Do I need a water tank with a combi boiler?

Combi boilers can struggle to meet large hot water demands, particularly where multiple appliances are running hot water at once. For example, you may find a significant reduction in water pressure when multiple showers are being run. It may also have to choose between heating the home and supplying hot water, as it cannot do both. This means that, occasionally, you may have to sacrifice your heating in order to meet your hot water demands.

If you want additional hot water storage, but you don’t want a conventional boiler system with a hot water tank, there are two other types of boiler that may be right for you: a storage combi boiler or a system boiler. Both of these options allow hot water to be stored and used later. Below, we examine the storage capacities of both and some of the pros and cons.

How do system boilers store hot water?

A system boiler is a sort of combination of a conventional and a combi boiler. Like a conventional boiler, a system boiler has an external hot water cylinder that stores your hot water until it’s needed. However, unlike a conventional boiler, it doesn’t require a header, or cold feed, tank in the loft.

A system boiler takes water from the mains supply, like a combi boiler, and heats it in the heat exchanger. Once the water has reached the correct temperature, it is pumped to the hot water tank where it can stay warm for a couple of hours until you need it.

This is sometimes a better option than a combi boiler, particularly for large homes, because it allows for multiple taps or showers to be in use at the same time without affecting the temperature or the pressure of the water. 

It’s now a requirement for new boilers in homes to be condensing models, as opposed to non-condensing ones. This is to ensure that households are running as efficiently as possible, with condensing boilers achieving up to 90 per cent efficiency or more.

Unlike combi boilers, a system boiler can run out of hot water once the stored water in the tank has run out. This means that you’ll need to wait for more to be heated. So, when you’re choosing a cylinder for your system boiler, you need to choose the right size. If your cylinder is too small, you’ll run out of hot water frequently. If your hot water tank is too big, your gas bill may be very high, as you’re heating water that isn’t being used. If you’re unsure what size hot water tank you need, you can use this guide from the Hot Water Association.

How do storage combi boilers store hot water?

If you want a way to store the water that your combi boiler heats, you could invest in a storage combi boiler.

A storage combi boiler merges the technology of a combi boiler with that of a system boiler. However, you don’t need space to house a hot water tank as the boiler has a small storage tank built into it. This allows for high-volume water consumption but without the hot water tank that could accompany another form of boiler, such as a conventional boiler.

These boilers used to be quite large and were only available in floor-standing models. However, as the technology has improved, the boilers have become smaller and can now be wall-hung. They’re usually available in a number of sizes, depending on the size and water needs of your household. Most models offer a water storage capacity from 46 litres to 170 litres. 

Which boiler is best for me?

You may be wondering which system is preferable for your home.

We’d recommend a system boiler if you:

• Don’t have enough space for a header tank in the loft
• Have a large household with high hot water demands
• Have enough space for a large hot water tank
• Want to run multiple showers, taps or appliances at once
• Don’t mind spending a bit more on your boiler system

We’d recommend a storage combi boiler if you:

• Don’t have the space for a hot water tank
• Want hot water on demand as well as stored water
• Want to run two showers or fewer at once
• Have a lower budget for a boiler