Storage Combi Boilers Explained

If you have limited space but use a lot of hot water, a storage combi boiler
could be the solution for you.

Save up to £590 PER YEAR a year on bills.

Storage Combi Boilers Explained

Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: December 4, 2023. Updated: June 17, 2024

A storage combi boiler is a combi boiler with a storage tank for hot water. They work pretty much the same way as combi boilers, but store hot water internally for times of higher demand rather than in a separate hot water cylinder – saving space. The central heating function remains the same.

We’re specialists in storage combi boilers and have installed thousands across the country in the past few years. Our most recommended and installed storage combi boiler is the Viessmann Vitodens 111-W.

It’s worth remembering that storage combi boilers are more expensive than standard combis. However, they provide hot water more like a system boiler and hot water cylinder, giving you the best of both worlds.

What is a Storage Combi Boiler?

In simple terms, a storage combi boiler is a combi boiler with a storage tank inside the boiler itself (a thermal water store). This means it has a hot water cylinder housed within the boiler unit.

Storage combi boiler = Combi boiler with storage tank for hot water.

  • A standard combi boiler draws cold water directly from the mains supply, heating it on demand (when you turn on a tap or shower).
  • Storage combi boilers also draw cold water from the mains supply, but hot water is fed from the internal tank. The boiler then refills it is used. This means hot water can be fed to taps and showers more quickly than the cold water flow could handle.
Viessmann Vitodens 111-W - Storage Combi Boilers

The efficient thermal store is shown in the rear-right of the boiler in this cross-section of the Vitodens 111-W.

Storage combi boilers have the benefits of both combi and system boilers. They deliver pressurised hot water like a system boiler and external hot water cylinder without the external 150-500 litre tank in your home. You can read more about the differences between system and combi boilers in our guide.

These boilers can deliver up to 20 litres a minute flow rate which is enough for up to three showers. Viessmann has floor-standing and wall-mounted models. They have different-sized internal hot water tanks which allows high hot water flow rates for longer periods of time.

Another great benefit of storage combi boilers like the Viessmann Vitodens 111-W or 222-F is their efficiency. The efficient operation of central heating and the thermal water store allows the boiler to get up to 98% efficient.

This video shows two of our engineers installing the Viessmann Vitodens 111-W storage combi boiler.

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How Does A Storage Combi Boiler Work?

A combi boiler system provide heat for the central heating system and hot water taps in your home. The difference is how the hot water taps are supplied.

  • Combi boilers work by heating cold water from the mains supply through a heat exchanger and allowing it to flow through your home to a tap or shower.
  • Storage combi boilers provide hot water from an internal hot water cylinder whilst continually filling it up. This allows a higher hot water flow than without the efficient thermal store for hot water.

What is CLS (Cylinder Loading System) Technology?

Storage combi boilers use a cylinder loading system (CLS) to keep water hot. It takes cooler water from the bottom of the internal cylinder and pumps it through the plate heat exchanger to keep the water hot. A traditional stand-alone hot water cylinder will use a heating coil to heat the water.

Read more about heat exchangers here >

The Components of a Storage Combi Boiler

Storage combi boiler - Vitodens 111-W breakdown

Storage combi boiler – Vitodens 111-W breakdown

Using the Viessmann Vitodens 111-W as an example, the diagram shows the components of a storage combi boiler system:

  1. 46-litre stainless loading cylinder/storage tank
  2. Stainless steel Inox-Radial heat exchanger
  3. Variable speed combustion fan
  4. Integral expansion vessel
  5. High-efficiency pump

Why Choose a Storage Combi Boiler?

You don’t have the space for a hot water cylinder

With an integrated hot water tank a storage combi boiler can provide hot water levels equivalent to a 150-litre cylinder without needing the cylinder. This means you can save space in your kitchen, garage, airing cupboard or loft and still get the hot water you need.

