What Is an Immersion Heater and How Does It Work? Running Costs and How Long to Leave It On

What Is an Immersion Heater?

An immersion heater is a self-contained electric water heater fitted directly inside a hot water storage cylinder. The clue is in the name: the heating element is immersed in the water it heats. This makes it fundamentally different from a gas boiler, which heats water indirectly via a heat exchanger. Think of it like a giant kettle element sitting inside your tank.

In the UK, around half of all households use an immersion heater as either a primary or backup hot water source. In all-electric homes — those without a gas connection — the immersion heater is often the only way to produce hot water. In homes with a gas boiler, the immersion typically acts as a backup during boiler breakdowns, or as a top-up during periods of high demand.

A standard UK hot water cylinder holds between 120 and 200 litres, though sizes range from around 80 litres up to 500 litres in larger properties. Many cylinders are fitted with two immersion heaters: a top-mounted element that heats a small volume of water quickly for immediate use, and a bottom-mounted element that slowly heats the whole tank — ideal for running overnight on a cheaper off-peak electricity tariff.

The three key components you need to know about are: - The heating element — the metal rod (copper, Incoloy, or titanium) that converts electrical energy into heat - The thermostat — a temperature-sensing switch that turns the element off once the water reaches the target temperature, and includes a safety cut-off - The isolating switch — a dedicated mains switch wired alongside the unit to allow safe isolation

How Does an Immersion Heater Work?

The working principle is electrical resistance. When you switch the immersion heater on, electricity flows through the metal element. The element resists the flow of current, and that resistance generates heat — exactly the same process as a toaster or electric oven element, just submerged in water.

Heat spreads from the metal sheath into the surrounding water through direct contact, and natural convection then distributes warmth through the rest of the cylinder. Hot water rises to the top of the tank, which is why the top of the cylinder fills with usable hot water first. The thermostat monitors the water temperature continuously, and once it reaches the set point (usually 60–65°C), it cuts power to the element and waits.

If you have two elements, the sequence differs slightly depending on which one is active. The top element heats just the upper portion of the tank — useful for a quick boost when you only need a small amount of hot water. The bottom element heats the full volume, which takes longer but gives you more hot water overall. Using the bottom element overnight on Economy 7 or a similar off-peak tariff is the most cost-effective way to run an immersion heater as a primary source.

Element material matters too, particularly in hard water areas: - Copper elements are the most common and the least expensive, but limescale builds up quickly in areas with hard water - Incoloy (a nickel-iron alloy) is more resistant to limescale and corrosion, and is the most popular choice for replacing worn-out elements - Titanium elements offer the best resistance to hard water, making them ideal in areas like the South East, East Anglia, and the Midlands, though they cost more upfront

Is an Immersion Heater Expensive to Run?

Yes — compared to a gas boiler, an immersion heater is significantly more expensive to run for hot water production. Gas costs roughly 6p per kWh, while standard-rate electricity in 2026 sits at around 26–28p per kWh under the Ofgem price cap. That difference adds up fast when you are heating a full cylinder of water every day.

A typical immersion heater element is rated at 3kW. At current electricity prices, running one for an hour costs approximately 74–78p. That might not sound like much, but the costs compound quickly: - Running for 2 hours per day costs roughly £1.48–£1.57 per day - That adds up to around £44–£47 per month - And approximately £540–£570 per year for moderate use - A household heating a full 180-litre cylinder from cold twice a day could pay as much as £1,700 or more annually

Prices vary by region — expect the top end (or 20–30% more) in London and the South East, and less in the North and Scotland.

