James Elston

Published April 9, 2024

The type of boiler you need will depend on your home size (including amount of bathrooms) and your central heating needs. It will also be determined by the location of your home, and whether you’re connected to the national grid. 

There are three main boiler types: combi, system and conventional. Boiler types can also be further subdivided into fuel types used to produce heat, like electricity, gas, oil, LPG or biomass.

In this article, we will focus on combi, conventional, and system boilers, as well as electric boilers and heat pumps. Each type has its unique features and benefits, tailored to different home sizes, water usage patterns, and heating requirements.

Combi Boilers

Combi boilers are the most popular boiler type found in UK homes.

Combi boilers, short for ‘combination boilers’, are space-savers compared to conventional boilers. These compact boilers heat water directly from the mains, when needed, and don’t require the installation of a bulky water storage tank. This makes them a good option for smaller homes and apartments.

Since it heats water instantly and only when you need it, you’re looking at a more efficient and cost-effective way to manage your home’s heating and hot water needs.

How combi boilers work

Combi boilers have two outputs – one diverts water to the central heating system and the other to the taps. This output is managed by the diverter valve, which diverts the flow of water. 

Let’s start with how it works to heat your home. 

The boiler’s thermostat sends a signal to the boiler that it needs to heat up water, either because the thermostat was just switched on or because the temperature has dropped. The boiler uses its fuel source (most commonly gas or electricity) to switch on the burner or element that heats the water. 

This hot water is then sent to your radiators. 

When you turn on the tap, the diverter valve redirects the flow of water to the tap, giving you hot water on demand. 


  • Space-saving: Combi boilers are compact and don’t need separate hot water tanks.
  • Neat appearance: They’re not just about function; they look good too. With a neat appearance, they seamlessly blend into your home.
  • Instant heating: Say goodbye to waiting for water to heat up. With a combi boiler, hot water is there when you need it, on demand.
  • Energy efficient: These boilers are kind to your wallet and the planet. They’re 90%+ efficient, meaning less energy wastage and lower bills.
  • Cost-effective installation: Thanks to their simpler design and lack of complex pipework, combi boilers are more wallet-friendly to install. To find out more, read our guide on combi boiler installation costs.


  • Water pressure issues: Combi boilers rely on good mains water pressure. In homes where the pressure is low, they might struggle to perform effectively.
  • Limitation on simultaneous use: If you have a busy household with several bathrooms, a combi boiler might struggle to provide hot water to multiple outlets simultaneously.
  • Complicated electronics: If the boiler malfunctions because of a technological failure, it can be expensive to fix.

Is a combi boiler right for you?

Wondering if a combi boiler is the right fit for your home? There are a few things you’ll need to consider.

House size

Combi boilers are a win for smaller homes or flats where space is limited, so you’ll first have to ask yourself “What size boiler do I need?“. By ditching the cylinders and tanks, you free up precious space. 

If you have a two or three-bedroom home without a loft, and you don’t need multiple hot showers running simultaneously, a combi boiler can be a great choice. We have an entire guide on the best small combi boilers if you’re considering one for your smaller home. 

However, they may not be the best fit for every situation. If your home has low mains water pressure or is a larger household with many occupants and several bathrooms, a combi boiler might not be able to keep up with the hot water demand.


When it comes to combi boiler costs, you need to consider the upfront costs, installation, and maintenance costs.

Combi boilers can range from £500 to £3,000, depending on the brand and model, and the installation can add another £1,500 or more to the bill, depending on whether you’re replacing a combi or changing from a different boiler to a combi. 

We’ve got a guide to some of the cheapest combi boilers if you want to save a few pounds. 


Since combi boilers don’t have any water storage tanks, they’re much simpler to maintain. The best combi boilers on the market should not give you any issues, as long as you do your annual service. 

Water pressure

Combi boilers heat water directly from the mains. If you don’t have strong mains water pressure, a combi boiler might not be the best pick for you. Other types of boilers that use external water storage tanks are better suited to homes with poor mains pressure. 

Number of radiators

Even the best electric combi boilers can only do so much! You need to consider your boiler output and the amount of radiators you have at home. 

A combi boiler with an output of 25 kW will be sufficient for up to 10 radiators. If you have 20+ radiators, you’re going to need the most powerful combi boiler on the market at 40 kW+, or consider a system boiler to meet your heating needs. 

