection, flueing, and compliance to your plumber. If you ever smell gas, don’t try to investigate the unit yourself. Turn off the supply if it’s safe to reach, keep clear, and call your gas provider’s emergency line.
A good installer will confirm the right size for your home, handle the connection and venting to standard, and dispose of the old unit correctly.
How to make the decision
- Confirm your fuel: natural gas or LPG, and whether it’s connected.
- Map your peak usage: list what runs at the same time on your busiest morning.
- Pick the type: continuous flow for compact spaces and steady demand, storage for heavy simultaneous use.
- Match the size: litres for storage, litres per minute for continuous flow, weighted for bathroom overlap.
- Check placement: outdoor space, clearances, and pipe run to your main taps.
- Get a licensed plumber to verify sizing and quote the install.
The takeaway
The right gas hot water system is the one sized for how your household actually uses water, not the one that happens to match the old unit. Sort out fuel, peak demand, type, and placement first, then let a licensed plumber confirm and install it. If you’d rather not wait for the current system to fail before deciding, it’s the same logic behind booking regular plumbing servicing: dealing with it early beats a cold-shower emergency. When you’re ready to compare options for your home, get in touch with the Jasco team for current advice and sizing.