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Last updated: 28th March, 2024


Combi Boilers vs Conventional Boilers

Combi Boilers vs Conventional Boilers

Key takeaways

  • Combi boilers provide on-demand hot water without needing separate storage, optimising space.
  • Conventional boilers are suited for larger households with a higher demand for hot water supply.
  • Both systems have unique installation considerations, efficiencies, costs, and maintenance needs.

Combi boilers, also known as combination boilers, are compact units that serve as both a central heating system and a high-efficiency water heater.

Choosing the right type of boiler for a home is essential for ensuring efficient heating and hot water supply. Combi boilers, also known as combination boilers, are a popular choice due to their ability to provide hot water on demand without the need for a separate hot water cylinder. 

This makes them an ideal solution for homes with limited space and a lower demand for hot water at any one time.

On the other hand, conventional boilers, sometimes referred to as regular or heat-only boilers, work on a different principle.

They require a cold water storage tank, usually in the loft, and a hot water cylinder, often found in the airing cupboard, to store and provide hot water. 

This can be beneficial for larger homes with higher hot water needs where multiple bathrooms may be in use simultaneously, as it allows for a steady supply of hot water.

Considering a boiler's space and storage requirements, efficiency, costs, performance, maintenance, and the size and type of home they're used in is essential for making an informed choice between a combi and a conventional boiler.

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Understanding Boiler Basics:

When discussing the core components of a home's heating system, one quickly encounters two primary types of boilers: combi boilers and conventional boilers. Each plays a pivotal role in delivering comfortable living conditions, yet they operate on decidedly distinct principles.

Combi boilers, also known as combination boilers, are compact units that serve as both a central heating system and a high-efficiency water heater. They heat water directly from the mains when a tap is turned on, which means there is no need for a separate water storage tank. This on-demand hot water feature and the lack of a cold water tank make combi boilers particularly appealing for smaller homes and spaces where efficiency of space and energy is essential.

On the other side of the spectrum sit conventional boilers. These systems utilise a separate cold water tank (typically in the loft) and a hot water cylinder. They are adept at providing hot water to multiple taps simultaneously, which makes them more suitable for larger houses with multiple bathrooms. A conventional boiler's ability to supply a steady stream of hot water to several outlets is a key point of distinction.

Boiler Type

Combi Boiler

Conventional Boiler

Space Savings

No tanks required, compact

Requires cold and hot water storage

Water Delivery

Immediate hot water provision, no storage

Provides hot water in bulk, hence some waiting


Smaller homes/flats, fewer occupants

Larger homes, high water demand

Energy Efficiency

Generally more energy-efficient as they heat on demand

Less efficient due to heat loss from the tank

Lastly, the system boiler bridges gaps between the two; it requires a cylinder like a conventional boiler but omits the need for a cold water tank. Many of these are also condensing boilers, meaning they capture heat from exhaust gases that would otherwise be wasted, thus increasing efficiency. It is vital to choose the boiler type that aligns with a home's size and hot water needs.

Types of Boilers Explained

Choosing the right boiler for your home involves understanding the main differences between combi and conventional boilers, including considerations about space, energy efficiency, and the mechanics of how they operate.

Combi Boiler Overview

A combi boiler, short for combination boiler, is a high-efficiency unit that serves as both a water heater and a central heating boiler in a single compact unit. They tend to be space-saving as they do not require a hot water cylinder or cold water storage tank, freeing up valuable space in homes. The heat exchanger within a combi boiler ensures that water is heated instantly on demand, which can lead to reduced energy bills due to their only heating water when it's needed.

Conventional (Heat Only/System) Boiler Overview

A conventional boiler, sometimes referred to as a heat-only or system boiler, typically necessitates more space as it includes a separate hot water cylinder and a cold water tank, which is often placed in a loft or attic. This boiler type is beneficial for homes with multiple bathrooms because it can provide a steady flow of hot water to several taps simultaneously. However, because water is stored in a cylinder, there must be enough heated water in reserve, requiring planning and potentially leading to more complex pipework.

Heating and Hot Water Functionality

The core difference between combi boilers and conventional boilers lies in their approach to providing heating and hot water to homes. Each system is designed to meet these needs efficiently, but their internal operations and components vary significantly.

How Combi Boilers Provide Heating and Hot Water

A combi boiler, short for 'combination boiler', is a compact system that provides both central heating and instant hot water directly from the mains supply, eliminating the need for separate hot water storage cylinders or cold-water tanks. At the heart of a combi boiler lies a heat exchanger, which rapidly heats water from the cold mains supply. When a hot water tap is turned on, the boiler activates, and the heat exchanger goes to work, delivering hot water on demand.

