How a Combination Boiler Works

There are several different types of boilers available to heat your home and provide hot water. The combination boiler, known often referred to as the combination boiler, has become very popular because it works in a different manner to other boilers. They don’t use a water tank and are generally mounted on the wall, taking up a great deal less space than other boilers. The advantage of combination boilers is that you don’t have to heat up water you won’t need which gives big savings on both money and energy.


A combination boiler delivers hot water as soon as it's needed. The operate to a pressure that is very close to water mains pressure which makes them fast-acting, efficient and energy-saving options. These boilers are ideally suited for smaller houses and apartments and cost less to install than conventional boilers. They also create less mess during the installation process.


When two faucets are opened at the same time, the water flow can be significantly reduced and in general, the delivery rate of hot water can be lower than with conventional boilers anyway. If you do already have a hot water tank, it will increase the cost of installing a combination boiler.


A combination boiler is fully pumped, exactly as it would be with a conventional boiler, but the system is completely sealed. Because there are no open vents in the system, it doesn’t require a feed and expansion tank as part of the system. The boiler does actually incorporate a small expansion tank (usually about 8 liters), but this, too, is part of the sealed system.

A condensing combi boiler will extract heat from the gases exhausted to the flue. This lets the gas condense in the boiler and release latent heat into the system.

The expansion and contraction of the water in the central heating is accommodated in the pressure vessel, which is inside the boiler casing. Since there’s no expansion cistern for the system, it doesn’t have an open safety-vent pipe but utilizes as pressure-relief valve instead. The combination boiler will also have an overheat cut-out. This stops the water boiling in the event of the internal thermostat on the boiler failing. The gauge on the boiler will show the pressure in the system and offer an indication that the system isn’t admitting air. At times, you’ll need to re-pressurize the system.

Hot Water

A very efficient plate heat exchanger in the combination boiler produces hot water on demand. No hot water is produced until the faucet is opened which means no energy or money is wasted. When faucets are turned on, water from the sealed unit that runs the central heating is automatically heated and sent to the heat exchanger for the hot water.

This is followed by cold water from the mains, or from another water source, which passes over the heat exchanger in the boiler where it is heated. From there, water passes through to the faucet. The combination boiler gives priority to supplying hot water. As soon as the faucet is turned off, no more hot water is produced and the water for the central heating system returns to its usual circuit.