Several factors affect how much you actually pay: - Tank size: a larger cylinder needs more energy to heat and holds heat for longer - Thermostat setting: higher settings use more electricity - Insulation: a poorly insulated tank loses heat faster and needs reheating more often - Timing: heating water during peak-rate hours is the most expensive approach - Usage habits: long showers and heavy hot water demand increase run times

The single biggest way to cut costs is to use a timer and, if possible, switch to an off-peak electricity tariff. Economy 7 tariffs, for example, offer around 7 hours of cheaper electricity overnight — typically at 12–14p per kWh, roughly half the daytime rate. Setting your immersion to run during those hours and relying on the tank's insulation to keep the water warm through the day can cut your hot water bill by up to 50%. Pairing an off-peak tariff with a well-insulated cylinder and a programmable timer is the gold standard for all-electric hot water.

How Long Should You Leave an Immersion Heater On?

As a rough rule of thumb, allow approximately one minute per litre of tank capacity to heat water from cold (around 10°C) to a usable 50–60°C. In practice: - A 80–120 litre cylinder typically takes 1 to 1.5 hours - A 120–180 litre cylinder usually needs 1.5 to 2.5 hours - A large 200+ litre cylinder can take 3 hours or more from completely cold

The placement of the element also matters. A top-mounted element heats a smaller volume much faster — you might have enough hot water for a shower in 30–45 minutes — but the lower two-thirds of the tank stays cold. A bottom-mounted element heats the whole tank but takes longer.

You should never leave an immersion heater on continuously. Running it 24 hours a day is one of the most expensive mistakes a homeowner can make, and it also accelerates element wear and limescale buildup. The best approach is to switch it on about an hour before you need hot water, or to programme it with a timer so it comes on automatically before your usual shower or bath time.

Watch out for these signs that something may be wrong: - Rumbling, hissing, or banging sounds from the tank — this can indicate the water is overheating or there is heavy limescale on the element - The water never seems to run out — a stuck thermostat may be keeping the element on indefinitely - Frequent tripping of the circuit breaker — this suggests a fault with the element or thermostat that needs professional attention

Immersion Heater Costs: Replacement and Repair

When an element or thermostat fails, the cost to put it right depends on what needs replacing. Elements typically last five to ten years, though in hard water areas limescale can burn one out in as little as three years. Choosing an Incoloy or titanium replacement can extend that lifespan considerably.

Typical UK costs for 2025/26 (supply and fit, typical UK range): - Element swap: £130–£280 - Thermostat replacement: £100–£170 - Combined element and thermostat: £180–£320 - Full cylinder replacement (if the tank is corroded or leaking): £450–£900+ - Adding a programmable timer: £50–£150

Prices vary by region — expect the top end (or 20–30% more) in London and the South East, and less in the North and Scotland.

Immersion heater installation and replacement is notifiable electrical work under UK Building Regulations. This means the work must be carried out by a qualified and competent electrician or plumber, and it must meet the requirements of Part P (electrical safety) and Part G (sanitation and hot water). Before booking anyone, you can check their registration on the NICEIC register online.

Common Problems and What to Do

Most immersion heater faults fall into a small number of categories. Here is what to look for and what you can safely do yourself before calling a professional:

• No hot water at all: check your consumer unit (fuse box) first. A tripped circuit breaker is the most common cause. Reset it once. If it trips again immediately, there is a more serious electrical fault — stop and call a professional.
• Water not hot enough: if the element has partially failed or is heavily scaled, it may not reach temperature. Try the reset button on the thermostat first (usually a small red or black button under the cover). If the problem persists, the element likely needs replacing.
• Water too hot or running constantly: a thermostat stuck in the on position will allow the water to overheat. This is a safety risk — turn the heater off at the isolating switch and arrange for a thermostat replacement.
• Heater keeps tripping the circuit breaker: heavy limescale on the element causes localised overheating, which triggers the electrical safety cut-out. Repeatedly resetting a tripping breaker without investigating the cause is not safe.
• Strange noises: rumbling and banging usually point to limescale on the element. This does not always mean immediate failure, but it is a sign the element is working harder than it should and may be nearing the end of its life.

Regular maintenance helps avoid most of these problems. Flushing the tank periodically to remove sediment, fitting a limescale filter in hard water areas, and booking an annual check with a qualified plumber or electrician will catch problems early and keep running costs down.