Conventional Boilers

Conventional boilers, also known as heat-only or regular boilers, have long been the classic choice for many British homes. 

These boilers rely on two tanks. One is a cold water storage tank, usually tucked away in the loft, which feeds the boiler. The other is the hot water cylinder, where hot water is stored for use in your central heating system and taps. 

The hot water cylinder is often housed in an airing cupboard that is part of either an open-vented system with a small tank in the loft or an unvented system. The unvented system can save you some loft space as it fills your radiators directly from the mains, without the need for that extra loft tank.

How conventional boilers work

It all starts with the cold water tank in the loft, which is fed by the mains. This water flows to the boiler, often under gravity, where it is heated.

Water is heated by a heat exchanger. This device transfers heat from the boiler’s burner to the water, without them coming into direct contact.

Once the water is heated, it’s stored in a separate hot water cylinder, ready and waiting for when you need it. This water is circulated through the central heating system and is redirected to the taps when needed.


  • Greater hot water use: Perfect for busy households, they can easily handle the demands of several hot water outlets all at once.
  • Compatibility with solar: They work well when paired with solar thermal panels, offering an eco-friendly option for heating your water.
  • Back-up heating: They often include an immersion heater in the hot water cylinder. So, even if the boiler has issues, you can still get hot water.
  • Compatible with old heating system: If you have an older property, a conventional boiler can often be integrated with minimal changes to your existing pipework.


  • Takes up space: These boilers need a fair bit of room. You’ll need loft space for the cold water tank and an airing cupboard for the hot water tank, which might be challenging in smaller homes or flats.
  • Waiting time for hot water: If you use up all the hot water in the tank, you’ll have to wait for it to refill and reheat.
  • Heat loss: The hot water in the cylinder can lose heat over time and make it less efficient. Proper insulation is needed.
  • Complex installation: Setting up a regular boiler system can be quite complex and costly.

Is a conventional boiler right for you?

When deciding if a regular boiler is for you, you’ll need to consider a couple of key factors, including what type of boiler you have at the moment, and if you need to change it.

House size

For one, a conventional boiler can be great if your home has two or more bathrooms. With its ability to store hot water in a cylinder, it can better meet the high water demand of multiple bathrooms.

Bigger homes can also benefit from the capacity and efficiency of a regular boiler. Unlike combi boilers, regular boilers are a great choice in areas with low water pressure as they don’t rely on mains pressure for hot water.


Conventional boilers costs are much higher than combi boilers. These trustworthy boiler systems can cost more than £4,000, excluding installation. And the installation will also set you back a bit, costing on average £1,800 to £3,200. So, if you want to get the best conventional boiler, you’re going to have to pay up!


You should always get your boiler serviced annually, no matter the type. However, since a conventional boiler has a lot of different parts, you should also visually inspect your boiler once a month and check that there are no leaks or a drop in pressure. 

If you don’t have time to check on your boiler frequently (and research some common boiler problems) then a conventional boiler might not be your best choice. 

Water pressure

Because conventional boilers make use of a cold water tank to feed the system, they work well in homes with poor mains water pressure. 

Number of radiators

Conventional boilers work well in larger homes with more radiators. A boiler with a larger output of between 26 kW and 40 kW can heat a home with up to 20 radiators. 

If your home is smaller (in terms of size and the number of radiators), it will make more sense to consider a compact combi boiler. 

System Boilers 

System boilers are the middle ground in the boiler world, combining elements of both conventional and combi boilers.

Just like conventional boilers, system boilers have a separate cylinder for storing hot water. Water for the taps is heated and stored in this hot water cylinder, ready for use. Water for the radiators comes from the mains, is heated, and then circulated. 

This setup makes system boilers a bit more integrated and space-efficient compared to conventional ones.

Plus, they’re compatible with solar water heating systems, adding an eco-friendly touch and the potential for savings on your energy bills.

How system boilers work

A system boiler works by drawing cold water directly from the mains into its unit, and the heat exchanger warms the water to your set temperature. This heated water is then circulated to your home’s radiators through a central heating pump.

Simultaneously, water in a separate hot water tank is also heated and ready for use. When you turn on a tap, you’re provided with immediate warm water.


