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Fixing a combi boiler - diagnosis and repair


General Method of Gas Combi Boiler Operation

Before starting to repair a combi boiler it is important to be familiar with the way the boiler should function under normal operational conditions.

The mechanical operation is directly linked to the electric/electronic operation and by understanding how far into the process the boiler has progressed before the operation fails, should greatly assist in the diagnostic fault finding process

The operational process follows a cycle where the combi looks for an operation to occur, and be confirmed as satisfactory, before moving onto the next stage of the cycle. Failure of any part of the cycle to be confirmed will result in the process going no further or on some models the combi going to "Lock Out".

The term 'lockout' refers to the combi stopping the process at any given point through its cycle because the last part of the process has not been confirmed. On certain models the combi does not stop the process entirely but stops at the point where the cycle failed this is sometimes referred to as passive lockout.

The correct functioning of any particular make or model may well differ from manufacturer to manufacturer but for the purpose of explanation of the method of operation, we will assume a typical modern combi with a fanned flue and electronic ignition sequence.

Before starting to diagnose any fault on a combination boiler the basics must be checked as much time and effort can be wasted searching for a technical fault that does not exist.

Initial combi boiler repair checks:

  1. Is the system filled with water and bled of air? Most pressurized systems require 1 bar pressure when the system is cold

  2. Is there an adequate gas supply available at the boiler?

  3. Is there an adequate electrical supply available at the boiler? See basic electric checks.

Once the basic checks have been done we are now at a point where the boiler can be put through its paces.

The first step in the boiler cycle is a demand for the cycle to commence this is typically generated by the opening of a domestic hot water tap. The demand is sensed, usually by flow switch or sensor which then allows the fan to run at full speed.

The fan is proved to be running by the operation of the air pressure switch. Once the air pressure switch has sensed the air movement generated by the fan the next stage of the cycle commences.

The ignition circuit is energized which allows a pilot solenoid valve to open and feed gas to the pilot burner. Alternatively, a low rate gas supply can be fed directly to the main burner, dependant on the model of boiler. At the same time the ignition unit generates a continuous spark at the pilot head or directly onto the main burner. The boiler at this point now needs confirmation that the pilot or low rate gas supply has ignited successfully this is generally done by a process known as flame rectification (see separate sheet). At this point if the flame is not detected within a preset time period the boiler will lockout on certain models.

On satisfactory detection of the low rate flame or pilot flame, as the case may be, the main gas solenoid is energized allowing the main gas flame to be established.

Additional controls and devices are added by certain manufacturers which will affect the cycle of operation. Therefore in addition to the broad method of operation outlined above the instructions supplied with the boiler will need to be consulted for such devices as pump flow switches and low water pressure switches and their effect on the operation of the boiler.

When the boiler has completed its initial " lighting up process" the controls on the boiler as well as the external controls of the system take over the control of the boiler.

The boiler controls include thermostats and thermistors, while the external controls include the time clock and room thermostat when fitted

The above information should help to identify the area of the cycle where the fault might exist and when the cycle of operation is tied to the combi boiler's electric/electronic operation the fault-finding process should become much easier. For instance, there would be no point investigate a problem with the ignition cycle if on the initial boiler demand the fan had not started as the fan running and being proved to be running comes before the ignition process in the cycle of operation.

Normal Boiler Operational
Flow Chart

  1. Demand for heat (Hot water tap on or demand for heating)

  2. Fan runs at high speed

  3. Air pressure switch makes

  4. Pilot/Low rate gas solenoid opens

  5. Ignition spark at pilot/main burner

  6. Pilot lights/Main burner lights at a low rate

  7. Pilot/Low rate gas sensed by rectification probe

  8. Second solenoid (main gas) opens

  9. Main burner established

  10. Boiler controls & external controls monitor and control boiler operation

Ignition Systems

There are two basic types of ignition system on a combi boiler, the practically permanent pilot light obsolete amd the intermittent electronic ignition

The permanent pilot is rarely seen, having a flame burning constantly inside a boiler wasted a considerable amount of gas, and in all new combi boilers, it has been replaced by electronic ignition systems. The electronic system removes the need for a permanent pilot and eliminates the age-old problem of pilot outage in draughty conditions.

This problem often leads to customers struggling or not being able to relight the pilot light. The knock-on effect was wasted time and effort of the heating engineer having to visit site to relight the pilot light. All too often costing the heating engineer to lose valuable working time and not being practical to charge the customer to light the pilot light.

The intermittent electronic ignition of the combi boiler is now standard on all combi boilers taking the form of flame rectification.

