Last Updated: Sunday, 11th February 2024

How Does A Hot Water Tank Work?

If you find yourself wondering, "How does a hot water cylinder work?", you're in the right place. Understanding the mechanics of a hot water cylinder is essential for anyone who wants to make informed decisions about their home's heating system. In this comprehensive guide, we'll break down the elements, components, and even the science behind hot water cylinders.

What Is A Hot Water Cylinder?

A hot water cylinder is a great way to store hot water for use later. As you can imagine, these cylinders can come in various types and sizes depending on how much water you need to use in your home. They can be powered by different energy sources, including gas, electric, and solar.

Types Of Hot Water Cylinders

Vented Cylinders

Diagram showing how a gravity fed water system works inside the home

Vented hot water cylinders are the traditional cylinders that you find in gravity systems. These are low pressure systems that rely on a cold water tank (also known as a header tank) to feed water into the tank to then be heated, directly or indirectly (we'll come to that later). This cold water tank is usually high up in the house, typically in the loft, in order to increase the gravitational pull on the water to give the most pressure possible.

They are called "vented" as they have a vent which will release any build up of pressure in the tank and release it back to the cold water header tank. They are generally less expensive, reliable due to its simplicity, easier to maintain but generally are associated with low water pressure.

Homes with vented cylinders generally also install shower pumps to boost their water pressure inside their homes. We'll discuss this shortly.

Unvented Cylinders

Diagram showing how an unvented heating system works inside your home

These are modern hot water cylinders that connect directly to the mains. In this way, by removing the need for a header tank, they take up less space. They also heat and deliver at mains water pressure offering you much better water pressure in your home and better shower performance.

However, they are generally more expensive and require additional safety features due to storing water at high pressure. They are also only as powerful as the mains pressure you receive into your home. If you have low mains water pressure, you will still have poor pressure in your home. There is still a fix for this though - we'll come to that shortly.

Direct vs. Indirect Cylinders

There are two main ways to heat the water inside your hot water cylinder:

  • Direct Cylinders: These have an immersion heater inside the tank.

  • Indirect Cylinders: These are heated by an external source like a system boiler.

What Are The Benefits Of A Hot Water Tank?

Hot water tanks provide a reliable and flexible solution for meeting high hot water demands in your home. Ideal for households with multiple showers or outlets running simultaneously, these tanks ensure a consistent supply of hot water for everyone's needs.

Additionally, hot water tanks offer a safety net in case of boiler failure. If your is equipped with an immersion heater, these tanks can still heat water even when your boiler is out of commission. This is a distinct advantage over relying solely on a combination boiler, which leaves you without any hot water if it breaks down.

What Size Water Tank Do I Need?

Picking the right cylinder size is important. If it's too small, you'll run out of hot water a lot. If it's too big, you'll waste money heating water you don't use.

Figuring out the right size isn't simple. You need to know how much hot water you use when everyone's home and busy. You also need to know how fast the cylinder can make cold water hot again. It's a good idea to talk to an expert to help you decide.

There are some general size guidelines based on how big your home is. But these are just starting points. They might not be right if you have a big family or if someone takes really long showers. Here's a quick look at some suggested sizes:

Indirect Heating Hot Water Tank Estimated Sizes

 Size of Home

Minimum Capacity (litres)

Cylinder Coil Rating (kW)

1 Bedroom



2 Bedroom



3 Bedroom



4 Bedroom



5 Bedroom



Direct Heating Hot Water Tank Estimated Sizes

 Size of Home

Minimum Capacity (litres)

1 Bedroom


2 Bedroom


3 Bedroom


4 Bedroom


5 Bedroom


How To Improve The Performance Of Your Hot Water Tank

Modern hot water cylinders are built to save energy. Some even let you control them from your phone, which could help you save money on your bills. But the best thing you can do is wrap your tank in insulation. This keeps the heat in, so your water stays hot for longer and can reduce your bills.

Common Issues With Hot Water Tanks

Vented Hot Water Cylinders

As mentioned earlier in the article, vented cylinders, and thus gravity systems are usually associated with low water pressure. The most common fix for this is to install a shower pump. We talk about this in more detail, discuss other issues and other solutions in our article on improving gravity systems.

Learn how to improve water pressure in gravity systems

Unvented Hot Water Cylinders

Unvented Cylinders rely on good mains water pressure to deliver good water pressure to your home. If you have poor mains water pressure, even with an unvented cylinder, your shower will likely not perform well. In these cases you need to boost your mains water pressure, which can be done through mains water booster pumps or the installation of an accumulator tank.

Learn more about accumulator tanks here

Wrapping Up

Understanding your home's hot water system doesn't have to be a daunting task. From the basic mechanics of how a hot water cylinder works to the nitty-gritty details of types, sizes, and efficiency, we hope to have you covered in this guide. Whether you're dealing with low water pressure or pondering the right cylinder size for your home, the information here should help you make informed decisions. Remember, when in doubt, consulting a professional is always a good move. Most importantly, please let us know if you have any questions or comments on this guide.

Here's to a home filled with perfectly heated water and energy-efficient solutions!

Frequently asked questions

What is a hot water cylinder?

A hot water cylinder is a tank used to store and heat water for later use. It can be powered by gas, electric, or solar energy.

What are the types of hot water cylinders?

There are vented and unvented cylinders. Vented ones are low-pressure systems connected to a cold water tank. Unvented ones connect directly to the mains and can offer better water pressure.

How do I choose the right cylinder size?

There are general size guidelines based on the number of bedrooms and bathrooms in your home which we discuss in this article. However, for a tailored answer you should consult an expert.

What are the benefits of a hot water tank?

Hot water tanks provide a reliable supply of hot water for your home, especially if you use a lot of hot water. With an immersion heater fitted, they can also provide a backup if your boiler fails.

How can I improve the efficiency of my hot water cylinder?

Insulate your tank to keep the heat in, which makes your water stay hot for longer and can reduce your bills.

Any more questions?

If you have any queries or questions about the products we sell, or even your next project, give us a shout! We'll try our best to give you a hand.