How Does a Hot Water Cylinder Work?

Updated July 2026 · Emergency Plumber Chelsea

The short version. If your home has a boiler and a separate tank in a cupboard, you have a stored hot water system. The boiler heats water, the cylinder holds it, and your taps and showers draw from that store. This is different from a combi boiler, which has no cylinder at all and heats water on demand as it flows to the tap. Our guide to how a combi boiler works explains that system. Stored hot water is still very common in Chelsea's larger flats and townhouses, because a cylinder can supply several bathrooms at once in a way a single combi often cannot.

Cold in at the bottom, hot out at the top. Hot water is lighter than cold, so inside a cylinder it naturally settles in layers, with the hottest water at the top and the coldest at the bottom. The cylinder is plumbed to use that. The cold feed enters low down, and the hot draw-off leaves from the very top. When you run a bath, cold water flows in at the bottom to replace what you take out, and pushes the hot water up and out. This is why a cylinder can give you a full bath of properly hot water even while fresh cold water is coming in beneath it, and why the water only turns lukewarm right at the end, when the cold layer finally reaches the top.

How the boiler heats the water. Most cylinders in homes with a gas boiler are indirect. Inside the tank is a coiled copper pipe, the heat exchanger, and the boiler pumps its own hot water round that coil in a closed loop, called the primary circuit. The water in the coil never mixes with the water you use at the tap; it simply passes its heat through the copper into the stored water around it, then returns to the boiler to be heated again. That is why the heating water can be dark and full of inhibitor while your hot tap runs clean. A direct cylinder has no coil and is heated only by electricity, which you tend to find in flats with no gas supply.

The immersion heater. Most cylinders also have an immersion heater, an electric element that sits inside the tank like the element in a kettle. In a direct cylinder it is the main heat source, and some have two elements, one near the top for a quick top-up and one near the bottom to heat the whole tank on a cheaper night tariff. In an indirect cylinder the immersion is usually a backup for when the boiler is off or broken. Our guide on what an immersion heater is covers how to use one safely and what it costs to run.

The cylinder thermostat. Strapped to the side of an indirect cylinder, usually about a third of the way up, is a small thermostat with a dial. It reads the temperature of the stored water and switches the boiler's supply to the coil on and off. When the water drops below the setting it calls for heat, and once the water is up to temperature it stops the call. It is commonly set at around 60°C, which is hot enough to control legionella bacteria in stored water; turning it much lower to save money is not a good trade. If the thermostat has been knocked down, or it has slipped off the cylinder wall, you get lukewarm water or water that is far too hot, which our guide to hot water that is not hot enough walks through.

The valve that shares the boiler's heat. One boiler has to serve both the radiators and the cylinder, so a motorised valve decides where the hot water from the boiler goes. Some systems have a single three-port valve with a motor head that can send heat to the cylinder, the radiators or both at once. Others have two separate two-port zone valves, one for heating and one for hot water. Both are driven by your programmer and the thermostats. A failed valve motor is one of the most common reasons for having heating but no hot water, or hot water but no heating, in homes with a cylinder, which is why our guides to heating that works but no hot water and hot water but no heating both start with that valve.

Vented cylinders: fed by a tank in the loft. How the cold water gets into the cylinder is the other half of the story. An older, vented cylinder is fed by gravity from a cold water storage tank in the loft or on a high shelf. Water falls from the tank into the bottom of the cylinder, and an open vent pipe rises from the top of the cylinder and bends over the tank, so if the water ever got too hot it could expand safely up the pipe. The pressure at your taps depends on how far above them that tank sits, which is why top-floor showers in a tall Chelsea house can feel weak. If the tank itself is overflowing or noisy, see our guide to cold water storage tank problems.

Unvented cylinders: fed straight from the mains. An unvented cylinder has no loft tank. It takes cold water directly from the incoming mains, so hot water leaves it at close to mains pressure and showers feel strong on every floor. Because it is a sealed vessel under pressure, it needs safety equipment that a vented cylinder does not: a pressure-reducing valve on the inlet, an expansion vessel to absorb the water as it expands on heating, an expansion relief valve, and a temperature and pressure relief valve that discharges through a tundish to a safe drain. Fitting one is notifiable building work that must be done by an engineer holding the G3 unvented qualification. The differences in pressure, space and cost are covered in our comparison of unvented vs vented cylinders.

Reading the clues when something goes wrong. Once you know the parts, most cylinder faults point to a likely cause. No hot water at all, but the radiators work, points to the motorised valve, the cylinder thermostat or the programmer. Hot water only when the immersion is on points to the boiler side. Water that runs hot then quickly goes cold suggests an undersized cylinder or a coil furred up with scale, which is common with London's hard water. Water dripping from the tundish on an unvented cylinder means a relief valve is doing its job, often because the expansion vessel has lost its charge. Water on the floor under the cylinder is covered in our guide to a leaking hot water cylinder. Whatever you see, note it down, along with the cylinder's make from its label, before you call.

What you can safely check yourself. You can check the programmer is set to give hot water, look at the cylinder thermostat dial, make sure the immersion switch is on if you rely on it. Leave the safety valves, the expansion vessel and anything inside the motorised valve to an engineer, and never adjust or cap a relief valve that is discharging. If you are unsure whether the cylinder is still the right size for your household, our guide to what size hot water cylinder you need helps.

Getting help in Chelsea. If your cylinder is not heating, the water is too hot or too cold, a relief valve is discharging or there is water where there should not be, call Emergency Plumber Chelsea on 020 3002 6667. Our engineers diagnose and repair cylinders, thermostats, motorised valves and immersions as part of our plumbing repairs and boiler repair work across SW3, SW10, Knightsbridge, South Kensington and Fulham. There is no call-out fee on jobs over 3 hours.

Quick tips

• Hot water leaves from the top of the cylinder and cold enters at the bottom, so the water only cools right at the end of a long draw.
• Keep the cylinder thermostat at around 60°C; much lower saves little and lets bacteria grow in stored water.
• If you have heating but no hot water, or the other way round, the motorised valve is the first suspect.
• On an unvented cylinder, water from the tundish is a warning sign. Never cap or block a discharging relief valve.
• Note the cylinder's make and whether it is vented or unvented before you call; it speeds up the diagnosis.

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