System Boilers vs Combi Boilers: What’s the Difference?

A boiler is one of the essentials of a UK home. It serves a home in different ways, such as heating it and providing hot water to taps and showers. Most homes these days use a gas boiler, though electric or oil boilers are also used in remote areas where gas is not available.

While the type of boiler based on fuel is almost pre-decided, with a gas boiler being the first and prime choice, it is often difficult for homes to choose between a system and combi boilers. This classification of boilers is based on their purpose, performance, hot-water storage, and hot-water delivery. Each has specific advantages and disadvantages that make it tricky to choose the best option for a particular home. These are designed to serve different categories of homes based on the demand and use of hot water for each type.

Keep reading this blog to learn everything you need to know about the differences between a system boiler and a combi boiler, and, more importantly, which one is suitable for your home.

Understanding The Types of Boilers   

Generally, boilers are categorized by parameters such as fuel type and the method of heating water. Types of boilers on the basis of fuel used to fire them are:

1.       Gas-fired boiler

2.       Coal-fired boiler

3.       Electric-powered boiler

4.       Oil-powered boiler

These days, most homes use a gas boiler, while other types of boilers are found in older homes, especially in remote areas where gas is not available. Similarly, another classification is based on how boilers use energy to heat water. These are:

1.       Condensing Boilers – Condensing boilers use energy more efficiently, up to 98% AFUE (Annual Fuel Utilization Efficiency). Most of the modern boilers fall under this category.

2.       Non-Condensing Boilers – These are conventional boilers that are less energy-efficient. The AFUE of a non-condensing or conventional boiler ranges from 55% to 75%.

While this classification is somewhat more technical than most people may understand, another classification of boilers is more popular among UK homeowners. It is based on the operation of a boiler, i.e., how it heats water, stores it, and delivers it to radiators, taps, and showers. Whatever the type or category, the only purpose of a boiler is to heat the water for onward supply for:

·         Central Heating System.

·         Supply of Hot Water for Taps and Showers.

With that specific purpose in mind, the boilers are categorized in a different way as follows:

The Two Types of Boilers

Although boilers are mainly classified into two types based on their operation, they are actually of three types depending on how they heat and supply water to radiators, taps, and showers. These are:

1.       Conventional Or Regular Boilers. Also known as “heat-only” boilers.

2.       System Boilers

3.       Combination Boilers, commonly known as “Combi Boilers.”

A conventional or regular boiler is very similar to a system boiler in terms of basic technology, whereas a combi or combination boiler differs in many ways. In a broader sense, we can classify boilers into two main categories: system and Combi boilers.

System Boilers

Systems and conventional boilers are similar. These boilers store the hot water in a separate tank/cylinder, commonly known as a “hot water cylinder”. The difference is that a conventional boiler also requires an additional tank to store cold water, whereas a system boiler does not. Another difference is that some components, such as pumps and valves, are provided separately in a conventional boiler but are built into a system boiler. In reality, a system boiler is an upgraded version of conventional boilers.

A system boiler heats the water, which is then pumped to the storage cylinder. Heated water is then sent to radiators for the central heating system, and simultaneously to taps and showers. In this way, system boilers can supply hot water to taps, showers, and radiators consistently and instantly, regardless of demand. So, a system boiler must operate consistently to keep the storage cylinder full of hot water at all times.

The process of heating water in a system boiler can be summarized as follows:

·         A system boiler heats the water.

·         Transports heated water to the hot water storage cylinder.

·         Heated water remains stored in the cylinder until it is required for radiators, taps, or showers.

·         When the central heating is turned on, or when someone opens a tap or shower, hot water is supplied directly to them from the cylinder rather than from the boiler.

Combination Boilers (Combi Boilers)

A combi boiler is a compact, all-in-one unit. Unlike the system or regular boilers, a combination boiler does not require a cold or hot water storage tank or cylinder. It draws cold water for heating directly from the main supply line instead of a cold-water tank. Similarly, instead of storing heated water in a separate cylinder, a combi boiler heats water on demand. When the central heating system requires hot water, or when someone turns on a tap or shower, the combi boiler automatically turns on and starts heating the water. When there is no demand for heated water, it automatically stops heating it. So, a combi boiler only burns the fuel when there is a demand for heated water, either for central heating or for taps and showers.

Since it heats the water on demand, it may take a little time to produce hot water, unlike a regular or system boiler, which provides hot water instantly whenever needed.

The process and functioning of a combi boiler can be summarized as follows:

·         A combi boiler gets cold water directly from the main supply line.

·         It starts heating the cold water when hot water is needed.

·         Heated water is supplied directly to the point of use rather than being stored in a separate cylinder.

·         When the demand for heated water ends, the combi boiler stops heating the cold water.

System Boilers vs Combi Boilers - What’s The Difference?

·         Hot Water Delivery – A combi boiler delivers hot water on demand, whereas a system boiler stores it in a hot water cylinder.

·         Availability of Hot Water – Hot water is supplied instantly by a combi boiler. In a system boiler, hot water remains available for use, irrespective of demand.

·         Limitation of Hot Water Availability – Hot water availability is limited with a combi boiler, but a system boiler can supply it simultaneously to multiple users.

·         Space Required – A combi boiler is a relatively compact unit that can be easily mounted on a wall, so it requires minimal space. On the other hand, a system boiler requires additional space to install a hot water cylinder.

·         Cost of Installation – The cost of installation is significantly lower for combi boilers than for system boilers.

·         Suitable For – A combi boiler is best suited for small homes with only one bathroom. System boilers are suitable for larger homes with more than one bathroom.

·         Main Supply Pressure Relevance – Combi boilers require an appropriate mains water pressure, whereas system boilers are less dependent on it because they do not have to deliver heated water directly to taps and showers.

·         Energy Efficiency – Both are highly energy efficient, but there is always a risk of heat loss in system boilers.

FAQs

1.      Which one is better, a combi or a system boiler?

It depends on the size of the home and the number of users. A combi boiler is better for small homes with one bathroom. A system boiler is best for larger homes with multiple users.

2.      Can we use two showers at a time with a combi boiler?

Not recommended. A combi boiler is most likely to struggle when more than one person uses hot water at the same time.

3.      Is the energy consumption the same in combi and system boilers?

Both a combi and a system boiler are highly energy efficient. However, when multiple users need hot water simultaneously, the system boiler will have to work harder. Likewise, heat loss is also possible in system boilers. Due to these factors, a system boiler is more likely to consume more energy.

4.      Is the installation cost higher for a system boiler?

The cost of installation is typically higher for a system boiler than for a combi boiler because a system boiler also requires a hot water storage cylinder. It also required more labor, time, and materials to install a system boiler, making it more expensive to install.

5.      What if I want to replace my system boiler with a combi boiler?

It largely depends on the number of bathrooms and the size of your home. You need to consider your home's hot water usage before switching from a system to a combi boiler. Do take professional advice from a qualified boiler engineer while making this decision.

Conclusion

Choosing the best boiler type is critically important. While the combination boilers (popularly known as combi boilers) are very common these days, they may not be suitable for every home. Both the combi and system boilers have their own merits and demerits. If a combi boiler has lower installation costs and requires less space, it cannot supply hot water to multiple users simultaneously. On the other hand, a system boiler can provide hot water to multiple users simultaneously; it requires additional space and costs for installing a separate hot water storage cylinder. Hence, making an appropriate decision about the type of boiler required requires in-depth analysis and consideration of multiple factors.