The efficiency of a combi boiler's heating system hinges on the pump that circulates hot water to radiators throughout the home. One significant advantage is that the water pressure remains unaffected by simultaneous use, as combi boilers rely on mains pressure rather than a pump. Furthermore, since water is heated as needed, there is no additional energy cost for storing hot water, thereby reducing energy bills for many households.

Key Components:

  • Heat Exchanger: Rapidly heats water on demand.

  • Pump: Ensures even distribution of hot water and heating.


  • Instant hot water

  • No need for a separate hot water tank

How Conventional Boilers Operate

Conventional boilers, often known as regular boilers, utilise a more traditional approach to heating and hot water. They consist of a separate hot water cylinder stored in an airing cupboard and a cold water storage tank usually located in the loft. 

The boiler heats the water, which then is stored in the hot water cylinder until needed. When a demand for hot water is made, the water is then pumped from the cylinder to the taps.

Central heating is managed by the boiler heating water and sending it through to the radiators. A conventional boiler system might be better suited to homes with higher hot water demand, particularly those with multiple bathrooms, as they can provide a greater flow rate than combi boilers. However, the presence of the hot water cylinder and cold water tank means they occupy more space and hot water can run out, requiring time for the cylinder to refill and reheat.

Key Components:

  • Hot Water Cylinder: Stores a large volume of hot water.

  • Cold Water Tank: Feeds water into the cylinder, relying on gravity.


  • Ideal for homes with multiple bathrooms

  • Requires more space for storage components

Installation Considerations

When selecting a boiler installer, it is imperative to consider the complexities of your home's existing infrastructure. 

With combi boilers, installation is generally simpler; they do not require additional components like a cold water tank or a hot water cylinder, making them a more straightforward choice for most urban homes that may lack extensive loft space or an airing cupboard.

In contrast, the installation of a conventional boiler involves more intricate pipework and the integration of a hot water cylinder, which is often situated in an airing cupboard, and a cold water storage tank typically located in the loft. 

This setup can be more labour-intensive and may entail a more significant system upgrade for older houses or those with an existing conventional system.

Conventional boilers benefit from retaining water temperature for an extended period, which might be advantageous in households with multiple bathrooms requiring simultaneous access to hot water.

  1. Combi Boiler Considerations:

  2. Conventional Boiler Considerations:

    • May necessitate a new hot water cylinder or expansion of current pipework.

    • Loft space is required for the cold water tank.

    • Larger homes with several bathrooms may benefit from the stored water capacity.

It is critical, whatever your choice may be, to engage a certified and experienced boiler installer to not only ensure the adherence to safety regulations but also to guide you through the process efficiently. A professional installer can provide insight into the most suitable boiler type for your home while considering existing systems and spatial limitations.

Comparing Space and Storage Requirements

When assessing the space and storage needs of combi boilers versus conventional boilers, clear differences emerge. Combi boilers are noted for their compact design. They integrate the heating and hot water functions into a single unit, often small enough to fit within a kitchen cupboard. 

This space-saving advantage arises because combi boilers do not require separate cold water tanks or hot water cylinders.

In contrast, conventional boilers typically necessitate a cold water tank in the loft and a hot water cylinder housed in an airing cupboard or equivalent space. These additional components account for a larger footprint within the home. 

Houses with ample space, specifically those with a loft for the water tank, can accommodate these elements more readily.

Moreover, the absence of a storage tank with combi boilers translates to a direct supply of water from the mains. This ensures that hot water is available on demand without the need for a storage cylinder. Here’s a brief comparison:

  • Combi Boilers:

  • Conventional Boilers:

    • Pros:

      • Suitable for homes with existing tanks and cylinders

      • Can supply several hot water outlets at once

    • Cons:

      • Requires loft space for the cold water tank

      • Needs an airing cupboard or similar area for the hot water cylinder

Ultimately, the choice between these two boiler types may hinge on the existing or potential space available for installation. Those with smaller homes may lean towards the combi boiler for its space-saving benefits, while larger properties with sufficient space might opt for the storage capabilities of a conventional boiler.

Evaluating Efficiency and Costs

When assessing boilers, it's imperative to consider both the immediate financial impact and the long-term efficiencies that will affect energy bills.

Energy Efficiency and Bills

A-rated combi boilers are notable for their high energy efficiency, often operating within a 90-94% efficiency rating. In contrast, older heating systems might only utilise 55% of their energy for central heating. 

