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Posted: 10th September 2025 | Author: Adam Brown
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An immersion heater is a reasonably common alternative to a gas boiler, using electricity to heat up water. It’s often used as the main way to heat water in homes that don’t have a gas boiler, or as a backup if the main system ever goes down. It’s a simple, reliable bit of kit that’s been around for years.
Whether you’re installing a new heating system or just thinking about the current set-up, understanding how an immersion heater works is key to making the right choice for the job. In this guide, we’ll cover exactly what an immersion heater is, how it works and how long they take to heat up water.
An immersion heater is a type of electrical heating element that sits inside a hot water tank or cylinder. It works by warming the water from within - much like a kettle does.
The unit contains a metal heating element, typically made from copper or stainless steel. When you switch the heater on, an electrical current passes through the element, causing it to warm up, which in turn warms the water around the element.
A built-in thermostat controls the temperature. Once the water reaches the set level, the thermostat switches the heater off automatically. This helps prevent energy waste and keeps running costs down.
Many immersion heaters also include a safety cut-out. If the water overheats, the system shuts down as a precaution - adding an extra layer of protection.
Immersion heaters can be used as:
Typically, immersion heaters are installed in unvented or vented hot water cylinders. They can be controlled manually or with a timer, allowing you to heat up the house ready for when you come home.
If you’re trying to decide which heating system is right for your project - whether it’s for your own home or a client’s - it’s handy to know how long each system takes to do the job. This can help you to decide the best choice for the set-up.
With an immersion heater, it can take around two to three hours to fully heat a standard hot water cylinder. But that’s just a ballpark figure - there are a few things that can affect the timing. Things like the size of the cylinder, the power of the heater, how cold the water is to start with and whether you’re heating the full tank or just topping it up all come into play.
Here’s a closer look at what makes the difference.
Most hot water cylinders in UK homes are between 100 and 210 litres. A typical 120-litre tank takes about two to three hours to heat using a 3kW immersion heater. If yours is on the larger side, though - say 200 litres - it could take four hours or more.
You’d think that a bigger hot water tank is better, right? Well, not exactly. Because immersion heaters heat from the point of contact outward, a bigger tank just takes more time and energy. Insulation can help to make your water cylinder as heat-efficient as possible to limit this, but you should always consider the right size of cylinder needed to do the job.
The bottom line: The bigger the tank, the longer it takes to heat - it’s pretty straightforward.
Immersion heaters come with a power rating that basically tells you how quickly it can heat water, and most immersion heaters are 3kW. A 3kW unit will warm up a 120-litre tank in a couple of hours. If you go for a more powerful 6kW heater, you’ll get hot water a lot faster - but it’ll also draw more power while it’s running.
That said, not every setup can handle a higher-rated element, and it’s not always necessary unless you’re dealing with big demand or a lot of people in the house. If you're unsure what’s needed, best to check with a qualified electrician or heating specialist before making your choice.
In short: A higher power rating heats water more quickly, but that’s not always the safe or efficient choice.
In the warmer months, the cold water going into your tank might be around 15°C, but in winter, it can be closer to 5°C. That lower starting point means your heater has to work harder and for longer to get it up to a usable temperature.
If your immersion heater’s been off for a while - for example, you’ve been on holiday - or you’re using it first thing on a cold morning, it’ll probably take longer to heat up. Using a timer-controlled system can help to ensure you’ve got hot water when you need it. And again, good insulation on your tank can help keep the water warmer for longer between uses.
In other words: Cold water takes longer to heat - simple as that.
You don’t always need to heat the whole tank. Some immersion heaters offer two heating elements - one at the top and one at the bottom - allowing you to choose how much of the water you heat. The top one’s great for quick top-ups if you just need a small amount of hot water for a single shower or washing up. Heating just the top section of the tank can take as little as 30 to 60 minutes.
Using the bottom element will heat the full tank, which is ideal if you need a bigger supply - for example, if multiple people need showers or you’re filling a bath. But if you don’t need that much, you’re just using more energy than you have to.
In simple terms: Immersion heaters that allow you to only heat part of the tank can cut down heating times.
Choosing the right heating system for your home or project can seem overwhelming at first, but there are clear pros and cons to each option. Getting to grips with the key features and drawbacks of different heating systems can help you to make the right decision every time.
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The Author: Adam Brown
With a background as a Mechanical Engineer, I started Nationwide Supplies alongside my brother with a vision to modernise what seemed a stuck-in-its-ways industry in the builders merchant sector. Since 2011 it has been my goal to source quality products at the correct prices to be able to offer a saving to customers along with high quality customer service which offers quick delivery and superior technical knowledge in the industry.
