What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thinghot water cylinder
also cylinder, unvented cylinder, calorifier
A "tank" in British usage is the COLD storage cistern, usually in the loft — so "hot water tank" reads as a category error rather than a synonym for the cylinder.
Australia
Close, not identicalhot water unit
also HWS, hot water service, cylinder
Australian usage names the whole SERVICE rather than the vessel, and the unit is commonly outdoors — a siting difference that changes the pipe runs and the losses.
United Statesyours
Same word, different thingwater heater
also hot water tank, tank water heater
Names a vessel that HEATS as well as stores, usually directly fired or electric. The British cylinder is more often heated indirectly by a boiler through a coil.
Canada
Same word, different thingwater heater
also hot water tank
As the United States, and commonly rented from the utility rather than owned — which has no British or Australian equivalent and changes who replaces it.
The governing standard, by market
United Kingdom
Approved Document G / BS EN 12897
Hot water safety, and unvented storage water heater requirements.
United States
ASHRAE 90.1 / IPC
Water heating equipment efficiency and installation requirements.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is a hot water tank the same as a cylinder?
- The words point at the same job and carry different assumptions, and in British usage "tank" points somewhere else entirely. A traditional British system has a cold water storage TANK — an open cistern in the loft — feeding a hot water CYLINDER lower down, with the height difference providing the pressure. So a British plumber hearing "hot water tank" reaches for a clarification rather than a spanner. American usage has no equivalent loft cistern in most housing, the supply is mains-pressure throughout, and the storage vessel is a tank that heats its own contents. Both deliver hot water; they are different system architectures with different failure modes and different components.
- What is the difference between vented and unvented?
- Where the pressure comes from and what contains it, and it is the distinction with the most safety weight. A vented cylinder is fed from an open cistern above it and is open to atmosphere, so it cannot be pressurised and its flow depends on the height of the cistern. An unvented cylinder is fed directly from the main, sealed, and therefore contains mains pressure and the expansion of heated water — which is why it requires an expansion vessel or air gap, a pressure relief arrangement and a temperature relief device, and why installing or servicing one is a competence-restricted activity in several jurisdictions. A sealed vessel with a failed relief path is a genuine hazard, not merely a leak.
- Why does an expansion vessel exist at all?
- Because water expands when heated and a sealed system has nowhere to put the extra volume. In an open vented system the expansion simply pushes water back up toward the cistern, which is why no vessel is needed. Seal the system — an unvented cylinder, or a pressurised heating circuit — and the same expansion has to be absorbed somewhere, or the pressure rises until a relief device opens and discharges. An expansion vessel gives it a compressible gas cushion behind a diaphragm to push into. Its size follows from the system volume and the temperature rise, and the commonest fault is not a failed vessel but a lost gas charge, which lets the diaphragm bottom out and the pressure spike on every heating cycle.