A gas tankless water heater can provide continuous hot water without storing a heated tank, but it needs correct sizing, a suitable gas supply, compliant flueing, condensate management for condensing models and professional installation. It is not automatically the lowest-cost or lowest-emissions option.
Compare the building’s peak hot-water demand, climate, fuel cost and electrification plans before choosing gas. A heat-pump water heater may use substantially less delivered energy and avoids combustion inside the property.
How a gas tankless heater works
When a hot tap opens, a flow sensor starts the burner and the heat exchanger warms water as it passes through. There is no large storage tank, so standby losses can be lower. Maximum delivery still depends on burner input, incoming-water temperature and the requested outlet temperature.
ENERGY STAR explains that certified gas tankless units use a secondary heat exchanger and can use less energy than conventional non-condensing tankless equipment. Its technology overview is useful for understanding condensing designs.
Size for simultaneous demand
| Input | Why it matters | How to assess it |
|---|---|---|
| Peak flow | Several fixtures may run together | Add realistic simultaneous fixture flows |
| Temperature rise | Cold inlet water requires more heating | Target outlet minus winter inlet temperature |
| Gas capacity | The burner may need a larger supply | Licensed gasfitter checks meter, pipe and pressure |
| Minimum flow | Small draws may not trigger the burner | Check the model’s activation rate |
| Location and flue | Combustion gases must discharge safely | Installer checks approved clearances and route |
| Water quality | Scale can reduce heat transfer | Test hardness and follow descaling guidance |
Do not size from household headcount alone. Two bathrooms used simultaneously can matter more than the annual litres consumed. Ask the installer for the assumed winter inlet temperature and the flow available at that rise.
Condensing or non-condensing
Condensing units recover more heat from exhaust gas and generally achieve higher efficiency. They need corrosion-resistant venting and a compliant condensate drain. Non-condensing units may have simpler condensate requirements but discharge hotter exhaust and can use more fuel.
Installation is part of the purchase
- Gas pipe or meter upgrades
- Approved flue material and termination clearances
- Condensate neutralisation and drainage where required
- Power for ignition, controls and freeze protection
- Isolation valves and service access
- Water-pressure and flow compatibility
- Commissioning, compliance documentation and warranty registration
In Victoria, gasfitting work must be performed by an appropriately licensed or registered gasfitter. Energy Safe Victoria’s gasfitter guidance specifically includes hot-water-system installation.
Carbon monoxide and combustion safety
Carbon monoxide is colourless and odourless. Faulty installation, blocked flues, inadequate combustion air and damaged heat exchangers can create serious danger. Never install, alter or service a gas water heater as a DIY project.
Energy Safe Victoria recommends that gas water heaters and other gas heaters be serviced every two years by a qualified gasfitter. Its gas-safety guidance also describes carbon-monoxide testing during a comprehensive service.
Efficiency, emissions and alternatives
Tankless gas can reduce standby losses, but it still burns a fossil fuel and may create methane emissions elsewhere in the supply chain. Compare local energy prices and expected grid changes over the heater’s life. If solar electricity or electrification is planned, a heat-pump system may fit the long-term direction better.
Also consider water use. Continuous hot water can encourage longer showers, which can offset some energy savings.
Maintenance
- Follow the manufacturer’s professional service interval.
- Descale the heat exchanger when water quality and instructions require it.
- Keep air intakes and flue terminals clear.
- Clean inlet screens and inspect isolation valves.
- Check condensate drains on condensing models.
- Investigate error codes, soot, unusual smells or unstable temperatures promptly.
Questions for quotes
- What peak flow and winter temperature rise did you calculate?
- Does the gas supply or meter need upgrading?
- Where will the flue and condensate terminate?
- What efficiency rating applies to this exact model?
- What service and descaling schedule is required?
- How does the installed lifetime cost compare with a heat-pump system?
Our guide to electric tankless water heaters covers another on-demand option. Before leaving any system idle, read when to turn off a water heater.
Bottom line
Choose a gas tankless water heater only after professional sizing and a comparison with heat-pump and storage alternatives. Require compliant installation, model-specific efficiency evidence, accessible servicing and a clear plan for gas supply, flueing, condensate and carbon-monoxide safety.