  • No need for a cold water tank: This is perhaps the most appealing benefit. It makes them a great option if you don’t have a loft or want to free up some space in your home.
  • Easier and quicker installation: Thanks to many central heating and hot water components being built into the boiler itself, they’re generally easier and quicker to install.
  • Fast response: With a built-in pump, system boilers react quickly, meaning you get hot water and heating without delay.
  • Great for multiple uses: Need to run several taps at the same time? No problem! A system boiler stores hot water in a cylinder, allowing for multiple uses simultaneously without a drop in water pressure.


  • Mains water pressure reliance: Just like a combi boiler, a system boiler depends on good mains water pressure to work well.
  • Hot water limits: The amount of hot water you can use is capped by your cylinder’s size. Once it’s empty, you’ll have to wait for it to refill and reheat.
  • Cylinder space requirements: Even without a cold water tank, they still need space for a hot water cylinder, so they’re a bit bulkier than combi boilers.

Is a system boiler right for you?

Here are some things to consider to determine whether a system boiler is right for you.

House size

A system boiler could be the right solution for you if you have a larger home or find yourself needing plenty of hot water. On the other hand, a system boiler may not be the best fit if your home is smaller and you’re looking to keep installation costs down – especially since you’ll need space for the hot water tank. 

Also, if you already have a conventional boiler, switching to a system boiler could mean additional expenses and effort in refitting pipework and radiators.


When it comes to costs for the best system boilers on the market, you can expect to pay upwards of £1,300 (excluding VAT). However, the fuel type will influence the cost, with oil system boilers costing the most – in excess of £3,000. For installation, you can add another £1,000 to that price.


Your boiler should be serviced once a year.

System boilers have more parts than combi boilers, but less than conventional boilers, so you don’t have to check your boiler all the time to look for issues. 

Because of fewer parts, there are also fewer potential points of failure. 

Water pressure

System boilers are the perfect blend between combi and conventional boilers when it comes to water pressure requirements. 

The hot water cylinder means you’ll have hot water in your taps, even if you have a lower mains water pressure. 

However, since the water for the radiators is heated from the mains, you do require a fairly strong pressure for your central heating.

Number of radiators

The output of a system boiler will determine how many rooms and radiators it can serve. The larger the output, the more radiators it can heat up. The hot water cylinder also means it can provide hot water to several taps at the same time, making it good for two to four-bedroom homes. 

The best 40 kW system boiler will easily be able to work in a four-bedroom home with up to 20 radiators. 

Electric Boilers

Electric boilers are a viable alternative for home heating and hot water (read our guide on electric boilers vs gas boilers to see exactly how these boilers differ from standard gas systems).

Electric boilers heat your home’s water using electricity instead of gas. They provide the same type of heating as gas boilers but are ideal for homes that don’t have access to a gas supply or where gas boilers aren’t allowed.

How do electric boilers work?

Electric boilers work by heating the water with a heating element. Water from the mains passes over this element, which is then circulated through your central heating system or provided to your taps. 


  • Environmentally friendly: Electric boilers don’t produce any harmful gases and are also not reliant on fossil fuels. It can be coupled with solar panels to be completely off the grid. 
  • Highly efficient: These boilers convert almost all the energy they use into heat. 
  • Quiet: Electric boilers operate almost completely silently. 
  • Compact: Since there’s no need for additional water storage tanks or fuel lines, this boiler type can fit in compact spaces. 


  • Running costs: Electricity is often more expensive than gas, which can cause higher heating bills when compared to a gas boiler. 
  • Limited home size: These boilers are not suitable for larger homes with a high hot water demand. 
  • Power outages: If there’s no power and no backup battery or solar system, you won’t have hot water. 

Is an electric boiler right for you?

Considering getting an electric boiler for your home? Here’s what you should know. 

House size

Electric boilers are best for smaller homes with three or fewer bedrooms, as they provide hot water on demand and can’t cater to multiple hot water users at once. 


As for the cost, electric boilers are often cheaper and more economical initially. You can get a boiler for as little as £900 while more expensive models can cost closer to £4,500

However, electricity per unit is generally more expensive than gas.


We recommend getting a heating specialist to check out your boiler once a year. 

Water pressure

An electric boiler is less dependent on specific water pressure than gas boilers; however, they still need a minimum water pressure to ensure that enough water flows through the system.

Number of radiators

If you have lots of radiators or rooms, an electric boiler is not for you. That’s because this boiler type generally has a lower output than gas boilers. 

1.5 kW of boiler output is required per boiler. So, if you have 10 radiators, you’ll need a 15 kW boiler.

