What type of water system do I have — combi, cylinder or unvented?

This is the question underneath most other plumbing questions. Why the shower is weak, whether a power shower will help, why the hot tap runs harder than the cold, whether you get airlocks, what happens in a power cut, what a plumber is allowed to touch — all of it changes depending on which of three arrangements your house has. It takes five minutes to establish and it is worth doing before you read anything else.

Step by step

1. Look for a hot water cylinder.A tall insulated copper or steel cylinder, usually in an airing cupboard, sometimes in a loft, a garage or under the stairs. If there is no cylinder anywhere and hot water appears within seconds of opening a tap, you almost certainly have a combi boiler.
2. Look in the loft.A large plastic or galvanised cistern, typically holding a couple of hundred litres, with a ball valve. That is a cold water storage cistern and it means a vented, gravity-fed system. There may be a much smaller one beside it, which belongs to the heating rather than the water.
3. Put the two answers together.No cylinder, no tank: combi. Cylinder plus a big loft tank: vented gravity. Cylinder but no loft tank at all: unvented, fed straight off the main. A cylinder with no tank and no visible expansion vessel may be a thermal store instead, which behaves like a cylinder but heats the water as it passes through a coil.
4. Confirm with the hot tap test.Open a hot tap upstairs fully and watch it. If it runs as hard as the cold tap beside it, hot water is at mains pressure, so the system is combi or unvented. If it runs noticeably softer, the hot side is gravity-fed from the loft.
5. Look at what is attached to the cylinder.An unvented cylinder carries hardware a vented one never has: a red or blue expansion vessel nearby, a control group of valves on the incoming cold pipe, and a small open funnel called a tundish with a pipe running outside. A vented cylinder has none of that — just a vent pipe going up to the loft.
6. Check the boiler if you still are not sure.A combi is a single wall-hung unit with five small pipes underneath, one of which is a pressure gauge circuit, and it has no cylinder anywhere in the house. A system or heat-only boiler works with a cylinder and usually has a pressure gauge (sealed) or a feed and expansion tank in the loft (open-vented).
7. Write the answer down somewhere.In the meter cupboard or inside the boiler cupboard door. It is the first question every plumber, every shower manufacturer and every troubleshooting guide asks, and the answer never changes until the system does.

Tips

• The single most useful diagnostic in the house is running the hot and cold taps side by side. Equal force means mains-pressure hot.
• A house can be a hybrid: an unvented cylinder heated by a heat-only boiler, a combi with an added accumulator, or a gravity system converted with pumps. Identify the hot water arrangement and the heating arrangement separately.
• A cold tank in the loft does not automatically mean the hot water is gravity fed — some houses store cold water and then pump it. If there is a pump near the cylinder, look at what it feeds.
• In flats, look for the cylinder in a hall cupboard. Loft tanks are rare and communal systems are common, so if there is neither, ask the managing agent.

Alternatives

• Combi: Heats water on demand as it flows through the boiler. No stored hot water, no cylinder, no tanks. Hot and cold both at mains pressure. Flow rate is limited by how fast the boiler can raise the temperature, so hot water gets noticeably worse when two outlets run at once, and much worse in winter when the incoming main is cold.
• Vented (gravity / open-vented): Cold water is stored in a loft cistern which feeds both the cold taps and the hot cylinder. Hot water is pushed out only by the height of water above the tap, so upstairs showers are weak. Airlocks are possible, pressure is low but flow can be generous, and it works in a power cut.
• Unvented (mains-pressure stored): A sealed cylinder fed directly from the main. Strong hot and cold at every outlet, several outlets at once, no loft tanks. Carries pressure and temperature safety devices, needs annual attention, and any work on it requires a specific qualification.
• Thermal store: Looks like a cylinder but works the other way round — the stored water is heating water, and your mains cold passes through a coil inside it and comes out hot. Gives mains-pressure hot water without the unvented safety regime. Less common but not rare.

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