A system boiler isn’t suitable

Due to the space required with system boilers and cylinders, storage combi boilers are an ideal substitute for households where a system boiler cannot be installed.

You have a property with multiple bathrooms

If your property has more than one bathroom, storage combi boilers are a great option between a combi and a full system setup. Not sure if they’re right for you? Get an instant and free online quote with Boilerhut, and we’ll call you back to discuss your options.

You have a large family and high hot water demand

A storage combi boiler will be perfect if you have a large or growing family. With multiple family members in one household, you will inevitably find that multiple people want hot water simultaneously. Whether the hot water demands are from the shower, the bath, or the kitchen sink, a storage combi boiler is equipped to deliver hot water as and when you need it through multiple outlets.

They are easy to install

Storage combi boilers are easier to set up and install for a heating engineer than a full system but more complicated than a traditional condensing combi boiler. This helps reduce installation time, getting your heating and hot water as soon as possible.

Viessmann combi boilers

We specialise with Viessmann because we believe they provide a premium product with superior parts, warranty and servicing options. Viessmann storage combi boilers offer industry-leading efficiency and can save you money on your gas bills, paying back their initial installation cost in just a few years.

Get A Quote For A Storage Combi Boiler.
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Is a Storage Combi Boiler Worth it?

Storage combi boilers are highly efficient and ensure high hot water flow rates. Heated water is kept in a small, well-insulated tank inside the boiler, which can lose less heat than a separate hot water cylinder. Some storage combi boilers are 98% energy efficient. They’re ideal for larger properties and tend to be cheaper and easier to install than a full system boiler setup.

What Properties Are Storage Combi Boilers Suitable For?

Depending on how much hot water you need at any one time, a storage combi boiler could be the right choice for you. Storage combi boilers combine heating, hot water and hot water storage. They are excellent for mid-sized homes and large flats or apartments.

If a traditional combi boiler can’t provide the hot water needed for your property, a storage combi boiler is a great option if an entire system boiler is overkill.

If you have two bathrooms that might use hot water simultaneously, storage combi boilers could be a great option. You get to keep your cupboard space, and a model like the Viessmann Vitodens 111-W can be put on the wall like a standard combi boiler.

Advantages And Disadvantages Of A Storage Combi Boiler


  • Instant hot water
  • Efficient heating and hot water generation
  • Cheaper to purchase and install than open vent and system boilers.
  • Minimal space: no need for attic tanks or hot water cylinders.
  • Compact size; some models can fit inside a kitchen cabinet.
  • Easy to install and set up for the installer. They’re more compact than system boilers, which simplifies installation.
  • Options are available for properties of all sizes.
  • Stores hot water: they provide the ability to deliver hot water to multiple outlets simultaneously.
  • High-efficiency heating and hot water. The Viessmann Voitodens 111-W Storage Combi is up to 98% efficient / A-rated.


  • More expensive than a standard combi boiler.
  • Larger than standard combi boilers.
  • More complicated internal mechanisms.
  • May require two heating engineers to install due to the weight of the unit.
Get A Quote For A Storage Combi Boiler.
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What Size Storage Combi Boiler Do I Need?

When choosing a storage combi boiler, it’s essential to consider the size of the property in relation to how much water will need to be heated throughout. Don’t be afraid to take advantage of the space if you have it. Space-saving is one of the most important benefits of a storage combi boiler.

When thinking about size, there are two things to consider: how much water you’ll need, and how much room you have.

The Viessmann Vitodens 222-F 32kW is a large, floor-standing storage combi boiler with a 100-litre hot water tank that offers the equivalent hot water of a 300-litre system boiler cylinder. It’s perfect for homes with very high hot water demands.

The Viessmann Vitodens 111-W is a wall-mounted boiler and larger than combi boilers of similar power. It’s available in 25kW and 32kW versions. The 111-W has a 46-litre hot water tank, which provides water equivalent to a stand-alone 150-litre hot water tank.

