What Type Of Boiler Do I Need

Last Updated on February 11, 2025

Choosing the right type of boiler for your home depends on several key factors, including your home size, hot water demand, and number of radiators, which affect your central heating requirements. Your fuel type—whether you’re on mains gas or need an off-grid heating solution also plays a crucial role.

There are three main types of boilers: combi, system, and conventional (regular) boilers. These are further categorised based on fuel type, such as gas, oil, LPG, and electric boilers.

Understanding the efficiency ratings, installation costs, and compatibility with existing pipework is crucial when selecting the best heating system for your home.

In this guide, we’ll explore each boiler type, its pros and cons, and how to determine the most cost-effective and energy-efficient option for your needs.

Combi Boilers

Combi boilers (combination boilers) are the most common boiler type in UK households, offering an energy-efficient and space-saving heating solution. Unlike traditional systems, a combi boiler draws water directly from the mains supply and heats it on demand, eliminating the need for a separate hot water cylinder or cold water storage tank.

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. 

1️⃣ 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. 

2️⃣ This hot water is then sent to your radiators. 

3️⃣ When you turn on the tap, the diverter valve redirects the flow of water to the tap, giving you hot water on demand.

Benefits of a combi boiler

✅ Instant Hot Water & Central Heating – Since combi boilers heat water only when required, they ensure high energy efficiency, reducing heat loss compared to conventional boilers with stored hot water.

✅ Lower Energy Bills – By operating on-demand, combi boilers reduce gas consumption, making them a cost-effective heating solution.

✅ Compact & Space-Saving – Ideal for small homes, flats, and apartments, as there’s no need for a loft water tank or airing cupboard storage.

✅ High Water Pressure – Mains-fed water supply ensures stronger water flow compared to gravity-fed systems.

✅ 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.

However, combi boilers may not be suitable for larger homes with multiple bathrooms, as simultaneous demand can reduce water pressure. If your household has high hot water usage, a system boiler or conventional boiler might be a better fit.

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.

Costs

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. 

Maintenance

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 schedule an 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 boilers or regular boilers, have long been a trusted heating solution for UK homes, particularly larger properties with high hot water demand.

How conventional boilers work

These boilers work by using two separate water tanks:

• A cold water storage tank (typically located in the loft) that feeds the boiler.
• A hot water cylinder that stores heated water for radiators and hot taps.

Depending on the setup, conventional boilers can operate with either:
1️⃣ Open-vented systems – Uses a small feed and expansion tank in the loft.
2️⃣ Unvented systems – Connects directly to the mains water supply, offering better water pressure without requiring an additional loft tank.

The cold water storage tank in the loft receives water from the mains supply. This water flows under gravity into the boiler, where a heat exchanger transfers heat from the burner to the water without direct contact.

Once heated, the water moves to the hot water cylinder, ready for use in radiators and taps. This stored hot water is available on demand, making conventional boilers ideal for households with multiple bathrooms.

Benefits of a conventional boiler

✅ High Hot Water Capacity – Designed for homes with multiple bathrooms, conventional boilers can supply several taps simultaneously without reducing water pressure.

✅ Compatible with Solar Thermal Systems – Can be integrated with solar thermal panels, helping to reduce energy costs and lower carbon emissions.

✅ Back-up Heating Option – Many systems include an immersion heater in the hot water cylinder, allowing you to heat water independently if the boiler fails.

✅ Ideal for Older Homes – Works well with traditional radiator systems and existing pipework, reducing the need for costly upgrades.

Disadvantages of a conventional boiler

❌ Requires More Space – A loft tank and a hot water cylinder are essential, making this system unsuitable for smaller homes and flats.

❌ Waiting Time for Hot Water – Once the stored hot water is depleted, the cylinder needs time to refill and reheat, unlike a combi boiler which heats water instantly.

❌ Heat Loss from Stored Water – Even with good insulation, some heat escapes from the hot water cylinder, making this system less energy-efficient than combi or system boilers.

❌ Complex Installation & Higher Costs – Setting up a regular boiler system can be more labour-intensive, requiring additional pipework, tanks, and venting systems, increasing installation costs.

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.

Costs

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!

Maintenance

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 strike a balance between conventional and combi boilers, offering a reliable heating solution for homes with multiple bathrooms.

