Hot water cylinder: what you are really buying
The word is more common overseas, but the thing is familiar: an insulated tank that stores hot water. What matters is how it is built and how it is supplied.
A hot water cylinder is a storage tank that heats water and holds it ready for use. In Australia most people say hot water tank or storage system. The idea is the same: a vertical insulated vessel, heated by electricity, gas, a heat pump or the sun.
If you came here comparing a cylinder with a continuous flow unit, the short answer is this. A cylinder gives you a fixed volume of hot water and it keeps some heat in reserve. A continuous flow unit heats as you use it. Neither is better for every household.
How does a cylinder work?
Cold water enters near the bottom, heats, and rises. Hot water leaves from the top. An insulating layer around the vessel slows the heat loss, and a thermostat or burner control keeps the contents at a set temperature.
Safety parts sit around it. A relief valve guards against over pressure and a tempering valve limits the temperature at your taps. Our guides to the relief valve and tempering valves explain both.
What types of cylinder are there?
| Type | How it is built or supplied | Worth knowing |
|---|---|---|
| Mains pressure | Sealed vessel fed directly from the water main | Strong flow at the taps. This is the usual choice in Australian homes |
| Low pressure (vented) | Fed from a header tank, so pressure depends on height | Older design. Flow can be weak, especially in a single storey house |
| Electric | Heating element inside the vessel | Often paired with off-peak supply. See off-peak hot water |
| Gas storage | Burner under or beside the vessel with a flue | Recovers faster than electric. Gas work is for a licensed gasfitter |
| Heat pump or solar | Same storage vessel with a different heat source | See heat pump hot water |
What is the vessel made of?
Common materials are vitreous enamel lined steel, stainless steel and copper. Each resists corrosion differently, and the right choice depends partly on your local water. Hard or aggressive water shortens the life of some vessels.
Most steel vessels also carry a sacrificial anode that wears away so the tank does not. Ask your plumber about it at service time, since it is one of the main reasons a cylinder lasts or fails early. Our page on how long a hot water system lasts covers the rest.
Is a cylinder the right choice for you?
It suits a household that uses hot water in busy bursts, such as morning showers and an evening wash. The stored volume copes with that. It is less suited to a home with very little space, or one where hot water is used only now and then.
Size matters more than most buyers expect. Too small and you run out. Too large and you pay to keep a big tank hot all day. Our guide to what size system you need sets out how to think it through.
Questions to ask before you buy
- Is it mains pressure? Confirm it matches the supply to your house.
- What is the warranty on the vessel and the parts? Terms differ, and often depend on a licensed installer.
- What does the anode look like at service? Ask for it to be checked.
- Where will it sit? Check access, drainage for the relief valve and, for gas, the flue clearances.
Cylinder or continuous flow: which suits your household?
Both give you hot water, but they behave differently. This table compares them on the points that decide a household's choice.
| Question | Storage cylinder | Continuous flow |
|---|---|---|
| How does it heat? | Heats water and holds it ready | Heats as you use it |
| Hot water in reserve? | Yes, a fixed volume | No stored volume |
| Busy morning showers and an evening wash | Copes well, because of the stored volume | Depends on what the unit can heat as you go |
| Little space | Less suited, as the vessel is large | Better suited |
| Hot water used now and then | Less suited, as you pay to keep a big tank hot | Better suited |
| Size risk | Too small and you run out, too large and you heat water all day | Sized on how much you use at once |
For the full comparison read instantaneous vs storage hot water. Then use what size you need to avoid the biggest buying mistake.
What to ask at a cylinder service
A cylinder can last for years or fail early, and the difference is often what gets checked at service time. Take this list with you.
- How is the anode? Most steel vessels carry a sacrificial anode that wears away so the tank does not. Ask for it to be checked.
- Is the relief valve draining properly? It guards against over pressure and needs somewhere safe to drain.
- Is the tempering valve limiting the temperature at the taps? It is a separate safety part.
- How hard is my water? Hard or aggressive water shortens the life of some vessels, so the material of your vessel matters.
- Is the vessel mains pressure? Confirm it matches the supply to your house.
- Does the warranty need a licensed installer? Terms often depend on it, so keep the service records.
Everything on this list is a question for a licensed plumber, not a job to do yourself. Our page on the sacrificial anode explains why it matters.
Frequently asked questions
Is a hot water cylinder the same as a hot water tank?
In practice, yes. Cylinder is the term used more often overseas. In Australia the same vessel is usually called a storage tank or storage hot water system.
What is the difference between mains pressure and low pressure cylinders?
A mains pressure cylinder is fed straight from the water main, so flow at the taps is strong. A low pressure cylinder is fed from a header tank, and its flow depends on the height of that tank.
Can I replace a hot water cylinder myself?
No. Replacing one is licensed plumbing work in every Australian state and territory, and any gas or electrical connection needs a licensed gasfitter or electrician.
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)