System Boiler Vs Combi Boiler: Which Is Best For Your Home

Last Updated on February 11, 2025

Choosing the right boiler impacts your home’s comfort, energy efficiency, and running costs. Boilers provide hot water and central heating, making them essential for everyday living. But should you go for a system boiler or a combi boiler?

This guide will break down the key differences between the two, helping you make an informed decision. We’ll compare efficiency, cost, installation, and suitability for different home sizes. Whether you’re a homeowner or landlord, this guide will help you find the best boiler for your needs.

Now, let’s dive into the comparison.

System Boiler Vs Combi Boiler: Which Is Better?

When it comes to choosing between a system boiler and a combi boiler, the primary difference lies in how each unit provides hot water to your home.

System boilers: Ideal for Larger Homes with High Hot Water Demand

A system boiler stores hot water in a separate storage cylinder, meaning it has a readily available supply of hot water for multiple taps and showers simultaneously. This makes system boilers particularly suited to larger households or homes with multiple bathrooms and high hot water demands.

Unlike combi boilers, which heat water on-demand, system boilers provide hot water instantly once it’s been heated and stored. However, once the water in the storage cylinder runs out, there will be a waiting period while the boiler refills and reheats the water.

Another key benefit of system boilers is their ability to work well in homes with low water pressure. The hot water storage cylinder can provide an additional boost of water pressure, making it an excellent choice for properties that experience weak mains pressure.

Pros and Cons Of System boilers

Pros

• Reliable Hot Water Supply: A system boiler provides hot water to multiple faucets simultaneously, making it ideal for larger households. Once the storage tank is heated, you can access hot water instantly whenever needed.
• Improved Water Pressure: System boilers boost low mains pressure by using a storage cylinder, providing consistent water pressure throughout your home.
• Energy Efficient: Fully compatible with solar panels, system boilers can help reduce your energy bills by integrating with renewable energy systems.

Cons

• Space Requirements: System boilers require extra space for the hot water cylinder and associated pipework, which may not suit smaller homes.
• Higher Maintenance: With more components, system boilers require more frequent maintenance due to additional potential points of failure.
• Hot Water Delays: If the storage cylinder is emptied, you’ll need to wait for it to refill and heat up before more hot water is available.

Additional factors to consider

• Boiler Insulation: For optimal performance, ensure the storage cylinder is well insulated to maintain heat and reduce energy waste.
• Boiler Location: A burst or leak could damage your property, so it’s crucial to position your system boiler and storage tank in a safe location to minimise risk.

Combi boilers: A Compact, On-Demand Solution

Combi boilers are a popular choice for UK homes due to their efficiency and space-saving design. Unlike system boilers, combi boilers do not require a separate hot water storage tank.

Instead, they heat water directly from the mains supply as it is needed, making them a great option for those looking for instant hot water without the need for additional storage.

However, combi boilers can struggle to provide hot water to multiple taps simultaneously, especially if several bathrooms or appliances are being used at once.

They are best suited for smaller homes with fewer occupants or properties with moderate hot water demand. Additionally, combi boilers rely on the pressure of your mains water supply, meaning if you have low water pressure, they may not be the best option for your home.

Pros

• Combi boilers require much less space than other boiler types.
• There is much less maintenance required as there are no additional storage tanks.
• You have instant hot water as they heat water directly from your mains water line.
• The warranty that comes with your combi boiler will cover your entire water system, saving you potential additional expenses should something go wrong with your boiler.
• Easy to install.
• Lower running costs save you a lot of money on your energy bill.

Cons

• There is a few seconds delay in providing water after you open the tap as the unit needs to heat the water first.
• The unit will struggle to supply sufficient hot water to multiple taps at the same time.
• Not suitable for homes with low mains water pressure.

System Boiler Costs Vs Combi Boiler Costs

The best value boiler comes down to your home heating needs. Combi boilers offer overall great value when it comes to running costs and installation costs. However, if you have a larger household with a high hot water demand, you may find that the initial savings on a combi boiler aren’t worth it. 

Constantly having to heat water directly from the mains could mean higher heating bills for your household over time. This is where the system boiler outshines the combination boiler.

Let’s compare how system boilers and combi boilers measure up against each other in terms of their value. We will look at the initial installation costs first and then compare the two boiler types’ running costs to see their impact on energy bills and how they can help you save money.

