System Boiler vs Combi Boiler: Which One Is Right for You?

Understanding the System Boiler

A system boiler, sometimes referred to as a sealed system boiler, operates by heating water and storing it in a separate hot water cylinder. This design allows it to provide heating and hot water to multiple bathrooms simultaneously without a drop in water pressure. Since a system boiler is connected directly to the mains supply, it doesn’t require a cold water storage tank in the loft—only the separate hot water cylinder, which can be installed in an airing cupboard or utility space. This setup makes system boilers a great solution for medium to large homes with high hot water usage and multiple taps or showers in use at once.



System boilers are ideal for properties with existing pipework that includes a hot water tank, or for households that already have sufficient space to accommodate a water cylinder. One of the key advantages is that the stored hot water allows you to enjoy long showers, run baths, or use several appliances at the same time without experiencing weak mains pressure or running out of hot water too quickly. However, once the supply of stored hot water is used up, you’ll need to wait for the cylinder to refill and reheat before using hot water again. In exchange for this performance, you may face slightly higher maintenance costs, and the requirement for additional space may make them less practical for smaller homes.

The Convenience of a Combi Boiler

Combi boilers, or combination boilers, are a compact, space-saving solution for modern households. Unlike a traditional boiler or system boiler, a combi boiler does not use a hot water cylinder or a cold water tank. Instead, it heats water directly from the mains as soon as you turn on a tap or shower. This design means there’s no stored hot water—hot water is produced instantly through a hot water heat exchanger, offering immediate access without the need to wait.

Combi boilers offer excellent energy efficiency, as they only produce hot water when it’s needed. They also help reduce energy bills by eliminating the heat losses associated with storing water in a tank. This makes a combi boiler cheaper to run for many smaller properties. Because of their compact size, combi boilers can be installed in kitchen cupboards, making them perfect for flats or homes with limited loft space. However, they’re generally not recommended for homes with more than one bathroom or where multiple outlets are in use at the same time. High demand can cause a drop in water pressure or an inconsistent supply of hot water performance.

Key Differences: Combi Boiler vs System Boiler

The biggest distinction between a combi and a system boiler is how they deliver hot water. A system boiler heats water in advance and stores it in a hot water tank, making it better suited to homes with multiple bathrooms and high demand. A combi boiler, by contrast, heats water instantly using a heat exchanger and is better for homes where hot water usage is lower or more predictable.

In terms of installation, a combi boiler is generally easier and quicker to install, particularly if the home has no existing hot water cylinder. A system boiler requires more space and more components, including the cylinder, but it’s often the better choice for households that need hot water from multiple taps or showers simultaneously. When comparing energy efficiency, combi boilers tend to edge ahead slightly, as there’s no energy wasted in keeping stored water hot. However, well-insulated modern system boilers are also highly efficient, especially when used with underfloor heating or high-demand central heating setups.

Is a Combi Boiler Cheaper to Run Than a System Boiler?

Combi boiler running costs tend to be lower for households with modest hot water usage, thanks to the on-demand heating function. Without a hot water cylinder, there’s no standing heat loss, and that contributes to lower energy bills over time. This makes a combi boiler a more economical option for smaller properties or flats with one bathroom.

On the other hand, system boilers may be more cost-effective in large households where the simultaneous use of hot water is common. While there is a slight energy loss from storing hot water, the system’s ability to meet high demand without affecting performance or pressure makes it highly practical. If the boiler is combined with a solar thermal system, this efficiency improves even further, as solar panels can preheat the water in the cylinder, reducing the need for energy from the boiler itself.

Installation Considerations: Space, Pipework and Demand

When choosing between a combi and system boiler, your home’s size and layout play a crucial role. A system boiler requires space for both the boiler unit and the hot water tank, which may not be feasible in properties without a loft or airing cupboard. In contrast, combi boilers are perfect where space is tight and existing pipework can accommodate a direct mains-fed solution.

If your home has more than one bathroom or you frequently use multiple outlets at once, a system boiler offers the performance needed to supply hot water consistently. However, if you live alone or with a small family and only require hot water at one outlet at a time, a combi boiler may offer everything you need with added savings on installation and maintenance.

Solar Compatibility and Renewable Energy Options

Both heating systems—combi and system boilers—can be compatible with renewable technologies, though system boilers are generally more flexible. A system boiler works especially well with solar thermal systems because it stores preheated water in the hot water cylinder, improving overall energy efficiency and reducing your reliance on traditional fuel sources. While some combi boilers are compatible with solar panels, they typically require more specialised components to work efficiently with renewable energy setups.

Choosing a solar-compatible boiler system can enhance your home’s sustainability and may make you eligible for grants or support schemes. Whether you go with a combi or system boiler, integrating renewable energy can help lower your carbon footprint and further reduce your heating bills over the long term.

Combi Boiler or System Boiler: Which Is Best for Your Home?

Ultimately, your choice between a system boiler and a combi boiler will depend on the specific needs of your household. A system boiler is best suited to larger properties with multiple bathrooms, high hot water usage, and enough space to install a hot water cylinder. It provides consistent performance, making it ideal for family homes where showers and taps are often in use at the same time. A combi boiler, however, is a more compact and energy-efficient option for smaller homes, single-bathroom properties, or where space-saving is a priority.

It’s also worth considering future plans—if you expect your hot water demand to increase, or you plan on adding underfloor heating or a new bathroom, a system boiler may be a more future-proof option. On the other hand, if reducing your energy bills, cutting down on maintenance, and simplifying your heating system are your top priorities, a combi boiler offers a compelling solution.

Let Plumbcall Help You Find the Right Boiler

At Plumbcall, our expert heating engineers are here to help you find the right boiler for your Peterborough home. Whether you’re upgrading from a regular boiler to a combi or exploring the benefits of a new system boiler, we’ll assess your home’s heating and hot water needs to recommend the best fit. We offer installation of leading brands, all fitted to the highest standard, and we ensure that your new boiler system is energy efficient, reliable, and tailored to your lifestyle.

From medium-sized homes with multiple bathrooms to compact city flats, we have the experience and expertise to guide you through the decision on combi boiler vs system boiler. Contact us today for personalised advice, free quotes, and professional boiler installation. Let us help you enjoy a warm, comfortable home with a dependable hot water supply all year round.