Like conventional boilers, they use a hot water cylinder to store heated water. However, unlike conventional systems, system boilers draw water directly from the mains, eliminating the need for a cold water storage tank in the loft.

This compact design makes system boilers more space-efficient while still providing high hot water output, making them an excellent choice for larger households. Additionally, they are compatible with solar thermal systems, helping to reduce energy bills and lower carbon footprint.

How system boilers work

A system boiler functions by drawing cold water from the mains and heating it using a heat exchanger within the unit.

1️⃣ The heated water is then circulated to the radiators, providing central heating.
2️⃣ Simultaneously, the hot water cylinder stores heated water, ensuring instant hot water whenever a tap is turned on.
3️⃣ A built-in pump ensures efficient water circulation, allowing for faster heating response times compared to traditional systems.

Benefits of a system boiler

✅ No Cold Water Tank Required – Unlike conventional boilers, system boilers don’t require a loft tank, freeing up valuable attic space and reducing installation complexity.

✅ Faster & Easier Installation – With built-in expansion vessels and pumps, system boilers are quicker to install and require fewer external components compared to conventional systems.

✅ Consistent Water Pressure – Since system boilers draw water from the mains, they provide stronger water pressure than gravity-fed systems, ensuring a steady flow of hot water to multiple taps.

✅ Supports Multiple Hot Water Outlets – The hot water cylinder ensures that multiple taps and showers can be used simultaneously without any drop in temperature or pressure, making them ideal for larger households.

✅ Eco-Friendly Option – Compatible with solar thermal panels, system boilers can reduce energy consumption and cut heating costs.

Disadvantages of a system boiler

❌ Dependent on Mains Water Pressure – Like combi boilers, system boilers rely on mains water pressure to function effectively. If your home has low water pressure, you may need a booster pump.

❌ Limited Hot Water Supply – The amount of hot water available is determined by the cylinder size. Once the stored hot water runs out, you’ll need to wait for it to refill and reheat.

❌ Space Required for Hot Water Cylinder – Although system boilers don’t need a loft tank, they still require space for a hot water cylinder, making them 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.

Costs

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.

Maintenance

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 provide efficient home heating and hot water using electricity instead of natural gas. They function similarly to gas boilers but are ideal for homes without access to gas mains or where gas appliances are restricted.

These boilers are a clean energy alternative, producing zero emissions at the point of use. If paired with solar panels or a renewable energy source, electric boilers can provide sustainable, off-grid heating with significantly lower carbon footprint.

How do electric boilers work?

An electric boiler heats water using a resistance heating element instead of a gas burner.

1️⃣ Cold water from the mains supply flows into the boiler unit.
2️⃣ The heating element transfers heat to the water, raising its temperature.
3️⃣ The hot water is then circulated through radiators or delivered to taps and showers for on-demand use.

Unlike gas boilers, electric boilers don’t require a flue or gas pipe, making them simpler to install and more compact. 

Benefits of an electric boiler

✅ Eco-Friendly Heating – Electric boilers produce zero carbon emissions at the point of use, making them greener than gas boilers. They can be paired with solar panels for an off-grid energy solution.

✅ High Efficiency – These boilers convert 99-100% of electricity into heat, unlike gas boilers, which lose energy through flue gases.

✅ Quiet Operation – Since there’s no combustion process, electric boilers run almost silently, making them ideal for flats, apartments, and noise-sensitive areas.

✅ Compact & Space-Saving – Without the need for a gas supply, flue, or water storage tank, electric boilers fit easily into small homes, apartments, or utility rooms.

✅ Lower Installation Costs – With fewer components and no gas connection needed, installation is often cheaper and faster than gas boiler setups. 

Disadvantages of an electric boiler

❌ Higher Running Costs – Electricity is more expensive per unit than gas, leading to higher heating bills if usage is high.

❌ Not Ideal for Large Homes – Electric boilers typically have a lower output than gas boilers, making them unsuitable for homes with multiple bathrooms or high hot water demand.

❌ Vulnerable to Power Cuts – Unlike gas boilers, electric boilers completely shut down during power outages unless backed by a battery storage system or solar energy solution.

👉 Want to compare options? Read our guide on electric boilers vs gas boilers to see which system best suits your needs.

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. 

Costs

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.

Maintenance

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.

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.

FAQs

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.

Conclusion

Choosing the right boiler is like fitting a missing piece into your home’s comfort puzzle.

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.