Heat Pumps

Heat pumps are an innovative and eco-friendly choice for home heating and cooling. 

How do heat pumps work?

These boilers work by moving heat from external sources (either by air, ground, or water) into your home, using a compressor and circulating refrigerant.

The most appealing feature of these boilers is that they can reverse this process to cool down your home in warm weather. They’re versatile for both heating and cooling your home, so you get the best of both worlds.


  • Highly efficient: Heat pumps transfer heat from outside (mainly the air or ground) and use a lot less energy to heat up water than more traditional heating systems. 
  • Heating and cooling: In summer, heat pumps can cool your house, providing year-round temperature control. 
  • Environmentally friendly: No reliance on fossil fuels and no greenhouse gas emissions.
  • Government incentives: There are several incentives, like ECO4, that provide financial assistance to buy a heat pump.


  • Costly: Heat pumps have very high upfront costs. 
  • Struggles in cold weather: Although they can still work in winter, they’re better suited to moderate climates. 
  • Complex installation: The amount of pipework and ductwork required makes this a very complex heating system to install.
  • Uses electricity: Heat pumps require electricity to function and can cause a spike in your electric bills. 

Are heat pumps right for you?

Here’s what you need to consider to determine whether a heat pump is a good fit for your home. 

House size

Heat pumps are typically well suited for homes with up to three bedrooms that are not located in frigid climates. Larger homes will require more substantial heating systems (especially if they’re poorly insulated).


When it comes to purchasing one, you can expect to pay a pretty penny. They typically cost around £8,000 to £45,000, which can be quite steep.


Heat pumps generally have lower maintenance requirements than gas boilers. You just need to regularly check and clean the filter and get a professional to make sure everything is still fine once every year or two. 

Water pressure

Water pressure is not a requirement for heat pumps, so they can work well in homes with weaker pressure. 

Number of radiators

The amount of radiators your heat pump can run will depend on the heat pump’s capacity. For example, if you have radiators that each have a heat output of 1 kW, and your heat pump has an output of 10 kW, you can heat 10 radiators. 

Energy-Efficient Boilers

Energy-efficient boilers, also known as green boilers, are your smart choice for a warmer home without burning a hole in your wallet or harming the environment. These green boilers come packed with features that make your heating system efficient and eco-friendly.

Here are just some of the features that make energy-efficient boilers so appealing:

  1. Condensing technology: Energy-efficient boilers are condensing, meaning they capture and reuse heat that would typically be wasted.
  2. Incredible efficiency: These boilers are required to be at least 92% efficient, which means they’re like the energy-saving superheroes of home heating. For every unit of fuel they consume, a whopping 92% (or more) of it transforms into cosy warmth for your home.
  3. Modern features: Many of these boilers come with sleek modern features like user-friendly touch screens, optional controls that put you in charge, and even nifty wireless smart controls.
  4. Ecodesign rules: These boilers play by strict rules set by Ecodesign regulations. This means they’re not only good for your home but also great for the planet, with reduced emissions and energy use.

When it comes to costs and benefits, while the initial cost might be a bit higher, these boilers can pay you back in the long run through lower energy bills. Plus, you’ll be doing your part for a greener future.


How do I know what kW boiler I need?

When choosing a boiler, the kilowatt (kW) size is an important factor to consider. The size of your property, including the number of radiators, bedrooms, and bathrooms, significantly influences the required kW size. As a rough guideline, you should allocate about 1.5 kW for each radiator and 3 kW for each bathroom in your home.

How much does a new boiler cost?

The cost of a new boiler in the UK typically ranges from £500 to £2,750, depending on the type and complexity of the installation. It’s important to remember that a gas boiler must be installed by a Gas Safe registered heating engineer, adding to the overall cost. Including installation, the price can rise to as much as £6,250.


Choosing the right boiler is like fitting a missing piece into your home’s comfort puzzle. We’ve explored the main boiler types (combi, conventional, and system boilers) as well as electric boilers and heat pumps, each with their unique benefits. You should now be able to decide which type of boiler is the perfect fit for your needs!

Just remember, don’t just purchase the first reasonably-priced and appealing boiler you find. Think about the energy efficiency, cost, and how its features align with your home’s needs.Your choice of boiler impacts your home’s comfort, energy efficiency, and environmental footprint. Make an informed decision and reach out to Eco Happy for a cosier, eco-friendly home!

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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