The higher the efficiency, the more cost-effective the boiler is in the long run, as it converts more of the energy it uses into heat, resulting in lower energy bills. The key advantage of choosing an energy-efficient new boiler lies in its ability to reduce the home's carbon footprint and operate more economically.

Initial and Long-Term Costs

The initial cost of a new boiler can be substantial. Combi boilers, while technologically advanced, may come with an affordable price when compared to traditional systems, especially when considering installation complexities. 

A system upgrade to a combi boiler means dispensing with cold water tanks and hot water cylinders that are synonymous with conventional setups. This might result in lower installation costs.

However, one should also look beyond the purchase price. Long-term costs encompassing maintenance and potential repairs are equally pivotal in the decision-making process. 

Typically, combi boilers have fewer components, which can translate into less maintenance over time and thus more savings. It is prudent to evaluate both the initial layout and the ongoing costs to decide on the most cost-effective solution for one's home.

Comparative Analysis of Performance

Evaluating the performance of combi and conventional boilers hinges on understanding their capabilities in delivering hot water and managing heating with varying water pressure demands.

Water Supply and Pressure

Combi boilers directly utilise mains water pressure, which typically facilitates better pressure for showers and taps. They excel in delivering hot water on demand, making them an ideal choice for homes with a solitary bathroom. However, when multiple taps or showers operate simultaneously, combi boilers may struggle, leading to pressure issues.

  • Mains pressure: A combi boiler uses it directly, ideal for households with strong mains pressure.

  • Fewer pressure issues: Typically for single-use, with potential for reduced performance under multi-use scenarios.

In contrast, conventional boilers rely on a gravity-fed system with a cold water storage tank, usually situated in the loft. This setup can maintain water supply and pressure even when multiple outlets are used, benefiting homes with multiple bathrooms.

  • Low water pressure area suitability: Better suited for regions with inadequate mains pressure as they utilise a pump to boost flow.

  • Multiple users: Capable of handling several open taps without significant loss in water pressure.

Heating and Hot Water Output

Heating and hot water are where these two systems distinctly diverge. Combi boilers provide heating and hot water instantaneously from a single unit without the need for a storage cylinder. This on-demand feature ensures efficient use of energy but can be limiting during times of high demand.

  • On-demand hot water: Limitless supply for singular usage but may falter with concurrent demands.

  • Space-saving: Lack of a storage tank frees up space, an advantage in smaller properties.

Conventional systems are often more suitable for larger households where hot water demand from multiple sources is common. They store hot water in a cylinder, which allows for simultaneous access across various outlets without a drop in temperature.

  • Hot water storage: A sizable volume is kept ready, providing a steady and reliable supply.

  • Multiple taps: Can supply multiple users at once without noticeable temperature fluctuations.

In homes where water is frequently needed from various outlets at once, a conventional boiler may provide consistent performance. Conversely, for smaller homes or ones with lower hot water usage, a combi boiler’s prompt response can be more energy-efficient and equally reliable.

Suitability for Home Types and Sizes

Choosing the right boiler for a home hinges on various factors, such as the number of bathrooms and the home's size. Combi boilers, sleek in design and efficient, are an excellent match for smaller homes and flats. They directly heat water from the mains on demand, eliminating the need for storage tanks. This makes combi boilers suitable for homes where space is at a premium.

Conventional boilers, on the other hand, are typically found in larger houses. They use a separate hot water tank, which can cope with the high water demand of multiple bathrooms. This feature secures a steady supply of hot water, even when multiple taps are in use concurrently. Here is a brief comparative snapshot:

Boiler Type

Home Size

Number of Bathrooms

Space for Tanks


Small to medium

Single bathroom

Not necessary



Multiple bathrooms


While combi boilers thrive on high mains water pressure to deliver water without delays, conventional boilers can perform well even in areas with lower mains pressure. Thus, for homes with an older heating system that can't handle high pressure, conventional boilers might be the preferred solution.

It's essential for homeowners to consider their household's water usage habits. If a house frequently requires water in several locations at once, a conventional boiler with its stored hot water may prove more reliable. 

Conversely, for those prioritising energy efficiency and space-saving, a combi boiler might be the pragmatic choice.

Pros and Cons of Each Boiler Type

Choosing the right boiler for a home involves weighing the benefits against the potential drawbacks. Factors like space availability, cost implications, and hot water demands guide this decision.

Combi Boilers


  • Compact: Combi boilers are known for their space-saving design, making them an ideal choice for homes with limited space.

  • Efficiency: These boilers provide hot water on demand, leading to less heat loss and potential energy savings.


