How Does a Hot Water Cylinder Work? A Plain Guide

The basic idea: store it now, use it later

A combi boiler heats water the moment you ask for it. A cylinder works the other way round. It heats a fixed volume in advance, commonly somewhere between 120 and 250 litres in a house, and holds it hot for hours thanks to a thick layer of foam insulation. That is why a home with a cylinder can run a bath and a shower at the same time without either going cold, and also why it can run out: once the stored volume is used, you wait for the next lot to heat.

Every hot water cylinder, old or new, copper or stainless steel, has the same four things: a cold water inlet low down, a hot water outlet at the top, at least one heat source, and a thermostat.

How the water gets heated

Most cylinders in homes with a gas boiler are indirect. Inside the tank is a coil of pipe, and the boiler pumps its own hot water through that coil. The coil heats the stored water around it, but the two never mix: the boiler water goes round the coil and back to the boiler, and the water you wash in stays in the cylinder. Our direct vs indirect hot water cylinder guide explains the difference in more detail.

Almost all cylinders also have an immersion heater, an electric element screwed into the tank, usually as a backup for when the boiler is off or broken. A direct cylinder has no boiler coil at all and relies on one or two immersions, which is common in flats with no gas. Our guide, what is an immersion heater, covers how that element works.

Why the hot water stays at the top

Hot water is less dense than cold, so it rises and forms layers, with the hottest at the top and the coldest at the bottom. Engineers call this stratification, and it is the reason a cylinder is tall and narrow rather than short and wide. The hot outlet is taken from the top so you always draw the hottest water first, and the cold inlet enters at the bottom, often through a spreader, so the incoming cold water does not stir up the hot layer above it.

It is also why a cylinder can feel hot at the top and cold at the bottom and still be working properly, and why the hot water can seem to run out suddenly rather than fading gradually. When the cold layer rises to the level of the outlet, the tap goes cold quickly. If yours runs out sooner than it used to, our guide on why hot water runs out quickly goes through the reasons.

The controls: thermostat, valve and timer

On a boiler heated cylinder, a cylinder thermostat is strapped to the side of the tank or pushed into a pocket in it. When the stored water reaches its setting, usually around 60 degrees, it tells a motorised valve near the cylinder to close, so the boiler stops sending heat to the coil and sends it to the radiators instead, or switches off. A programmer or timer decides when the hot water is allowed to heat at all.

This is why so many hot water faults are really control faults. If the motorised valve sticks, or the cylinder thermostat fails, the radiators can work perfectly while the cylinder stays cold, which is the pattern described on our page about no hot water but the heating works.

Where the pressure comes from: vented and unvented

Cylinders come in two families, and the difference is how the water gets into them.

• Vented, also called tank fed or open vented: a cold water storage tank in the loft feeds the cylinder by gravity, and an open vent pipe from the top of the cylinder runs back up over that tank. Pressure at the taps depends on how high the loft tank is, which is why showers on these systems are often weak. Expansion simply goes up the vent pipe, so there is nothing to build pressure.
• Unvented: the cylinder is sealed and connected straight to the mains, so the hot taps run at close to mains pressure. Because it is sealed, it needs a set of valves to reduce and control the incoming pressure, an expansion vessel or internal air gap to absorb the expansion as water heats, and safety devices that release water through a tundish if the pressure or temperature ever gets too high.

The safety devices on an unvented cylinder

An unvented cylinder holds a large volume of hot water under pressure, so it is built with layers of protection. The cylinder thermostat switches the heat off in normal running. If that fails, a manual reset thermal cut-out stops the heat source. If that fails too, a temperature and pressure relief valve on the tank opens and dumps water before it can get anywhere near boiling. On the cold inlet, a pressure reducing valve, a check valve and an expansion relief valve keep the pressure where it should be.

Anything those valves release runs through the tundish, a short open section of pipe you can see into, so a discharge is visible rather than hidden. A dripping tundish is the cylinder telling you one of those valves is working, and our page on a tundish dripping explains what that means. Because these parts have to work together, fitting or altering an unvented cylinder is notifiable work for a G3 qualified engineer, and the cylinder should have an unvented cylinder service every year. Our unvented vs vented cylinder guide compares the two in more depth.

Thermal stores and Pulsacoils work differently

Some cylinders look like any other but work the other way round. In a thermal store, the tank holds boiler water, not the water you wash in, and mains cold water passes through a coil inside it and is heated on its way to the tap. The Gledhill Pulsacoil is the best known example in London flats. They fail in different ways from a standard cylinder, and our guide to how a Pulsacoil works explains the difference.

What this means when something goes wrong

Knowing how the parts fit together makes most faults easier to read:

• No hot water, radiators fine: a control fault on the boiler side, usually the motorised valve or cylinder thermostat.
• No hot water on the immersion only: the element, its thermostat, or a tripped safety cut-out. See how to reset an immersion heater.
• Water too hot, or banging from the cylinder: a thermostat has failed and the heat is not switching off. Treat it as urgent.
• Dripping tundish or water from a pipe outside: one of the relief valves on an unvented cylinder is opening.
• Water under the cylinder: a joint, a valve, or the tank itself. Our page on a hot water cylinder leaking shows how to tell which.

If your cylinder is not doing what it should

Hot water cylinders are our core work. If yours has stopped heating, is leaking, or the tundish is dripping, call us on 020 4652 5111 or send an enquiry with a photo of the label on the cylinder. We have 20 engineers across London, and the label tells us whether it is vented or unvented and which parts it takes. Our rate is on the pricing page and agreed before anyone attends.