Temperature Settings and Legionella Safety

Your immersion heater thermostat should always be set to at least 60°C. This is not just about comfort — water stored below 60°C can allow Legionella bacteria to multiply. Legionella thrives between 20°C and 45°C, and storing water at lower temperatures to save money creates a genuine health risk. Most immersion heaters leave the factory set to 60–65°C, which is the safe and recommended range.

Do not set the thermostat above 65°C. Higher temperatures waste electricity without any practical benefit, and they significantly increase the risk of scalding at the tap — especially dangerous for young children and elderly people.

If you notice any of the following, turn the immersion heater off at the isolating switch immediately and contact a qualified professional: - Very hot or near-boiling water from taps - Loud bubbling or banging from the tank - Hot water coming from the cold tap - Steam, dampness, or water stains in the airing cupboard or roof space

These are signs that the thermostat or the safety cut-out has failed, and continuing to use the heater risks scalding or, in extreme cases, a pressure build-up in the tank.

Step by step

1. Check your cylinder type and element position

   Before using your immersion heater, confirm whether you have a vented or unvented cylinder, and whether your element is top-mounted, bottom-mounted, or both. This tells you whether you are heating the whole tank or a smaller top portion. Check the label on the cylinder or the documentation left by the previous owner or installer.

2. Set the thermostat to the correct temperature

   Locate the thermostat control, usually hidden under a plastic cover on the side of the cylinder. Using a flat-head screwdriver, set the dial to 60°C. Do not go below this — 60°C is the minimum required to prevent Legionella bacteria from multiplying in stored water. Do not exceed 65°C, as this increases scalding risk and wastes electricity.

3. Fit or programme a timer

   If your immersion heater does not already have a timer, consider having one fitted by a qualified electrician (typical cost £50–£150 fitted, typical UK range). Set the timer to switch the heater on roughly 1 to 1.5 hours before your main hot water demand — usually before your morning shower or bath. This avoids running the element unnecessarily and reduces your electricity bill.

4. Switch the heater on at the isolating switch

   Turn on the dedicated isolating switch — this is usually a double-pole switch mounted near the hot water cylinder, often in the airing cupboard. Do not use an extension lead or a standard plug socket. The immersion heater must run from its own dedicated mains circuit.

5. Wait the appropriate heating time

   Allow roughly one minute per litre of tank capacity for a full heat from cold. A 120-litre tank takes approximately 1.5 to 2 hours; a 180-litre tank may need up to 3 hours. If you only need a small amount of hot water quickly and have a top element, 30–45 minutes may be sufficient.

6. Check the hot water temperature at the tap

   Once the timer period is up, run the hot tap briefly. The water should be comfortably hot but not scalding. If it is barely warm after the expected heating time, you may have a failing element or thermostat. If it is dangerously hot or boiling, switch the heater off immediately and call a professional.

7. Switch off after use (or let the timer do it)

   Never leave the immersion heater running continuously. If you do not have a timer, remember to switch it off at the isolating switch after use. A well-insulated cylinder will keep water hot for several hours without any power input.

8. Reset a tripped safety cut-out (once only)

   If you have no hot water and the element is not heating, check the consumer unit for a tripped breaker and reset it. Also check the thermostat cover — many thermostats have a small red or black reset button you can press with a screwdriver. Press it once. If it trips again straight away, do not reset it a second time. Call a qualified electrician or plumber.

9. Know when to call a professional

   Contact a Gas Safe registered engineer or a qualified electrician or plumber if: the circuit breaker keeps tripping; the water is overheating or you can hear loud banging from the tank; you smell burning near the cylinder; there is any dampness, dripping, or corrosion around the tank; or the element or thermostat needs replacing. Immersion heater replacement is notifiable electrical work under UK Building Regulations and must not be attempted as a DIY job.

Typical costs

Typical UK ranges as a guide only — prices vary by region (expect the top end, or 20–30% more, in London and the South East) and by how accessible your system is. Always get a written quote.

Frequently asked questions