  • Pressure issues: In residences with multiple bathrooms, combi boilers might struggle with water pressure when multiple taps are in use simultaneously.

  • Limited hot water supply: Large households may find the instantaneous water heating feature insufficient during peak times.

System Boilers


  • Steady pressure: With a system boiler, homeowners can use several taps at once without experiencing a drop in water pressure.

  • Space-saving: Although they do require space for a cylinder, they don't need a separate cold water tank, saving some room compared to a traditional heating system.


  • Requires cylinder: The need for a hot water cylinder means it's not as compact as a combi boiler.

  • Heat loss: As water is stored in a cylinder, there may be some heat loss, which could lead to less efficiency.

Heat Only Boilers


  • Compatibility: These boilers are often compatible with older, traditional heating systems, which can make them suitable for older homes.

  • High capacity: They can meet the demands of homes with multiple bathrooms, providing hot water to various outlets simultaneously.


  • Space requirement: Heat only boilers require a separate cold water tank and hot water cylinder, hence need more space than combi or system boilers.

  • Less efficient: There can be more heat loss as hot water is stored and not produced on demand. This could result in higher operating costs.

Maintenance and Lifespan

Regular boiler systems often require a more involved maintenance routine, due to their additional components like the cold water storage tank and the hot water cylinder. 

A Heating engineer might be called annually for a check-up to ensure everything is functioning correctly.

Combi boilers, on the other hand, are typically less demanding in terms of maintenance. This ease is due to their fewer parts and the fact that they don't require tanks or cylinders. Both types of boilers may need similar basic maintenance tasks, such as keeping the system pressure at the correct level and having annual servicing to maintain the warranty.

The lifespan of a boiler can vary, yet most units are expected to operate effectively for up to 15 years. However, this period can be shorter or longer depending on several factors, including the frequency of maintenance, the quality of the installation, and the boiler's make and model. Manufacturers might provide warranties ranging from 1 to 10 years, although the warranty conditions will differ.

Regarding condensing boilers, which can be either combi or regular type, they tend to have a higher efficiency level and therefore could potentially have a longer lifespan due to less stress on the boiler's components. Still, they are complex systems and require an expert touch during scheduled maintenance to ensure they continue to operate at peak efficiency.

In summary, longevity and maintenance depend on various dynamics. The choice of whether to install a combi or a conventional boiler should weigh these considerations alongside the specifics of the household’s heating requirements.

When selecting a boiler for a home, the choice often revolves around combi and conventional models. Among these, certain brands stand out due to their reputation for quality and service.

Worcester Bosch is a prominent name, praised for models like the Worcester Bosch Combi Greenstar 4000. Ideal for smaller homes, these boilers are distinguished by their reliability and comprehensive warranty options, which cover parts and labour for seven years with a 10-year guarantee on the heat exchanger.

Another notable manufacturer is Baxi, a company with a rich history in heating solutions. They offer a range of boilers suitable for various household sizes, focusing on efficiency and user-friendliness.

Vaillant boilers are also a common sight in UK homes. Their units are celebrated for being eco-conscious without compromising performance. Vaillant's commitment to renewable energy solutions makes them a forward-thinking choice for homeowners.

Here's a quick overview:



Notable Features

Worcester Bosch

Greenstar 4000

- Energy efficient

- Suitable for small homes

- Long warranty


(Various Models)

- User-friendly

- Diverse range


EcoTec (Various Models)

- Eco-friendly

- High performance

In summary, each boiler manufacturer has its strengths, from the long warranties of Worcester Bosch to the eco-friendly designs of Vaillant, and the user-oriented approach from Baxi. The choice often hinges on the specific needs of the household, whether it's the size, the hot water demand, or environmental considerations.

Considerations for Replacing Your Boiler

When homeowners consider upgrading their heating system, they must carefully evaluate their needs before proceeding to replace the old boiler. A new boiler installation is a substantial investment, thus making an informed choice is critical.

Types of Boilers:

  • Combi Boiler: Provides heating and hot water from one unit.

  • Conventional Boiler: Needs a separate hot water tank and cold water storage.

Space Requirements: A combi boiler's compact size benefits homes with limited space, as it eliminates the need for a hot water cylinder. Conventional boilers, suitable for homes with higher hot water demand, require more space for the storage of water tanks.

Installation Costs:

  • Combi boilers might offer a simpler installation process, potentially reducing initial costs.

  • Conventional boilers may incur higher installation charges due to the complexity of the system.

Efficiency: Modern boilers tend to be more energy-efficient than older models. Replacing an outdated boiler can lead to significant savings on energy bills.

