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Combi Boiler vs System Boiler


As much as the prospect of purchasing and installing a new boiler is exciting, you sure have found the process of selecting the ideal boiler type hectic and tiresome. Either way, to be able to narrow your options down to a combi or a system boiler, you do possess a discerning eye.

Since you are reading this article now, you probably wonder which of the two is better suited to your needs. The good thing is, in this guide, we are going to lay bare the difference between these two boiler types, even to the extent of discussing the pros and cons of each.

Given the massive popularity of the combi boiler in the UK, where most households rely on it for their hot water and central heating needs, you may be tempted to go for it right away. However, there is no need to rush into choosing it as there are some situations where a system boiler would be the ideal choice for your home.

For clarity purposes, it is good to state that the intent of this guide is not to advocate for one type of boiler over the other. Instead, it is to equip you with the necessary information about each to pick up the right choice of boiler for your home.

That said, let’s dive in and take a look at the difference between the two types of boilers.

What is a combi boiler?

A combi or ‘combination’ boiler derives its name from its ability to combine the two tasks of hot water supply and central heating. Combi boilers are highly efficient and compact single units that instantaneously provide hot water and heating for your home.

The system has two independent heat exchangers, one linked to your radiators and the other to the hot water supply line. Once you request hot water, the combi boiler signals the boiler to burn the fuel. As the boiler kicks to life, the heat exchanger becomes hot and thus, heats the water flowing over it.

Combi boilers are fitted with two control valves, each facing different directions, one transporting water to the radiators and one transporting water to the hot water taps. The system can transport water in one circuit at a time, but if requests for hot water and central heating are made simultaneously, the system prioritises hot water supply.

Combi boiler heats the water directly from the mains when the hot water tap or the central heating system is turned on. Drawing water directly from the mains eliminates the need for hot water storage cylinders or cold water tanks in the roof space.

Benefits of a combi boiler

Highly efficient

Given that all new combi boilers in the UK must have a high efficiency rate of at least 92%, it is clear that with a new boiler, you will be able to lower your energy bills and reduce your carbon footprint significantly. Reports claim that you can even save up to £200 in annual energy bills by leveraging the efficiency of a combi boiler.

More compact and space-saving

Since one will only be finding space for a single combined unit, combi boilers are the go-to choice when space is limited. Moreover, a combi boiler eliminates the need to create space for bulky tanks or cylinders around your home.

Instant hot water

Unlike a system boiler that needs time to heat water and refill the hot water cylinder when it is empty, a combi boiler prides itself in instant hot water access as it takes water directly from the mains and heats it.  There is absolutely no waiting time as it heats the water and supplies it to the hot water tap or shower instantaneously.

Quick and simple installation

Since a combi boiler consists of a single compact unit, it is far less complex and cheaper to install compared to the system boiler. Moreover, as it has few parts, it is not easy for it to develop problems, and even if problems do occur, it is easy to find replacement parts as they are readily available.

Safe and clean water

Coldwater from a combi boiler is fed directly from the main’s water supply, so it is healthy to drink. Moreover, the water from a combi boiler is cleaner since the boiler accumulates less sludge compared to the system boiler. A central heating system with a combi boiler has less sludge buildup since the water isn’t stored in any tank that may have rust or debris.

Disadvantages of a combi boiler

Unsuitable for properties with weak mains pressure

The pressure coming out of a combi boiler is relative to the pressure on the mains water supply. If the pressure of the mains is low, then installing a combi boiler will be the wrong choice as it does not have a pump for pumping the hot water. As such, you won’t enjoy power showers as the pressure of the water coming out of the tap or shower is low.

Cannot sustain high demand for heating and hot water

Combi boilers are designed for households with low hot water demand; hence they will struggle to supply water to two taps or showers simultaneously.

The majority of combi boilers can supply up to 16 litres of hot water per minute. However, if you open two hot water outlets at the same time, say, run a washing machine and a hot shower simultaneously, the water supply capacity is halved. Therefore, only a maximum of 8 litres per minute will flow to each outlet, making the combi boiler inefficient for households with large hot water or central heating demands.

Has no immersion heater

The absence of a water tank is both a strength and a weakness in a combi boiler. If and when your boiler breaks down, you will be forced to without hot water and central heating until the repairs are done.

It might not be compatible with old pipeworks.

Since most older heating systems were designed to deal with a weak water supply line, they might not withstand the high mains pressure flowing from the combi boiler.

What is a system boiler?

On the other hand, a system boiler differs from the combi boiler in that a cylinder for storing hot water. Although the hot water is stored in a separate cylinder for this type of boiler, the central heating and hot water systems are built into the boiler – hence the name system boiler.

The system boiler takes cold water from the mains supply, heats it using the heat exchanger, and stores it in the hot water cylinder.  As the system draws water directly from the mains water supply, installing a cold-water tank on the roof is unnecessary.

Since the hot water in a system boiler set-up is stored in a tank, a large hot water reservoir is always ready and available for use whenever you need it. Also, the hot water cylinder ensures a steady supply of hot water even if multiple taps or showers are open simultaneously.

Benefits of a system boiler

Ample and consistent supply of hot water

Since the hot water is stored in the hot water tank, which serves as the reservoir, the system boiler can discharge the stored hot water to cope with the level of demand regardless of the number of outlets demanding water. Therefore, system boilers are ideal for homes with two or more showers, radiators, and hot water taps.

Compatible with solar and geothermal energy

With proper planning and implementation, a system boiler can be fixed to operate on solar or geothermal energy with ease. Installing such a system will be cost-effective as it eliminates recurrent fuel bills. It will also be an environmentally friendly way to heat your home and will reduce your carbon footprint.

Requires less space than a regular boiler

With the need for a cold water tank eliminated since the system water supply draws water directly from the mains water supply, a system boiler will occupy less space than a traditional boiler.

The system boiler also does not have an expansion tank or external expansion vessels, thus making it easy to install and lowers the need for space, unlike a traditional boiler.

Disadvantages of a system boiler

Need more space than a combi boiler

With the massive external hot water cylinder, the system boiler takes more space than the combi boiler, which can even fit into a kitchen cabinet. Therefore, if your home has limited space, you may find it hard to create room for the hot water cylinder.

It may require hot water insulation.

One thing responsible for lowering the efficiency of a system boiler is heat loss from the hot water stored in the hot water cylinder. As such, there may be a need to insulate the hot water cylinder to ensure maximum efficiency. Carrying out insulation is quite expensive and will drive up the overall overhead cost for the system boiler.

Hot water isn’t instantly supplied after it runs out.

Under normal operating conditions, the system boiler should be enough to meet your home’s hot water supply needs. However, during busy days, say when lots of visitors are around, the hot water usage is higher than usual, and the entire volume of the hot water cylinder may be depleted.  If this happens, you have to wait for 30-40 minutes for the boiler to reheat the boiler again.

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