The Best Storage Combi Boilers

In our opinion, Viessmann manufactures the best storage combi boilers available in the UK right now. Vaillant has an option, as do Worcester Bosch, and models of both combi boilers range power output. The most popular storage combi boilers are from the Vitodens 111-W range: a large but still wall-hung boiler.

1. Viessmann 111-W Storage Combination Boiler

Viessmann Vitodens 111-WThis is a high-performance storage combi boiler, perfect for a larger flat or a family home with high demand for hot water on a daily basis.  With its CLS technology (Cylinder Loading System) and 46-litre internal hot water cylinder, this storage combi boiler produces 18 litres of hot water per minute. It can supply multiple hot water outlets. For example, fill a standard bath in 8 minutes while supplying a sink or washbasin.

All Viessmann boilers modulate energy usage, so energy efficiency is always high on their agenda, saving you money on your energy bills.  The advanced sensor technology maintains constant output temperatures even under high demand. Modulation allows a boiler to reduce heating power from its maximum output; a Vitodens 111-W can modulate from 2.9kW to 32kW.

This storage combi boiler can also be controlled with a tado° thermostat or the new Viessmann smart thermostat and smartphone app.

Please read our full review >

Key Features:

  • Dimensions (H x W x D): 950mm x 600mm x 500mm
  • Cylinder loading system (CLS) and integrated stainless steel storage tank
  • User-friendly control via backlit LCD touchscreen
  • High hot water capability with a flow rate of 18 l/min
  • Available in outputs from 4.7 to 35.0 kW
  • Hot water performance equivalent to a separately installed 150L cylinder
  • User-friendly dials for rapid adjustment of heating and hot water
  • Energy efficiency class: A
  • Optional wireless remote control available
  • Optional solar thermal backup
  • Up to 10 years extended warranty available with Boilerhut

2. Vaillant ecoTEC plus Combination Store

Vaillant ecoTEC pro storage combiThis combi boiler with a hot tank from Vaillant delivers great performance with lower energy consumption.  It produces 20 litres per minute of hot water when needed, and then modulates to using only 16% power, saving you fuel and money.

Vaillant ecoTEC plus Combination Store Key Features

  • ErP A rated for both hot water and heating efficiency
  • Dimensions (H x W x D): 720mm x 440mm x 600mm
  • Integrated hot water storage device at the back of the boiler
  • Hot water within 5 seconds
  • Flow rate of up to 20 l/min
  • A wide range of heating control options are available
  • 5-year guarantee as standard, with 7 to 10 years available when you book a Vaillant Advance installer

The Vaillant isn’t as tall or wide as the Viessmann 111-W, but it is much deeper. It’s essential to take proper measurements and consult Boilerhut engineers when buying a boiler to ensure it will fit where you want it installed. This boiler may not fit in most cupboards.

Feel free to read our full ecoTec combi review >

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Storage Combi Boilers for Larger Homes

Our recommendations:

1. Viessmann Vitodens 222-F Storage Combi Boiler

The Viessmann Vitodens 222-F is a floor standing gas storage combi boiler, designed for providing heat and hot water to large homes thanks to its integrated 100-litre hot water tank.

The Vitodens 222-F is designed to easily fit into a kitchen or utility room, both in terms of its looks and in terms of its standard kitchen unit dimensions, which allow for easy integration

Viessmann Vitodens 222-F Key Features

  • Rated heating output: 1.8 to 35 kW
  • Internal 100-litre hot water cylinder
  • Inox-Radial stainless steel heat exchanger
  • Modulating Matrix cylinder burner
  • Compatible with Viessmann’s apps
  • Colour touch-screen with a graphic display of energy consumption
  • No LPG conversion kit needed
  • 10-year guarantee when installed by a Viessmann-trained installer like Boilerhut

Read our full Vitodens 222-F review >

Large floor standing storage combi boilers

2. Viessmann Vitodens 242-F Storage Combi Boiler (Discontinued)

The Viessmann Vitodens 242-F was a floor-standing gas storage combi boiler designed to use solar technology alongside gas. The Vitodens 242-F was equipped with an integrated 170-litre hot water storage tank.

