Homeowners Professionals

How does a hot water tank work?

Hot water is something most of us take for granted until it stops working. But have you ever thought about where your hot water comes from or how your hot water cylinder works?

You might be here because you’re wondering if your current system is the right one for you. If you’re thinking about changing your boiler system or if you’re just interested in how a heating system works, this post should help to answer your questions. We’ve covered how your hot water tank works, the benefits of a hot water tank, how much it could cost and what size you might need.

Firstly, how does a hot water tank work?

Hot water tanks, or cylinders, are a way of storing your hot water and keeping it warm for when you need it. There are two different types of cylinders that can be found in UK homes: direct and indirect. The water in a direct hot water cylinder is heated by an immersion heater within the cylinder. An indirect hot water cylinder requires an external device to generate the hot water, such as a boiler.

Once you’ve determined whether your cylinder is direct or indirect, there are two further types of cylinder you could have. It could be a vented cylinder or an unvented cylinder. The main difference is that one requires a cold water tank and one doesn’t.

A vented cylinder relies on a cold water tank to feed water into the cylinder for it to be heated by either the boiler or an immersion heater. The cold water tank will need to be situated in the loft as it’s a system that requires gravity to work rather than pressure. This usually means that your water pressure is better downstairs than upstairs, and it could mean that you’ll need an electric pump to maintain good shower pressure.

An unvented cylinder doesn’t need a cold water tank because water enters it from the mains supply. Mains pressure is better than pressure created in gravity-fed systems so your shower performance should be improved. The other benefit is that you’ll save space in the loft with the elimination of the water tank.

An unvented cylinder could be move expensive to install as it requires extra safety mechanisms. You might need  relief valves, which reduce the risk of the water becoming overheated, and twin thermostats to control the temperature of the cylinder. It will also need an expansion tank as the water will expand slightly as it’s heated. Find out more about expansion tanks further down the page.

The only system that can  run without a cylinder  or hot water tank is a  combi boiler, which heats the water internally.  

What are the benefits of a hot water tank?

There are lots of benefits to having a hot water tank. Many people think a cylinder takes up precious space that could be used for something else. However, you’ve got the potential to put the cylinder in a cupboard and turn the space into an airing cupboard. An  airing cupboard  is an excellent place to store your clean towels, bedding and clothes.

A system with a hot water tank can cope with a high demand for hot water. If you’re a family of four or more, you may find this kind of system more practical than other alternatives.

Hot water tanks are extremely versatile. They allow for different ways of heating water (direct or indirect), but the water can also be heated more sustainably via  solar power  or biomass fuels. Also, a backup heat source can be included in an indirect cylinder if the boiler fails, so the  chances of running out of hot water  are reduced.    

TOP TIP: Invest in a good insulation jacket for your hot water tank to keep the water warmer for longer and to reduce your heating bills.

What is an expansion tank on a hot water heater?

As previously mentioned, an unvented cylinder needs an expansion tank. An expansion tank is used to prevent pressure increases as the water heats and expands. As water is heated, it can increase in volume by up to four per cent. The expansion tank can absorb this extra water volume so that the pressure doesn’t build up in the system.

You’ll need an expansion tank with all cylinders that are 30 L or bigger. If your cylinder is smaller than this, you should get a plumber or engineer to help you decide if you require an expansion tank.

The reason you don’t need an expansion tank in a vented system is because there are special built-in taps that will do the job automatically.  


How long should a hot water tank last?

As with all appliances, your hot water tank won’t last forever. It can be hard to tell when a tank needs replacing, but there are some general signs you can look out for.  


A leaking hot water tank is usually irreparable as it’s a sign of internal damage. You should look into getting your tank replaced as soon as possible.


You should check for rust either in your hot water when it comes from the tap or around the tank itself. You should check all its valves for rust too. If you find rust on your tank, it’s a sign that it could start to leak soon, so you should start looking for a replacement.  

Cold water

Your hot water tank might no longer be able to heat your water, which indicates a problem with its internal heating element (if it’s a direct cylinder). The heating element usually stops working after a certain amount of time. You could try replacing the element that’s broken, which is a fairly easy job for a heating engineer to do. However, if your tank is between eight and 12 years old, it may stop working soon anyway and so fixing a heating element could be a waste of money.

As a general rule, you should replace your tank when it’s between eight and 12 years old. If you know that the tank has been properly maintained and looked after, then it could run for longer than this.  

What size hot water tank do I need?

The size of the cylinder you’ll need will depend on your hot water consumption. If your cylinder is too small, you’ll constantly run out of hot water, but if it’s too big, the tank will be storing and heating more water than you need, which is a waste of energy and money. The size of the cylinder also depends on whether it’s being heated directly or indirectly.

A cylinder is measured in litres, and the easiest way of working out which size you need is by looking at the number of bedrooms, bathrooms and family members you have.

How much does a hot water tank cost?

If you want to replace the hot water tank but not any other appliances within your heating system, it should cost around £500 for a new vented system tank and for a heating engineer to install it. The cost of an unvented system might be more. You could expect to pay about £700*.

Can I get a free hot water tank?

In some cases, you may  qualify for a free boiler  through government funding, however this additional funding doesn’t apply to hot water tanks.

*Figures taken from  Household Quotes. Correct as of April 2019.

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