What is a System Boiler? A detailed guide

A system boiler is a great choice for large homes with a greater demand for hot water. There are many advantages of installing a system boiler, but they are not suitable for all households. Read on to find out all you need to know about system boilers and if they are the right type of boiler for your property and needs.

Written by Kojo Prempeh

Last updated - 11/11/2022

Estimated reading time - 7 minutes

What is a system boiler?

A system boiler heats water directly from the mains water supply to be used for your home’s hot water and central heating.

When hot water is needed to warm up your radiators, your boiler will heat water directly from the mains water supply and circulate it through your central heating system. When you turn on your hot water taps or shower, hot water is heated in a hot water cylinder before being sent to your hot water outlets.

What’s the difference between a system boiler and a regular boiler?

A system boiler works in a similar way to a regular boiler, but there are many differences that set them apart. Just like a regular boiler, also known as a heat-only or conventional boiler, a system boiler heats and stores water in a hot water cylinder that is often located in an airing cupboard.

A key difference between the two is that a system boiler is fed water directly from the mains, whereas a regular boiler is supplied with water from a cold water storage tank in the loft.

Another difference is that many of the external parts needed for a regular boiler to operate, such as a pressure relief valve, circulation pump, and expansion vessel, are built into a system boiler. This makes installing a system boiler quicker and easier than installing a regular boiler, and it also frees up valuable storage space.

What’s the difference between a system boiler and a combi boiler?

A combi boiler provides both heating and hot water within a single compact unit. Combi boilers take cold water directly from the mains and heat it on demand. It is then sent to either your central heating or hot water outlets. Just like a combi boiler, a system boiler also gets its water supply directly from the water mains. Water is then heated and sent around your central heating, but the water used for your hot water outlets gets sent to the hot water cylinder where it gets heated and stored for later use.

What are the advantages of a system boiler?

Large supply of hot water

hot water cylinder
Hot water cylinder

In households with a system boiler, the hot water used for taps and showers is taken from the hot water cylinder. Some cylinders can hold as much as 300 litres of hot water for later use. This means that a system boiler can provide hot water to several outlets at the same time without experiencing a drop in pressure or temperature. This makes a system boiler a great choice for large households with high hot water demand.

You can still get hot water if your boiler breaks down

A hot water cylinder can be fitted with an immersion heater. Immersion heaters use a heating element (much like the coil inside your kettle, but much larger) to heat the water inside a hot water cylinder.

Immersion heaters are powered by electricity, so they can be switched on and off at your convenience without the need for your boiler. This is hugely beneficial in the event of a boiler breakdown. Households with an immersion heater connected to their hot water cylinder can still get hot water without the use of their boiler, unlike households with a combi boiler, which will have to wait until their boiler has been repaired to be able to enjoy hot water again.

Good water pressure


A system boiler is capable of delivering hot water to multiple taps and showers at the same time without any of them experiencing a loss in pressure.

In a conventional boiler system, water tanks located high in a property gravity feed water to the radiators and hot water cylinder. Without additional pumps, the pressure at which water comes out of taps and showers in properties with a regular boiler relies solely on gravity, which is much lower than mains water pressure.

Combi boilers receive, heat, and deliver water directly from the mains, which means that the water pressure at taps and showers will depend on the water pressure from the mains. If multiple taps or showers are in use, water pressure to each of them will drop significantly.

Although system boilers receive water directly from the mains, the water used for hot water outlets is sent to a pressurised hot water cylinder. When a hot water outlet is opened, the pressure of the water from the mains entering the hot water cylinder forces the hot water out of the cylinder and towards the open outlet.

Having a pressurised cylinder means that hot water can be delivered to multiple hot water outlets at the same time without losing any pressure.

Takes up less space than a regular boiler

As there is no need for a feed and expansion tank or a cold water storage tank, a system boiler will take up less space in a home than a regular boiler. This makes system boilers a popular choice for large properties with high hot water demand and with limited storage space.

What are the drawbacks of a system boiler?

Takes up more space than a combi boiler

Although a system boiler takes up less space than a regular boiler, it does take up more space than a combi boiler would. This is because a system boiler needs a hot water cylinder to operate. Domestic hot water cylinders can contain up to 300 litres of water and can be as tall as 2 metres, so owners of smaller properties should factor this in when considering their boiler choices.

You may have to wait for hot water

Hot water cylinders heat and store large volumes of water. If there is no demand for the stored water, the system will stop heating water. This means that after some time, the water in the cylinder will go cold. When this happens, depending on the size of the cylinder and the power of your boiler, you may have to wait up to an hour for the stored water to be heated again.

To get around this, most households with a system boiler set a timer at certain times of the day to instruct the boiler to start boiling water, for example, an hour before they wake up so they can have hot water for their morning shower or in the evening in time for a bath at night. This is great if you tend to use large amounts of hot water during set times. If you need hot water at a time when the boiler has been off for a while, you may have to wait a while before getting it.

Once the water has been heated, the amount of hot water you can use depends on the size of your hot water cylinder. Once the cylinder is empty, you will have to wait for more water to be heated.

How much does a system boiler cost?

The cost of buying and installing a system boiler depends on the manufacturer, model, size, and complexity of the installation, but you can buy a new system boiler for between £500 and £2500.

If you don’t currently have a hot water cylinder, or you do but it needs upgrading to suit a modern system boiler, you will have to install a new one when buying a system boiler. Hot water cylinders can be purchased for £300 to £1500, depending on their size and specs.

The cost of installing a system boiler varies depending on a number of factors, but mainly depends on the type of boiler that it will be replacing. A Gas Safe registered engineer will be able to assess your property and give you an estimate of how much it will cost to replace your current boiler with a new combi boiler.

Installing a system boiler could save you hundreds of pounds a year on your heating bill, depending on the heating system it is replacing. According to the Energy Saving Trust, replacing an old 70% efficient boiler with a 90%+ efficient boiler could reduce the fuel bills in a gas-heated mid-terraced house by up to £400 a year.

A new system boiler can last up to 15 years, and with proper maintenance and an annual boiler service, the lifespan of the boiler could be extended further, so the possible long-term savings could outweigh the initial purchase price of a new system boiler.

Call our free helpline.

For any questions that you have regarding your boiler, give our free helpline a call, Monday - Friday, 8am - 5pm. Let us do the hard work!

OR 020 8146 3960

Is a system boiler right for me?

System boilers are an ideal choice for larger homes with multiple bathrooms and a high demand for hot water. Water is heated and stored in a hot water cylinder, meaning multiple taps and showers can receive water at the same time without a reduction in temperature or pressure.

If you live in a small household or by yourself, you probably won’t use more than one hot water outlet at the same time that often, so a combi boiler will probably be adequate.

If you live in an area with low mains pressure, a regular boiler might be a better option, as they deliver hot water using gravity, so low mains pressure will have no bearing on water pressure.

Get a new boiler.

Has your boiler seen better days? With Smart Plan you can get a new boiler for as little as £30 a month, with a 10 year service plan included.

OR 020 8146 3960