Facebook pixel
new boiler

Get a new boiler

Fixed price boilers, next day

See boiler prices
new air conditioning

Air Conditioning

Get a quote
new heat pump

Heat Pumps

Coming soon

new ev charger

EV Chargers

Get a quote
boiler servicing

Boiler Servicing

Get a quote

Last updated: 2nd April, 2024


Combi Boilers - Advantages And Disadvantages

Combi Boilers - Advantages And Disadvantages

Key takeaways

  • Modern combis are amongst the most efficient boilers on the market.
  • Gas is one of the cheapest types of boiler fuel, as much as three times cheaper than electricity.
  • System boilers are better suited than combis for larger properties with multiple bathrooms.

Combi boilers are by far the most popular type of home heating system in the UK, and with good reason, providing many benefits to their owner and the properties they heat. A versatile boiler, combis are suited to many types of home, however, maybe not all types.

In this blog we’re going to be exploring some of the many pros and some of the lesser known cons of a combi boiler.

Combi Boilers Explained

Before we dive into the complexities of combis, if you aren’t completely sure what one is, don’t worry, allow us to explain.

A combi (combination) boiler is a type of domestic boiler that ‘combines’ a home’s central heating and hot water production into one succinct unit.

A combi takes water directly from the mains and heats it upon usage. After detecting a hot water request, the combi begins to burn gas in a combustion chamber. 

The hot gases rise and move through a series of pipes and into a component called a heat exchanger, the hot gas running through the exchanger then heats the surrounding cold water.

In a conventional boiler system, this process is done according to a programme and the water is stored in a separate cylinder until it’s needed. 

A combi only starts this cycle upon request (initiating the heating or turning on a hot water outlet), therefore the hot water is almost instant.

Combi Boiler Advantages

As mentioned, modern combis are an extremely popular type of boiler, this is down to their myriad of benefits, including:


Most modern combi boilers are extremely efficient in their heating of a property, with ErP ratings of ‘A’ and 90% conversion efficiencies, meaning hardly any energy used to produce heat is wasted.

Age of Boiler

Efficiency Percentage

Efficiency Rating (ErP)

0+ Years

90% +


10+ Years

85% +


15+ Years

80% +


20+ Years

70% +


25+ Years

60% +


The direct correlation between a boiler's age and its efficiency is clear to see in the newer systems’ ErP. 

Get a quote


Due to the compact casing and combined dual functionality of a combi boiler, essentially they perform the job of two appliances (conventional boiler and water storage tank), while occupying minimal storage space.

Most combis are small enough in dimension to fit easily into a standard kitchen cupboard, meaning they never detract from the decor or purpose of an internal space.

Water Pressure & Speed

Combi boilers take their water supply directly from the mains, this means the water circulating through the home’s central heating system retains a lot of the substantial flow pressure

A conventional boiler often relies on gravity to collect water, whereas a combi’s flow is much stronger and reduces the likelihood of water becoming stuck or ‘pooling’ in curved pipes due to the extra force behind the flow.

A combi boiler can produce instantaneous hot water, which is obviously beneficial for any property. 

Any Home with multiple bathrooms and an old, inefficient heating system may have to accommodate slower water heating speeds, increasing energy wastage, and as a result, energy bills.


Due to their modernity and consumer convenience based design, many combis can be synced with and remotely controlled by technology, such as smart thermostats or apps via a smartphone or tablet. 

This convenience in being able to wirelessly control your property’s temperature is game changing for homeowners, with some apps even being able to detect component faults and automatically notify an engineer.

Water Quality

Water produced from new combi boilers is clean and safe to drink, as it hasn’t been stored in a tank for any amount of time. This reduces the potential of sludge, rust or debris build up contaminating the water supply for a home.

Cheap To Run

Gas is among the most cost effective types of fuel for a heating system, coming in at almost three times cheaper than electricity as an operating fuel.

This indicates once the boiler unit and installation has been paid for, the boiler’s efficiency and low cost fuel will start to pay for itself in money saved much faster than you’d think!


Due to their popularity, combi boilers and their replacement parts are very prevalent. This means even if your combi has a problem, their superior availability ensures that there is always an abundance of replacement components that are cheap and easy to source.


Most modern combis have been designed with operational volume in mind, often excelling in minimising any sound output, with most top of the line boilers being silent when switched on.

Premium boiler brands often receive Quiet Mark awards for their success in acoustical consideration and volume reduction.


Many combi boilers can run on a variety of fuels, gas being the most popular, and can even operate on mains gas or LPG.

Potential Combi Disadvantages

While a combi boiler can be beneficial illustrated in the ways as mentioned above, they may not be practical for all households.

Poor Mains Pressure

Due to their reliance upon the mains water supply, combis as mentioned carry forth the level of water pressure from this connection. If the mains to a home has a particularly low water pressure, this means the boiler will operate at a much less efficient level, with slow flow rates and sub-optimal pressure.

Power Shower

In most cases if your property relies on a combi boiler for its hot water production, you will not be able to install and use a power shower due to them requiring a boiler pump, a component a combi is not compatible with. 

Multiple Bathrooms ( Or Larger Homes)

Certain property types like larger homes or homes with several bathrooms may not be suited to a combi boiler due to their high water demand.

In this instance a modern and efficient system or conventional boiler may be more suited due to its ability to serve numerous water outlets at one time.

