Combi boiler vs system boiler – all you need to know

Choosing between a combi boiler and a system boiler requires some research. After all, this is one of the most expensive home purchases you’ll make – and it’s a decision you’ll have to live with for at least a decade. 

In this post, we’ll examine the differences between the two boilers and explore the pros and cons of each to help you make that decision. 

If you’ve already started your research and you simply want a quick answer to your question, use the table of contents on the right to skip to the section you need. 

What is a combi boiler? 

As its name suggests, a combi boiler combines hot water and central heating. Everything required is contained inside a single unit – and because there’s no need for a hot water cylinder, it takes up much less space than a system boiler. 

Combi boilers heat water on demand. When you turn on a tap or your central heating, the boiler kicks into action and heats up the water you need. We’ll cover the pros and cons of the combi boiler later.

Learn more: What is a combi boiler, and how does it work? 

What is a system boiler? 

The main difference between a combi boiler and a system boiler is the hot water cylinder. A system boiler takes cold water from the mains supply, runs it through a heat exchanger, and stores it in a hot water cylinder until it’s required. The boiler connects directly to the radiators, and the hot water is stored separately. 

Having a tank full of hot water increases the capacity of a system boiler, allowing it to provide hot water to multiple taps simultaneously. However, the water supply is finite, and this is one of its downsides, which we’ll come to later. 

Pros and cons of a combi boiler 

It’s the combi-boiler’s space-saving credentials that really set it apart from its rivals. Some combi boilers are small enough to fit inside a kitchen cupboard, so if space is at a premium, this boiler has the advantage. 

Combi boiler advantages

• The combi boiler takes up less space than a system or conventional boiler. 
• It provides hot water on demand, so you’ll never run out! 
• The combi boiler is a single unit, so it’s easier to install than a system boiler, saving you time and money. 
• It has fewer parts, which makes it easier to maintain (and there’s less that can go wrong). 
• As the combi boiler takes water directly from the mains, it typically has a higher water pressure. 
• It’s incredibly efficient (modern combi boilers are up to 92% efficient), saving you money on your gas bill and lowering your C02 emissions. 

Combi boiler disadvantages

• A combi boiler doesn’t have the capacity to cope with the simultaneous demand for hot water from multiple taps. 
• When you turn on the tap, there is a short wait for hot water. It’s only a few seconds, but it’s more noticeable than a system boiler, which is instantaneous. 
• If the water coming from the mains supply isn’t high-pressure, you won’t benefit from a high-pressure water supply from your taps. 

Should I get a combi boiler? 

Combi boilers are better suited to small and medium-sized homes where space is limited and the demand for hot water is low. As a rough guide, a combi boiler is able to cope with a small or average-sized home with up to two bathrooms or a large home with one bathroom.

Pros and cons of a system boiler 

The standout advantage of a system boiler is its capacity. It can provide hot water to multiple taps and showers simultaneously in larger homes. 

System boiler advantages

• A system boiler has the capacity to meet a high demand for hot water in larger properties with several bathrooms.   
• It gives you instant access to hot water that comes directly from the cylinder. 
• System boilers are compatible with solar thermal panels. When combined, the two can reduce your gas bill and lower your C02 emissions. 
• With an immersion heater, you can increase the efficiency of your system boiler. 

System boiler disadvantages

• You need to find room for a hot water cylinder somewhere in your home, whether that’s in the loft or an airing cupboard. 
• System boilers are more complex to fit as they require the installation of the boiler and the cylinder. 
• There is only a finite amount of hot water in the cylinder, so if it unexpectedly runs out, you’ll have to wait while the tank re-fills with hot water! 
• More parts means more that can go wrong and, of course, higher maintenance costs (boiler cover is almost certainly an advantage here). 
• A system boiler requires high water pressure, which makes it incompatible with old pipes and radiators or areas with low water pressure.    

Should I get a system boiler? 

System boilers are better suited to a large home with more than one bathroom where there is a high demand for hot water. It has the capacity to fulfil the demand for several taps and showers running at the same time.

System boiler vs combi boiler – which is best?

There are no right or wrong answers when it comes to deciding between a system boiler and a combi boiler; there’s only what’s right for you. If it fits your needs, then it’s the right choice.

Consider these things before you rush to make a decision: 

The system you’ve got 

It’s easier to replace a boiler like-for-like than to swap one system for a different one. Replacing your old combi boiler with a new one is much more straightforward (and cost-effective) than switching from a combi to a system boiler. 

The size of your house

An average combi boiler is big enough to fulfil the needs of a small/medium-sized house with two bathrooms or a large house with one bathroom. System boilers are better suited to large homes with multiple bathrooms where there is a high demand for hot water. 

Your available space

Combi boilers take up less room than system boilers. If you opt for a system boiler, be prepared to find space for a hot water cylinder. That could mean giving up space in your loft or finding another suitable spot in your home. 

Before you rush to make the decision, speak to a Gas Safe engineer. An experienced installer will consider all of the above, as well as your water pressure and current radiators and pipework. 

System boiler vs regular boiler

A regular boiler (also called a conventional boiler) is a traditional heating system that works in much the same way as a system boiler.

It uses a heat exchanger to heat water, which is then stored in a hot water cylinder (like a system boiler). However, it also has a separate cold water storage tank (sometimes called a feed or header tank) that’s kept in the loft or attic.

A cold water storage tank in the loft provides the pressure needed for heating and hot water systems. With gravity on its side, it keeps the water flowing consistently, an advantage in areas with low water pressure.

Think of a regular boiler as a supercharged system boiler. It’s ideally suited to large homes with several bathrooms or older properties, which often have existing pipework and radiators better suited to regular boiler systems.

Older properties also feature large rooms with higher ceilings, which require a more powerful heating system to maintain a comfortable temperature.

The final verdict 

It’s not a case of pitting the two systems against one another: System boiler vs combi. It’s about finding the right boiler to meet your needs. The size of your property, your available space, and your current system are all factors that need to be considered before you make your decision. 

We’re on hand to help. For expert, impartial advice from experienced Gas Safe engineers, contact our team. If you live in Kent or its surrounding areas, we can send someone to look at your current system.