What Size Hot Water Cylinder Do I Need?

In our 5-minute guide to hot water cylinder sizes we’re going to explain:

Hot Water Cylinder Sizes

What Happens If Your Cylinder Is Too Small, Or Too Big?

If you don’t get the right size hot water cylinder for your property, you’re going to have one of two problems:

  • It’s too small – you run out of hot water and have to wait for more to heat up
  • It’s too big – your heating bill is going to increase, because you are heating hot water that you don’t need. And, bigger cylinders cost more money, so that’s a waste of cash.

What Size Cylinder Do You Need?

This is a general guideline when looking for a new cylinder installation.

If you want to make sure you get the perfect sized cylinder for your property, we’d recommend getting an inspection and quote from a qualified engineer.

Property Size Bathrooms Cylinder Size
1 Bedroom 1 120-150L
2 Bedrooms 1 150-180L
3 Bedrooms 1 or 2 180-210L
4 Bedrooms 2 210-300L
5 Bedrooms or more 2 300L or bigger

How Much Hot Water Do You Need?

No two properties are the same. One 2-bedroom property might need a 150L cylinder, another could need 210L.

So, what affects hot water usage?

  • How many baths are in the property
  • How many people are using hot water at the same time
  • Temperature settings
  • The time the boiler takes to heat more hot water
  • The size of the property
  • The number of sinks, basins and taps

How Much Water Do You Need To Fill A Bath?

Even baths come in different shapes and sizes. But, expect to use 75-85L to fill an average bath.

Does A Bath Use Less Water Than A Shower?

For the average 5-minute shower, expect to use approximately 50L of hot water.

How Much Hot Water Does The Average Person Use?

The amount of people in a property is going to determine the size hot water cylinder needed.

But, it also comes down to whether they are low, medium or high usage person.

Usage Hot Water (Litres Per Day)
Low 25
Medium 40
High 60


But, that doesn’t mean you take the usage and multiply that by hot water usage to get the correct cylinder size.

Remember, one person might have a shower in the morning, the other at night. The heating system can replenish water lost from the cylinder throughout the day.

What’s more important is thinking about hot water usage at peak demand.

Do I Need A Vented Or Unvented Cylinder?

If you are not converting a system, you’ll want whatever you already have.


If you have a boiler, hot water cylinder and a storage tank (usually installed in loft spaces), you’ll need a vented cylinder.


A unvented system will use a boiler and cylinder. As it connects to mains water, there is no need for a water storage tank.

What’s Next?

Thanks for reading how 5-minute hot water cylinder sizing guide.

If you’re looking for a quote, make sure you get them from a qualified heating engineer here.

Questions about cylinder sizes or installation? Leave a comment below and we’ll get back to you as soon as possible.

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