Like the Viessmann Vitodens 222-F, the Vitodens 242-F featured a compact width and depth, which make it ideal for installation in a kitchen or utility room.

Viessmann Vitodens 242-F Key Features

  • Integral solar cylinder
  • 5-inch colour touchscreen
  • Inox-Radial stainless steel heat exchanger
  • MatriX cylinder burner with Lambda Pro Control combustion controller for permanent high efficiency and clean combustion
  • Floor area of 0.4 m², so can fit into any recess
  • Compatible with Viessmann’s Vitoconnect 100 WLAN adaptor for smart home control
  • 10-year guarantee when installed by a Viessmann-trained installer like Boilerhut

Storage Combi Boiler Range

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Vitodens 111-W Gas Storage Combi Boiler - Installations in Swindon | Boilerhut

Viessmann Vitodens 111-W

  • Size: 25kW, 32kW
  • Efficiency: 94% / A-rated
  • Warranty: 10 Years
  • Stainless Steel Heat Exchanger
  • 46 Litre Hot Water Tank
  • Up to 21.2 Litres/Min Hot Water
222-F Floor-Standing Storage Combi Boiler

Viessmann Vitodens 222-F

  • Size: 32kW
  • Efficiency: 94% / A-rated
  • Warranty: 10 Years
  • Stainless Steel Heat Exchanger
  • 100 Litre Hot Water Tank
  • Up to 23 Litres/Min Hot Water
F54 Vaillant Error Code | Boilerhut

Vaillant ecoTEC plus combi store

  • Size: 25kW, 32kW
  • Efficiency: 89% / A-rated
  • Warranty: 10 Years
  • Stainless Steel Heat Exchanger
  • Up to 15.9 Litres/Min Hot Water

Get A Quote For A Storage Combi Boiler.
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Save £100s PER YEAR on your bills.

Yes. They’re great for properties that have low cold water flow rates, multiple bathrooms or no space for a more traditional boiler and hot water cylinder/tank. They aren’t for everyone, of course, but they’re a great option for some properties.

They work exactly the same way as a ‘normal’ combi boiler but they include a hot water tank. This allows them to push more water through the home than comes in through the cold water flow rate, for a temporary amount of time.

  • Combi boiler – an on-demnd heating and hot water unit that uses fuel, usually natural gas, LPG or oil. No need for separate hot water cylinders in the property.
  • Storage combi boiler – a combi boiler with an in-built hot water tank/cylinder for better hot water flow during high demand.
  • System boiler – a system boiler heats water and stores it in a large hot water cylinder, normally situated in an airing cupboard. Great for large hot water demand in large homes.
  • Storage combi boiler – a combi boiler with an in-built hot water tank/cylinder for better hot water flow during high demand, but not as high as a system boiler. A single unit: no external hot water tank.

A storage combi boiler is a ‘normal’ combi boiler that includes a hot water tank. This allows it to provide hot water instantly, with low cold water flow or when the demand is high.

Storage combi boilers come in differen sizes depending on your property’s requirements. They can be, pretty much, the same size as a ‘normal’ combi boiler. They can also be as large as a big single-door fridge – but these come with 170 litre hot water tanks.

We would pretty much always recommend the Viessmann Vitodens 111-W. This is a wall-hung storage combi boiler with a 46 litre hot water tank. It modulates 1:10 (meaning the 32kW version can run as low as 3.2kW saving you money) and is industry-leading rated as 98%/Class A efficient. Lastly, we can provide a 12 year warranty.

  • Size – They are generally larger than normal combi boilers so some people may struggle to put them in the same place as a previous unit.
  • Cost – They are generally more expensive due to the increase components and complexity.
Get A Quote For A Storage Combi Boiler.
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Save £100s PER YEAR on your bills.

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