System boilers

System boilers can meet the heating needs of larger homes. This inevitably means there will be more initial expenses tied to getting the boiler installed in your home. However, the boiler shows its value in its long-term usage. Check out our guide on the best system boilers to find a model best suited to your budget and needs.

Installation costs

A system boiler exists in two parts: the boiler itself and a hot water storage tank. The hot water storage unit will add to the fees and the time required to complete the installation of a new or replacement boiler.

System boiler installation costs can range between £1,500 and £4,500.

Running costs

System boilers are a cost-effective solution for a household with high hot water consumption. However, since hot water is stored at length, it will eventually start cooling down. This means that unused water will need to be reheated. If the extra stored water is not regularly used, it will add to your overall energy bill unnecessarily.

As there are more points of failure present on a system boiler, there are potentially higher maintenance costs for the device in the long run as well.

Combi boilers

Combi boilers bring a lot to the table concerning value. These compact units shine in single-bathroom households and they’re simple to maintain and cheap to run. However, if you push these units to their maximum with excessive water demand, the value they bring can quickly plummet.

Let’s take a look at what costs are attached to using a combi boiler in your home.

Installation costs

Combi boilers are a single-unit system that is quick and relatively simple to install. The average cost of a new unit as well as the installation can run between £1,500 and £3,000.

Running costs

Since combi boilers only heat water when needed, they’re much cheaper to run than system boilers. Depending on the make and model, you can save up to £200 annually on your energy bills as compared to a system boiler.

Additionally, if the boiler breaks down, there is no additional hot water cylinder that could cost you extra for repairs.

Which Boiler Is Best For Your Needs?

The size of your home and the number of bathrooms both play an essential role in determining which unit you need.

A system boiler is a must-have for larger homes with more than two bathrooms. The hot water storage tank adds much-needed capacity for daily usage. It also adds additional water pressure to the plumbing of larger homes.

Combi boilers, on the other hand, are the ideal heating solutions for small to average-sized homes. A combi boiler saves space, is low-cost, has high energy efficiency and can still potentially service multiple bathrooms. Take a look at our guide on the best combi boilers to find the ideal model that fits your budget and meets your needs.

Modern combi boilers come in higher-capacity models with up to 54 kWh of output, which can deliver more than 20 litres of flow per minute. Unfortunately, the running costs will increase as well.

With this in mind, let’s talk about what type of boiler would best suit your particular property.

Pick the right boiler for the size of your home

As noted at the start of this guide, you can use the current boiler setup of your home as a gauge for which unit will be the best for you. Below you will find a concise list of what boiler to use in what size home to best meet the water and heating needs.

System boiler

• Average-sized home with more than 3 bathrooms
• Large-sized homes with 2 or more bathrooms

Combi boiler

• Small homes with up to 2 bathrooms
• Average-sized homes with up to 2 bathrooms
• Large-sized homes with 1 bathroom

Should You Choose A System Or Combi Boiler For Underfloor Heating?

Both system and combi boilers work great for underfloor heating. That said, system boilers have a slight advantage as they are more powerful than combi boilers.

Depending on the boiler type and the setup in your home, additional adjustments may be required for them to work optimally.

For example, most combi boilers would require the installation of a two-port valve. This will allow the boiler to supply underfloor and central heating. The valve controls the flow of water from the boiler to the other areas of your home that need access to hot water.

FAQs

Which boiler is more eco-friendly?

Modern boilers are required by UK law to run at a minimum of 92% energy efficiency. At their baseline, both boiler types are much more eco-friendly than old conventional boilers. System boilers are fully compatible with solar energy solutions, while combi boilers only heat water on demand, meaning they can use less energy overall.

Which boiler is easier to install?

A variety of factors can influence installation costs. Generally speaking, a combi boiler is easier to install since it only consists of the boiler unit itself. On the other hand, system boilers require an additional storage cylinder which will add time and costs to an install.

 Conclusion

In general, we find that combi boilers cover a much wider range of applications in UK homes. The country is growing in size, with smaller households being more common today than in the past thanks to densely populated urban areas. 

This trend is only likely to grow, making combi boilers the perfect choice for most homes due to their cheap and efficient delivery of hot water and heating.

When we look at bigger households with multiple family members or tenants, a combi boiler simply cannot compete against a system boiler and its much larger capacity.

To pick the perfect boiler for you and your family, always start by checking your current boiler installation. Then, measure if your current boiler is meeting your needs concerning its capacity and cost.

From here, you can consult a heating engineer for their expert opinion. Find the absolute best boiler deals by contacting Eco Happy today.