Combi Boiler Costs

As with any boiler type, the unit cost and installation rate for a new combi will be manufacturer and merchant dependent.

Average iHeat combi boiler Installation list: (guide only)

Installation Type

Price (inc VAT)

Combi to combi swap


Get a quote

System to combi conversion


Get a quote

New boiler install


Get a quote

Back boiler to a combi


Get a quote

When Should You Replace A Boiler

As a general rule of thumb, any boiler 15 years or older should be replaced. In the time elapsed since the boilers installation the constant advancement in modern technology means upgrading to a newer boiler will work out more efficient, cheaper and safer.

Any boiler with signs of serious degradation, function failure or that has been ‘condemned’ by an expert heating engineer must be immediately disconnected and replaced for safety reasons as well as heating convenience.

Back Boiler To Combi Conversion

An old, inefficient back boiler can actually be converted to either a combi or system boiler, systems can work out cheaper in terms of the conversion due to a lot of the pipework remaining the same. 

However, most people opt for the combi conversion as an upgrading of the entire system could be more efficient in the long run, and may ensure less future faults, reducing repair or maintenance costs.

Secondly, you will need to decide upon the new flue placement of a combi for example, you could keep the same vertical flue position, releasing gases out of the chimney or move it due to the fact combis can operate with a vertical or horizontal flue.

Thanks to the compact design of modern combi boilers, they can be positioned into smaller spaces like a standard kitchen cupboard, or they can be placed in the same spot as the boiler being removed (if you want your boiler moved to a new location there will be additional fees, especially if there is no piping in this area).

The fire will need to be removed, followed by the old boiler, flue, tanks and all relevant piping will be re-routed or installed. Post new boiler installation any area repairs to surrounding walls or where the fire previously was will need carrying out and safety checking.

How Long Does A Combi Installation Take?

Each combi boiler install is a unique case and therefore any time indication is intended as a rough guide only.

For a combi to combi swap we advise to allow for up to 1 full day and for a conversion from a different type of boiler to a combi, we advise to allow for up to 2 full days.

Where Can Combi Boilers Be Installed?

A combi boiler is a versatile appliance that can be installed in several different locations around the property.

Combi boilers are extremely space economic and as mentioned above the most popular place for a combi is in a kitchen or utility cupboard.

Providing the property has suitable insulation amongst other things, a combi can also be stored in a loft or garage.

Other common housing places for a combi boiler include the utility room, bedroom or a basement.


While there are some cons to owning a combi if you occupy a very large property with several bathrooms, in the vast majority of cases a modern combi boiler will be extremely advantageous.

Not only will a combi perfectly heat your home, but it will also start to save you money due to its efficient performance ensuring you will be less frivolous when it comes to heating bills.

Combi boilers are manufactured by the most trusted and celebrated heating brands and are available in a range of kW sizes, meaning you never have to sacrifice on home heating, no matter your budget or heating demands.

Current iHeat Combi Boiler Brands:

If you think a modern, efficient combi heating system is ideal for your home, contact us today for a Gas Safe registered installation as fast as the next day!

Get a quote

Frequently Asked Questions

What is the Downside of a Combi Boiler?

The main downsides of a combi boiler include:

  • Limited hot water flow rate: Combi boilers can struggle to supply hot water to multiple outlets simultaneously, which might not be ideal for homes with several bathrooms.
  • Dependence on mains pressure: Their performance heavily relies on the water mains pressure. If your home has low mains pressure, your hot water flow rate may be insufficient.
  • No backup hot water supply: Without a hot water tank, if the combi boiler breaks down, you lose access to hot water.
  • Not always suitable for larger homes: Due to the points mentioned above, combi boilers might not be the best choice for larger homes with high hot water demands.

Is it Still Worth Getting a Combi Boiler?

Yes, for many households, combi boilers are still worth considering due to their efficiency, compact size, and convenience. They are particularly suited to small to medium-sized homes with relatively low hot water demand. Their ability to provide hot water on demand without the need for a separate water tank can also save on space and energy costs.

What is the Best Combi Boiler on the Market?

As of the last update, some of the best combi boilers on the market include models from Worcester Bosch, Viessmann, and Vaillant. These brands are frequently praised for their efficiency, reliability, and performance. Models like the Worcester Bosch Greenstar i, Viessmann Vitodens 100-W, and Vaillant ecoTEC Plus consistently receive high marks from both users and industry experts.

What is the Number 1 Boiler in the UK?

Worcester Bosch often holds the title for the number 1 boiler brand in the UK, thanks to its combination of reliability, customer service, and performance. It's a popular choice among both homeowners and installers, with a range of models to suit various home sizes and needs.

Which Boiler Brand is the Most Reliable?

Worcester Bosch and Viessmann are often cited as the most reliable boiler brands in the UK. They offer high-quality construction, innovative technology, and excellent warranties, which reflect their commitment to reliability and customer satisfaction.

What is the Most Efficient Combi Boiler in the UK?

Viessmann's Vitodens 200-W is among the most efficient combi boilers available in the UK, with some models offering efficiency levels of up to 98% under ideal conditions. This high efficiency can lead to significant savings on energy bills, making it a compelling choice for energy-conscious homeowners. Worcester Bosch also offers highly efficient models, such as those within the Greenstar series, known for their energy efficiency and performance.

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