How do Combi Boilers work

Boilers come in various forms and sizes, but there’s actually one type of boiler that’s found in most homes in the UK: combi (or combination) boilers. While conventional boilers used to be the norm decades ago, combi boilers have taken over because they’re the most space-efficient option and can address all the needs of a regular household.

Read on to learn about what a combi boiler is, how it works, and why it’s such a popular choice today.

What is a Combi Boiler?

Combi is short for “combination,” which is exactly what a combi boiler does by producing either hot tap water or central heating from the same compact unit. It can do this very quickly as well. When you turn on the tap, hot water flows out right away. The tap water comes directly from the mains, with the combi boiler heating it as it passes through the pipework.

Because the combi boiler doesn’t rely on heating water beforehand and storing it, it saves plenty of space. Unlike conventional and system boilers with more complex setups, a combi boiler is small enough to fit inside a kitchen cupboard. Installation is also easier. One limitation, though, is you usually can’t have hot water and central heating running in full capacity at the same time.

Combi boilers are usually fuelled by gas, but oil and LPG are also possible. Gas is the default fuel because most homes are connected to gas mains, which deliver gas automatically to the boiler. Homes that don’t have access to the grid can use oil or LPG instead. Both of these are stored in a tank outside the home and will need to be refilled.

How Do Combi Boilers Work?

A combi boiler has all of its parts kept inside one casing, and it’s connected to the pipework that goes out to the shower taps and to the radiators for central heating as well as to the mains supply.

For the combi boiler to be activated, it has to draw from the fuel supply first, whether gas, oil, or LPG. The fuel goes inside the boiler, entering the burners and kicking the system to life. It remains in constant standby until it receives a signal from the valves in either the central heating or hot water system, which happens when you activate the thermostat or turn on the tap for hot water. The combi boiler will then start heating water from the mains.

To achieve this, the combi boiler relies on heat exchangers. A heat exchanger works by transferring heat from one substance to another without them making direct contact. In a combi boiler, the heat exchanger typically contains a pipe that’s filled with gas. When water rushes into the heat exchanger, it touches the outside of the pipe and absorbs the heat from the gas.

The combi boiler has two heat exchangers, with the primary heat exchanger handling water for central heating while the secondary heat exchanger deals with hot water through the tap. The pipework and water for these are kept completely separate. One reason for this is the water for central heating picks up several cleaning agents as it passes through radiators, so it wouldn’t be suitable for bathing.

Central Heating

In terms of central heating, the combi boiler works closely with the thermostat to regulate temperature. When you turn on the thermostat or it activates because of preset temperature settings, the water from the mains is heated up by the boiler. This flows out of the boiler and into the radiators to warm up your home. Eventually, the water circles back to your boiler for another round of heating. Once the temperature in the room already matches the desired settings on your thermostat, the boiler modulates itself so the temperature will stabilise.

Hot Water

On the other hand, when you turn on the tap for hot water in your shower, the combi boiler uses the secondary heat exchanger and directs the water through different pipes. Some switching is involved if central heating is already running. Before the combi boiler can heat tap water, it has to suspend central heating first, with a diverter valve opening to transfer the water from central heating to the secondary heat exchanger. Because of this, you might hear some clicks from your boiler to signal the switching. The process happens very quickly so there’s no lag with the hot water. Once you turn the tap off, the combi boiler switches back to focusing on central heating.

Should I Use a Combi Boiler?

The main advantage of a combi boiler is efficiency. Aside from saving space, it’s faster and cheaper to install because there are fewer components required. Combi boilers are also classified as condensing boilers, which have an efficiency rating of at least 92% because they minimise heat losses and cut back on fuel consumption. On top of these, using a combi boiler is very convenient because it provides central heating and hot water on demand. Controls are simple, so that you can easily choose the temperature and even set the timing.

However, combi boilers are better suited for homes that don’t have several bathrooms. A combi boiler functions only from one unit, so if two taps are turned on at once for hot water, it won’t heat very well. Since water goes to the tap because of mains pressure, you might experience pressure drops too. At the same time, combi boilers don’t come with backup heaters, and it’ll be your only strong source of heating. For houses with old pipework, installing a combi boiler might call for too much adjusting, so homeowners will stay with what was originally used instead.

Still, for most homes in the UK, combi boilers are a good fit. They’re available in a wide range of outputs and sizes, which cater to different heating needs. Simple, quick, and efficient, the best combi boilers will do a great job of warming up your space and providing hot water while conserving energy, and they’re becoming increasingly common as the modern boiler to have in your home.