Combi boiler: what is it & how does it work

combination boiler, or combi boiler as it is typically known, is a boiler which provides the heat for both the radiators and the hot water outlets in the home. Around 70% of British homes use a combi boiler, making them the most common boiler type in the UK.

The key aspect of combi boilers is that there is no need for water cisterns in the loft or a hot water cylinder. This is because the water for the central heating is supplied via a temporary connection to the mains instead of a feed and expansion tank. As for the hot water for the taps, combi boilers heat it on demand from the mains. Without the need for a hot water cylinder, there is consequently no need for a cold water storage cistern.

Combi boilers are fuelled by natural gas or oil. Common manufacturers include Ideal, Vaillant, Vokera, Potterton, Baxi, Glow-worm and Worcester Bosch. Gas combi boilers are usually wall-mountable, while oil-fuelled ones are often free-standing. It is illegal for an engineer or indeed any individual who is not on the Gas Safe Register to work on a gas-fuelled combi boiler.

Combi boiler history

The combi boiler was invented in the 1960s by Vaillant, the German heating, cooling and engineering giants based in Remscheid, Germany. Despite their ground-breaking invention, there was little reason to even import combi boilers to Britain. This was because, at the time, UK water regulations did not allow domestic hot water or central heating systems to be directly connected to the mains – hence the use of tanks in the loft.

However, combi boilers got a huge leg up in the British market in the 1980s, when water regulations changed to allow central heating systems to be filled up via a temporary connection to the mains.

How does a combi boiler work?


When the thermostat detects that the temperature in the home has fallen below a set level, the boiler’s circuitry sends a signal to open an internal gas valve and for the electronic ignitor to light the burner inside the combustion chamber. Heat generated in the combustion chamber is transferred to the water flowing through the primary heat exchanger. This hot water is then pushed through the radiators by the pump. The gases generated from the combustion process are expelled through the flue. Heat from the hot water is emitted into the home via the radiators. The cooler water then returns to the boiler before being reheated and pumped through the radiators again. This process continues until the temperature in the home has reached the desired level and the thermostat then ceases to request heat from the boiler.

Hot water

When a hot tap is opened, a sensor detects the movement of water through the pipe and lights the burner. Water from the central heating circuit is heated by the primary heat exchanger. This water is then pumped through a secondary heat exchanger. Here, heat from the primary circuit is transferred to cold mains water, which then goes on its way to the outlet that has been opened.

If a hot outlet is opened while the central heating is already activated, a combi boiler will always give priority to the hot outlet. A diverter valve controls whether water heated by the primary heat exchanger should flow through the radiators or through the secondary heat exchanger.

It is important to note that the two bodies of water – the domestic hot water and the water in the central heating – never come into contact with each other. The stale, recirculated water of the central heating will (or should) contain anti-corrosion chemicals which would contaminate the hot water.

Condensing combi boiler

When hydrogen gas is burned (oxidised) during the combustion process, water vapour, i.e. steam is formed. The steam is obviously hot, i.e., it contains heat energy. Condensing boilers draw so much heat energy out of this water vapour, it turns back into a liquid. In a non-condensing boiler, this heat would be wasted – the water vapour would simply be expelled out of the flue. For this reason, condensing boilers are always more energy efficient than non-condensing boilers.

The condensate is collected in the condensate trap before being discharged into the sewer by the condensate pipe. Like how the trap of a sink carries a small amount of water to block smells from the drain from entering your home, the condensate trap holds a small body of water so that any toxic gases produced by the boiler cannot travel down the condensate pipe.

Consequently, a condensing boiler is not a special type of boiler in itself, but rather, a boiler with a certain engineering feature. In fact, the term “condensing boiler” is becoming more and more of a misnomer, because all boilers installed in the UK after 2005 must be condensing boilers for environmental reasons.

Other combi boiler parts

Other important combi boiler parts include the following:

  • Pressure Release Valve or PRV. The water inside the central heating should be set to 1 – 1.5 bar when cold. Should the pressure in the system rise above a predetermined level (e.g., 3 bar), water will exit the system through this valve.
  • Venturi. This is a valve which controls the flow of air into the combustion chamber. Its purpose is to ensure the correct ratio of air to gas. It ultimately ensures the clean and complete combustion of the fuel.
  • Thermistor. The word is a portmanteu of ‘thermal’ and ‘resistor’. As the name suggests, thermistors control the flow of electrical current in a circuit based on the temperature they detect.
  • Expansion vessel. This accommodates the increase in volume of the water when it is heated. Hot water should never be contained without allowing for this, as it may have explosive results. It is easily identified by its red exterior casing.

Combi boiler pipes

A combi boiler will have the following pipes connected to it:

Flow & Return

These two 22 mm copper pipes carry water to and from the radiators. Hot water is distributed to the radiators via the flow, and the heat it contains is emitted by the radiators. The cooler water returns to the boiler via the return. It is then brought back up to temperature and pumped through the radiators again.

Cold main

A combi boiler gets its water supply from a 15 mm copper pipe fitted to the mains in the property.

Filling loop

The filling loop may be a piece of rigid pipe, or a flexible braided hose. Its purpose is to provide a temporary connection to the mains in order to fill the central heating system with water. This is necessary because, as mentioned, UK water regulations do not allow a permanent connection to the mains

The filling loop is normally connected to the central heating side via the return pipe. However, on some boilers it may be connected to the flow. A non-return valve must be fitted so that water from the central heating can never flow back into the mains.

Gas supply pipe

The boiler’s gas intake is normally 15 mm. However, the actual supply pipe itself from the gas main is likely to be wider. 22 mm is often encountered. In some cases, 28 mm may be necessary. The right size depends on a number of factors, such as the power of the boiler, its distance from the gas main, and the number of bends in the pipework. The engineer installing the boiler must calculate the correct diameter to ensure that the boiler is not undersupplied with gas.

Hot outlet

Hot water for the taps is distributed to the house via this pipe. It is usually 15 mm copper.

Pressure release valve (PRV pipe)

If the boiler pressure exceeds a set figure – usually 3 bar – water will be discharged out of the boiler through this pipe. It’s usually 15 mm copper.

Condensate pipe

Waste water from the condensation process flows into the sewer via the condensate pipe. This is usually 22 mm plastic solvent weld. On the outside of the property, the diameter may increase to 32 mm or even larger. This is so that any water in the pipe doesn’t freeze and block it. For more information on this, see the following page: Frozen condensate pipe: how to fix.

Combi boiler problems

The most common combi boiler problem is low pressure. This is because combi boilers require a pressure of at least 1 – 1.5 bar when cold. Anything less than this, and the boiler won’t work at all. A combi boiler which keeps losing its pressure is highly indicative of a leak somewhere in the system.

Other problems may include strange noises, such as gurgling or fizzing. These sounds are collectively referred to as boiler kettling. They are usually a symptom of overheating caused by limescale. For a detailed look at these issues, see the following page: Common boiler problems: 10 common issues & causes.