Boiler & Central Heating System Components – What They Do and Common Faults

09 May. 2019

Written by Neil Marsh MCIPHE RP RHP EngTech

FORTHRIGHT have created a list of commonly used components within gas boilers and central heating systems, so you know what they are, what they do and some of the problems they may give you. This should be a good point of reference to question any recent diagnostic provided from your gas/heating engineer…

Remember to always use a Gas Safe registered engineer when you need a boiler repair. You can check if your engineer is suitably qualified and registered by going to the Gas Safe Register. 


This is used to extract products of combustion from the boiler and draw in fresh air from outside on room sealed appliances. 

Common faults are a noisy operation or the boiler not working. 

Where the boiler has a fault code/light sequence, it should show as a fan fault when checked against the boilers manual. 

Seized fans can often be saved by using a suitable lubricant (not WD40).


This is used to circulate heated water around the central heating circuit to the radiators, cylinder and other heat requiring components.

Common faults are a noisy pump, some of the heat emitting components, like radiators, are not getting hot or only partially heating (this can be due to other faults as well) or complete heating system failure.  Sometimes the boiler will bang or kettle heavily, depending on the make and model of the boiler, following a pump failure.

If the boiler has a fault code/light sequence then it may show as overheat, excessive temperature rise or pump fault in the manual.

Air Pressure Switch

A safety feature on boilers used in conjunction with the fan that is designed to prevent the boiler from firing following a failure with the fan.

Air pressure switches can stick and blockages within the fans venturi can prevent the switch from engaging also.

If the boiler has a fault code/light sequence, then it may show as a fan fault in the manual.


These are resistors that change their resistance with changes in temperature, of which a printed circuit board interprets and uses for such things as flame modulation, temperature readings, thermal cut out and other aspects of boiler control.

Common symptoms from faulty sensors are boilers behaving erratically, locking out at hot/cold temperatures, general glitching and complete failure. The fault system on the boiler does not always interpret sensor faults (if it has one). However, if the boiler has a fault code/light sequence, then it may show as a sensor fault in the manual.


This is a safety device that is used to cut the supply of gas to the burner following flame failure or high levels of carbon monoxide on boilers with a pilot light (boiler you have to manually light).

The common fault is that the boiler will not ignite. Generally, these were used on appliances before fault detection systems where used/developed.

Printed Circuit Board

The printed circuit board (PCB) is the brain of the boiler. They control the complete operation and sequencing of the appliance.

A PCB can present many faults, it can fail to send power to any of the components and it can glitch, which can cause erratic behaviour and other faults which can be misinterpreted by the fault diagnostic system.

Usually, if it fails to send power to a component, the fault system will flag up an alternative component as the issue although it is the PCB causing the issue.

Diverter Valve

A divert-er valve is used to divert the flow of water within a combination boiler, or heating system, that takes hot water priority. What this means is as the hot water is used/in demand, the divert-er valve will stop the flow to the circuit that is used to provide heat in the property.

Common faults are hot water not reaching full temperature, radiators and other heat emitters are coming on when only the hot water is in use and either no heating or hot water.

Fault systems on boilers generally do not detect faults with divert-er valves.

Hot Water Sensor/Diaphragm & Micro Switch

A combination boiler will use one of these methods to detect the flow of water and trigger the boiler to fire for hot water mode. There are generally three sorts that are used, one is a turbine that detects flow, one is a sensor that detects the change of water temperature and the other detects a difference in pressure.

Common faults are no hot water, and with some boilers (that use a diaphragm to detect pressure differences,) you can get partial hot water as some taps/draw points will engage the hot water and others will not.

Fault systems on boilers generally do not detect faults with any of these mechanisms.

Gas Valve

This component turns on/off the flow of gas to the appliances burner, and in some instances, modulates the amount of gas being used.

Common faults are no heating/hot water, a smell of gas from the flue outside and intermittent sticking of the gas valve causing the boiler not to ignite (flame failure).

Boilers fault detection systems (if any) will often show as a flame failure. In more modern boilers that premix gas with air before sending it through the burner to be ignited, funny noises can occur (spluttering, whining etc) when the gas valve is incorrectly calibrated (air to gas ratio).

Ignition Electrode

This component allows a spark that is generated by a PCB or spark generator to pass between two points on gas boilers with automatic ignition. This electrical arc is used to ignite the gas exiting the burner.

Common faults are the boiler failing to ignite and explosive ignition (lights with a bang). This will often show as a flame failure on boiler fault diagnostic systems.

Rectification Probe

This component registers that a flame is present at the burner on and is a safety feature.

Common faults are the boiler failing to ignite which will often show as a flame failure on the fault diagnostic system.

Plated Heat Exchanger

In most combination boilers this component is used to indirectly transfer heat from the system water to the tap water. Some combination boilers do not use a plated heat exchanger and diverter valve but use a twin pass heat exchanger that heats the tap water directly through the main heat exchanger. 

Common faults are cycling of the hot water temperature (temperature not stable), which is due to blockages/restrictions in the plated heat exchanger.

Fault diagnostic systems (if any) will often not detect this unless severe, then they may show an overheat or temperature rise fault.

Heat Exchanger/Heat Engine

This is where the ignited gas (flames) transmit heat to fluid (often water) inside to boiler to the central heating system. Heat engines are usually robust, although, they can leak and some modern versions can be more susceptible to issues caused by sediments like sludge or scale.

Aluminium heat exchangers, which are commonly used in modern boilers, have weaknesses that most people are not aware about. Click here to find out more. 

Motorised Valves

Also known as zone valves, 3-port valves, and mid position valves. These valves are designed to control the flow of water to different parts of the heating system such as the cylinder and radiators.

These are external controls to the boiler and will not be detected with any current boiler fault diagnostic system.

Common faults are hot water getting too hot, central heating coming on with hot water only use, the heating/hot water will not switch off, there is no heating or there is no hot water.

Cylinder Stat

This is used to regulate the temperature within a hot water cylinder. These are usually robust components.

The common faults are no hot water or the hot water gets too hot.

These are external controls to the boiler and will not be detected with any current fault diagnostic system on the boiler.

Room Stat –

This is used to regulate the overall temperature of the heat emitting devices such as radiators, these are also robust components.

The common faults are that the heating fails to come on, or the temperature of the room does not align with the temperature setting on the stat. Radio frequency versions are less robust than hardwired versions. Their batteries should always be changed prior to ordering a replacement unit as old batteries can cause issued with the wireless transmission.

Information about smart heating controls can be found here. 

These are external controls to the boiler and will not be detected with any current fault diagnostic system on a boiler.

Programmer/Time switch/Time Clock

This is the device that is used to turn on and set a time function for the central heating and hot water (where applicable).

The common faults with these are the heating does not work, hot water (where it has a hot water setting) does not work and faults with the timings

A picture showing common boiler component names can be found by clicking here,

FORTHRIGHT excel in boiler and central heating system fault diagnostics and repairs. We have fixed boilers up and down the country for many years. We are highly experienced and well trained at boiler and central heating maintenance, there are few competitors that operate at our level. Do not take chances with a boiler repair or central heating repair, if you need a boiler repair in Cheltenham, Gloucester, Stroud, Tewkesbury or the surrounding areas of Gloucestershire, use us. Our prices are transparent, fair and affordable, plus our service is second to none.