Combi boiler

combination boiler, or combi boiler as it is typically known, is a boiler which provides the heat for both the radiators and the domestic hot water outlets in the home. Combi boilers are the most common type of boiler in the UK, accounting for around 70% of central heating system configurations in British households.

The most distinguishing aspect of a combi boiler is that it eliminates the need for a hot water cylinder and for water cisterns in the loft space. 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, there is no need for a boiler to heat water in a hot water cylinder – a combi boiler heats water from the mains on demand. 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, as UK water regulations did not permit either 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 predetermined 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 pushed through the radiators by the pump, and 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, and the cooler water returns to the boiler before being heated again and redistributed by the pump, until the temperature in the home has reached the desired level and the thermostat 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 boiler

When hydrogen gas is burned (oxidised) during the combustion process, water vapour, i.e. steam is formed. A condensing boiler condenses this steam back into a liquid, thus recovering the heat energy which would otherwise be lost. This increases their energy efficiency. The condensate, i.e., the water, 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 bad smells from travelling up drain, 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; rather, condensing is a characteristic, and is an extremely common characteristic of combi boilers. In fact, the term “condensing boiler” is becoming more and more of a misnomer, as all boilers installed in the UK after 2005 must have this feature for environmental reasons.

Other components

Other important combi boiler components include the following:

  • Pressure Release Valve or PRV. The water inside the central heating should be set to approximately 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 to ensure the correct ratio of air to gas, and consequently, to ensure 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. Easily identifiable by its red exterior, the expansion vessel 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.
  • Filling loop. This is a length of pipework with a valve at each end that, when opened, is used to fill up the central heating circuit with water from the mains. It must also contain a non-return valve so that water in the central heating cannot flow back into the mains and contaminate it. The filling loop may be a short length of rigid copper pipe, or a flexible braided hose.

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, after which it is brought back up to the target temperature by the boiler, 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

Depending on the make and model of the boiler, the filling loop may be a piece of rigid pipework, or it may be a flexible braided hose. Its purpose is to provide a temporary connection to the mains in order to fill the central heating with water. This is necessary because, as mentioned, a permanent connection to the mains is not permitted by water regulations.

The filling loop is normally connected to the central heating side via the return pipe, although 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 intake for supplying the boiler with gas on the boiler itself is normally 15 mm. However, the actual supply pipe itself is likely to be wider; 22 mm or even 28 mm, depending 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. When installing a combi boiler, the engineer must calculate the correct diameters so 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.

PRV pipe

Should the water in the central heating system exceed a set pressure (usually 3 bar), this will be discharged by the pressure release valve through a 15 mm copper pipe outside the property.

Condensate pipe

Waste water from the condensation process is discharged into the sewer via the condensate pipe, which is usually 22 mm plastic solvent weld. On the outside of the property, the diameter may increase to 32 mm or even larger. The purpose of the increase is to prevent any water in the pipe from freezing, which would block the pipe and disable the boiler.

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