Warranty Period: Look for boilers that offer a substantial warranty period, as some may provide up to 10 years of coverage.

Boiler Quotes:

  • Obtain multiple free boiler quotes to compare options.

  • Ensure quotes include installation fees and additional costs.

When replacing a boiler, having a detailed understanding of the household’s heating necessities is essential. It is also advised to seek the expertise of certified professionals for installation. This ensures homeowners not only invest wisely but also adhere to safety and regulatory standards.

Fuel Types and Compatibility

The type of fuel available to a household plays a significant role in determining whether a combi or conventional boiler is suitable. Both combi and conventional boilers can be engineered to work with varied fuel types, notably gas, oil, and LPG (Liquefied Petroleum Gas).

Gas-fired boilers are common due to the widespread availability of mains gas, which tends to be the most cost-effective option. Combi boilers often use natural gas, providing instant hot water and heating without the need for a separate cylinder.

In contrast, oil boilers are an alternative for properties not connected to the gas network. Oil is typically used in conventional boilers where it heats water stored in a tank. This system is particularly beneficial in areas with limited gas infrastructure.

For those seeking a middle ground, LPG serves as a versatile fuel, utilised in both combi and conventional systems. Its application is often seen in rural locations, where the ease of storage and energy-rich nature of LPG offsets its slightly higher cost compared to natural gas.

Here is a brief overview of the compatibility:

Fuel Type

Combi Boiler Compatibility

Conventional Boiler Compatibility


Widely used

Commonly utilised


Less common

Preferred for off-grid homes


Suitable for rural areas

Frequently adopted option

When selecting a boiler, it is essential to consider the infrastructure and storage facilities available at the property, alongside the long-term sustainability and cost of the fuel type. Each option carries its own set of benefits and constraints, hence understanding these dynamics assists in making an informed choice.

Regulations and Safety

When installing or replacing boilers, regulations are central to ensuring safety and efficiency. The United Kingdom has comprehensive rules overseen by qualified heating engineers.

For Gas-fired Boilers:

  • New installations and replacements must be condensing boilers.

  • They should possess at least a 'B' efficiency rating.

For Oil-fired Boilers:

  • Similar to gas boilers, replacements and new installations are required to be condensing.

Legislation mandates that boiler installers adhere to specific safety standards, often regulated by governmental bodies. This ensures that every installation is performed not only with expertise but also with an emphasis on maintaining a safe environment.

Heating engineers must be Gas Safe registered when working on gas boilers. They must ensure that all fittings and installations comply with current legal standards. It's also their responsibility to inform boiler manufacturers of recurring issues, allowing for safety refinements.

Manufacturers have a pivotal role in maintaining safety by:

  • Delivering units that meet rigorous safety standards.

  • Providing clear instructions for safe installation and operation.

  • Ensuring that all products undergo stringent quality checks.

Compliance with these regulations is not just a legal requirement; it also offers peace of mind for the end-users, knowing their systems are safe, reliable, and efficient.

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Frequently Asked Questions

Which is better, a combi or conventional boiler?

The better option depends on household requirements. A combi boiler is more suitable for smaller homes with limited space and a lesser demand for hot water. In contrast, conventional boilers are ideal for larger properties with multiple bathrooms.

Is a combi boiler cheaper to run than a system boiler?

Yes, generally, a combi boiler can be cheaper to run. It only heats water on demand, which means there's no energy wasted in heating a storage tank, leading to potentially lower energy bills.

Is it worth changing to a combi boiler?

Changing to a combi boiler can be worthwhile, particularly if your current system is inefficient, or when you're looking to save space and reduce hot water wait times. However, the transition should be carefully considered, taking into account the costs involved.

When should you not use a combi boiler?

Avoid using a combi boiler if your property has high water usage simultaneously from multiple outlets, or if you have an existing low-pressure water supply. Combi boilers might not be capable of meeting the high demand for hot water in these instances.

Is it advisable to switch from a conventional to a combi boiler?

Switching to a combi boiler is advisable if you desire a more compact system and if you're currently experiencing inefficient heating and hot water supply. Remember, this switch should align with your water usage patterns for it to be beneficial.

Will I save money switching to a combi boiler?

Potentially, you could save money by switching to a combi boiler due to its on-demand heating capability, which can lead to lower utility expenses, especially if your existing system is outdated or inefficient.

Which is cheaper to run, a combi or system boiler?

A combi boiler is typically cheaper to run compared to a system boiler. This is due to its design that eliminates the need to heat and store water in advance, which can result in energy savings. However, system boilers may be better at meeting high water demands, which might make them more cost-effective for larger households.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.