How to Choose the Right Sized Hot Water Cylinder

Choosing the right size hot water cylinder is important but a lot of people don’t know where to start. In this guide, we will explain how you can figure out how much hot water you are likely to use and therefore what sized tank you will need.

Hot water cylinders are an important part of the central heating system in any property that runs on a regular boiler. But like the boiler itself, it is vital that you choose the right sized cylinder for your property.

In this guide, we will show you how to do exactly that. We will explain the different types of cylinder that are available and ensure that you are well placed to make an informed decision.


Hot Water Cylinder Sizes (How to pick)

If you are not sure what size hot water cylinder you need, the best advice is to get a central heating engineer to visit your property and make a recommendation.

However, it is possible to make a rough approximation for most homes using the following table as a guide:

Size of Property No. of Bathrooms Hot Water Cylinder Size
1 Bedroom 1 120-150L
2 Bedrooms 1 150-180L
3 Bedrooms 1 / 2 180-210L
4 Bedrooms 2 210-300L
5 Bedrooms or more 2 300L +

Why Choosing The Right Size Hot Water Cylinder Matters

If you do manage to end up with the wrong-sized hot water cylinder, it will create problems. This is the case regardless of whether you end up with one too big or too small.

If you pick a hot water cylinder that is too small, it is quite likely that you will find yourself running out of hot water quite regularly and having to wait for more to heat up before you can have a bath or shower.

If you end up with a cylinder that is too big, you will be heating up water that you don’t need and as a result, your heating bills will go up. Larger cylinders also cost more money and this is an expense you can potentially avoid.

How Much Hot Water Will You Actually Need?

Judging how much hot water you actually need is not a precise science. Everyone uses different amounts of water which means that in the same sized property, hot water usage can vary significantly.

This is why our recommendation table above suggests a range of cylinder sizes for different properties. A 1-bed house with 1 bathroom could only need a 120L hot water cylinder or it might need a 150L cylinder.

There are a few factors you can use to make an educated guess about what sized hot water cylinder is right for you. This includes things like:

  • How many baths and showers are there in your property?
  • How many sinks and taps are there in your property?
  • How many people are using hot water at the same time?
  • What temperature is your hot water tank usually set at?
  • How long does your boiler take to heat up more hot water?

If you have more people likely to be taking more baths and they tend to do so at around the same time, you will probably need a larger cylinder than if there are fewer people a home who take showers at different times of the day.

How Much Water Will I Use Per Day?

A useful indicator of how much hot water you are likely to use is estimating how much you will run each time you have a bath or shower. Again, this can vary depending on how big your bath is and how long you shower, but on average, you can expect to use around the following amounts:

  • Bath – 75-85L
  • Shower – 50L

Working out how much hot water a person uses per day is even harder since there are even more variables that come into play. Some people will use small amounts while others use a lot more. But heating industry experts will usually work off the following approximations for the average adult:

Hot Water Usage Amount of Water Used Per Day
Low 25L
Medium 40L
High 60L

These figures are only rough guides, so don’t just add them up for the number of people in your home and then decide on the size of the hot water cylinder you need based on that. The calculation is more nuanced than that.

You need to think about how much demand there will be at peak times, which usually means in the morning and the evening. This is when there your hot water cylinder is most likely to run out.

Vented or Unvented Cylinders?

As well as size, the other decision you will need to make is whether to choose a vented or unvented cylinder. If you are not sure what the difference is between the two, it can be briefly described as follows:

  • Vented Hot Water Cylinder – This type of cylinder has to be fed water from a cold water cistern tank. This is usually located in the loft of a property with water reaching the cylinder through a vent pipe by means of gravity. When the water is heating in the cylinder it expands, but the vent pipe means excess water is just pushed back into the cold water tank.
  • Unvented Hot Water Cylinder – This type of cylinder is connected directly to the mains water supply rather than a tank in the attic. There is no vent pipe to deal with water expansion so instead, they are usually equipped with in-built features such as an expansion unit at the top.

If you are replacing an existing cylinder, the best advice is to stick with the system you have unless there is a particularly strong reason to change it.

If you are installing a new cylinder, a vented system will prove to be slightly cheaper and easier to install but an unvented system doesn’t require a tank in the roof, which makes it slightly safer, and also is likely to give you higher water pressure.


Choosing the right-sized hot water cylinder is important. There aren’t too many different considerations but those that there are can vary a lot, so it is important to consider them carefully.

In this guide, we have offered guides on what size of cylinder is best for different-sized homes and also given some average indicators of how much hot water you are likely to use. We have also explained the difference between a vented and unvented hot water cylinder.

Armed with this information, you should be well placed to make an informed decision about the right hot water cylinder for you. But if you are in any doubt, it is a good idea to take advice from a qualified professional.

Do you have any questions about choosing the right sized hot water cylinder that we haven’t covered in this guide? Do you have any advice or tips for our readers that we haven’t mentioned?

It is always helpful to get feedback and thought from all our readers, so please do share yours with us today using the comment box below.

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