Hot water cylinders and immersion heaters explained

If your home has a hot water cylinder, usually a copper tank in an airing cupboard, it stores hot water so several taps can run at once. It is heated by a boiler, an electric immersion heater, or both. This guide explains how a typical vented cylinder works, what the immersion heater is for, and what you can safely check when the hot water goes cold.

Everything electrical on a cylinder is for a qualified electrician. This page does not give wiring steps or tell you how to replace an immersion heater.

How a vented cylinder works

A vented cylinder is part of a gravity-fed system:

• A cold water storage cistern, usually in the loft, feeds cold water into the bottom of the cylinder.
• Hot water rises and is drawn off from the top of the cylinder to the hot taps.
• An open vent pipe runs from the top of the cylinder back up and over the cistern. It lets air out and gives hot water a safe route if the cylinder overheats. Never block it.

The cylinder is heated in one of two ways, often both:

• Indirect: a coil inside the cylinder carries hot water from the boiler. The boiler heats the coil and the coil heats the stored water. The boiler's water and your tap water do not mix.
• Direct: the stored water is heated directly, most commonly by an electric immersion heater.

Many cylinders have a cylinder thermostat strapped to the side, which tells the heating controls when the stored water is hot enough. It is usually set by the installer.

Hot water at the tap comes out at gravity pressure, so it depends on how high the loft cistern is above the outlet. If the cylinder has no loft tank and has safety valves and a tundish, it is probably an unvented cylinder. That is a different system with different rules. See unvented cylinder basics. If you are not sure what you have, see combi, system and regular boilers.

What an immersion heater is

An immersion heater is an electric element that goes into the cylinder through a boss on the top or the side, like a large kettle element. It has its own thermostat and an over-temperature cut-out, and it is switched by a dedicated switch near the cylinder.

It can be used:

• as a backup, for when the boiler is off, broken or being serviced;
• as the main heat source in homes without a boiler for hot water, often on an electricity tariff with cheaper night-time rates.

Approved Document G's guidance (available from GOV.UK) says fixed immersion heaters should comply with the relevant British and European safety standard. It is one more reason to buy from a reputable supplier and have it fitted by a qualified person.

Single, dual and "economy" immersions

You may see two switches, or a switch marked "bath" and "sink". In plain terms:

• A single immersion heats the cylinder from one element, usually from the top.
• A dual arrangement has either two elements, or one element with two heating zones. One heats a smaller amount near the top for a quick top-up ("sink"). The other heats more of the cylinder from lower down ("bath").
• Economy or off-peak setups link the lower element to a timer or tariff so the whole cylinder heats overnight on cheaper electricity. The top element is used during the day only for a boost.

How your controls are wired depends on your installation and your tariff. If you are not sure what a switch or timer does, ask a qualified electrician rather than experimenting. Leaving an immersion on all day, every day, when it is meant to be a backup can cost a lot. Check what yours is set to do.

Hot water stopped: what you can check

Work through the outside checks first. None of these involve opening the immersion cover or touching wiring.

1. Is it the boiler or the immersion that normally heats the water? If the boiler does it, check the programmer is set to give hot water, and look for a fault code. A boiler fault is for a Gas Safe registered engineer if the boiler is gas.
2. Is the immersion switch on? Look for the switch near the cylinder, often with a neon light. Check any timer too.
3. Has the fuse or circuit tripped? If the immersion circuit has tripped and trips again when switched back on, stop. That is a fault for an electrician.
4. Has the over-temperature cut-out operated? Many immersion thermostats have a manual reset for the cut-out, sometimes under a cap or cover. Only follow the maker's instructions for your model, with the power isolated as they describe. If you are in any doubt, or it trips again, call a qualified electrician. A cut-out that keeps tripping is a sign of a fault, not a nuisance to work around.
5. Is the water warm but not hot? On a dual setup, you may only be heating the top section. The boost may not have run long enough.

If none of that brings the hot water back, call the right person. That means a qualified electrician for the immersion and a heating engineer for the boiler side.

Electrical work and Part P

Replacing an immersion heater involves draining at least part of the cylinder and working on a fixed electrical circuit. We do not give instructions for it. In England, electrical work in homes falls under Part P of the Building Regulations, and some work has to be notified. Approved Document P on GOV.UK explains when. Use a qualified electrician, ideally one registered with a competent person scheme. In Wales, Scotland and Northern Ireland, check the rules where you live.

Looking after a vented cylinder

• Insulation. A factory-foamed cylinder is already insulated. An older bare copper cylinder can take a jacket, and the first stretch of hot pipe from the cylinder is worth lagging. See pipe lagging.
• Limescale. In hard water areas, scale builds up on immersion elements over time. See limescale.
• Leaks. Copper cylinders can eventually weep at joints or through the copper. If you see water around the base, turn off the immersion and the boiler's hot water, and call a plumber.
• Scalding. Stored hot water can be hot enough to scald. Some baths and basins have a thermostatic mixing valve to limit the temperature at the outlet.
• The loft cistern. It should have a fitted lid and be insulated, and the vent pipe over it should be clear.

Is it cheaper to heat water with the immersion or the boiler?

It depends on your fuel prices, your tariff and how the system is set up. An immersion on a cheap overnight rate can make sense in some homes. An immersion left on all day at the standard rate is usually an expensive way to heat water. Check your tariff and your timer settings.

Should I leave the immersion on all the time?

Not usually, if your boiler normally heats the water. The immersion is then a backup. If it is your main heat source, it is normally run on a timer to suit your tariff. Ask an electrician if you are not sure how yours is set up.

Can I replace an immersion heater myself?

We do not recommend it. It involves draining part of the cylinder, a fixed electrical circuit and a heavy, hot vessel. Use a qualified electrician, and a plumber if the cylinder needs draining.