The ignition cycle forms a vital part of the boiler cycle, as should any part of the ignition system fail or not be detected then the boiler will, "fail to safe", often going into a, "lock out" mode.

The electronic ignition or spark is generated on demand via the printed circuit board (PCB), sometimes called a motherboard.The spark electrode is positioned to allow the spark to ignite the pilot burner or directly onto the main burner. When igniting the main burner directly, initially the burner is fed with a low rate/ignition rate gas supply. Once this has been detected then a higher gas rate is fed to the main burner to satisfy demand.

Hydraulic Valves - Diverter Valves

Diaphragm Type

This type of diverter valve is used by many different manufacturers and is produced by Berretta and Inter Albion in Italy. When the domestic hot water tap is opened a differential pressure is created and acts on the diaphragm. Once the differential pressure has been set up the diaphragm flexes and causes the central spindle to lift and thereby closes the port within the valve which feeds the central heating system circuit. At the same time the spindle rises through the top of the diverter valve and operates a double micro switch, which informs the boiler to operate in domestic hot water mode.

Wax capsule type

This type of diverter is used on early Vaillant boilers and on the Potterton Puma. The diverter is operated on temperature, when a hot water tap is opened the 80% of the wax thermostat situated in the cold mains supply causes the wax thermostat to contract. The diverter valve drop causing the closing the central heating ports and allowing the hot water with the boiler to be directed into the calorifier where it is used to indirectly heat the domestic hot water.

Wax Capsule Type Diverter Valve

Venturi type

This type of diverter valve uses the Venturi principle, similar to that of the instantaneous water heater. Once the hot water tap is opened the resultant flow through the Venturi creates a pressure difference to allow the diaphragm to lift and overcome the tension of the spring arrangement acting down on it. As the diaphragm lifts it also moves the spindle and valve to close the central heating ports and divert the hot water in the boiler to a calorifier to allow the production of domestic hot water. Once the tap is closed the tension of the spring forces the valve/spring assembly back to is original central heating position.

Venturi Type Diverter Valve


A sensor is a device that detects one type of energy and converts it into a different kind there are a number of transducers in boilers.


Displays are intended to provide some kind of information.
A display may give a reading that tells us a value that it is measuring. Watches, meters and calculators give direct numeric information but a bar or pointer may simply move over a scale to indicate a value. A second use of a display would be to give the state of a circuit or appliance. It often tells us whether the power is switched on or off if it is above or below a certain value or to provide a warning.

Displays are often classed as analog or digital. Analog displays have a pointer or a bar that moves over a display - similar to the hands of a clock. Digital displays simply indicate an on or off state or they may give numbers.

Types of display

Filament lamps

These have a fine tungsten wire that is heated by the current flowing through it. A typical indicator lamp may be rated at 12 volts, 2.2 watts. The current would be about 0.2 amp. They do provide a bright light.

Light-emitting diodes

These are semiconductor materials which are clearest when providing red light but yellow and green are also common. They require about 10 milliamp (0.01 amp) for normal brightness. They are common on mains powered clocks and meters. A line of LED's can form a bar chart or level indicator.

Liquid crystal displays

Liquid crystals depend on reflected light to give a visual display. However, they can be illuminated by a backlight. When a voltage is applied to the appropriate parts of the display the crystals are realigned so that some regions reflect the light and some do not. This provides the contrast that makes the display readable. The current they require is just a few microamps (0.000 001 amp), making them ideal for battery equipment.

Flame Rectification

Flame rectification requires a 240-volt ac electrical supply and is a very quick, safe and reliable way of detecting the presence of a flame.

The alternating current (ac) produces a sine wave which alternates from positive to negative and back again. Each complete wave is called a `cycle' and a 240-volt ac supply produces 50 cycles per second, hence 240 volts ac 50 Hertz.

Full ac Wave

Flame rectification occurs when the burner of the combi ignites and the ions in the flame allow the current to flow. The flame works as a rectifier creating a small dc current which is sensed by the electronics of the boiler.

The flame acting as a rectifier works in a similar way to that of a non- return valve only allowing flow in one direction in this case current only flows in the positive half of the cycle and is therefore known as half-wave rectification.

Rectified a.c. Half Wave

In effect what will happen is that the burner of the combi will become positively and negatively charged in turn as the current alternates.

The pilot or main burner has a substantially larger surface area than the ignition electrode so when the burner is ignited positive ions are attracted to the negatively charged burner and so the current flows through the lit flame and on to earth.

The original ac is rectified to dc through the flame, and this is known as partial or half-wave rectification.

The small dc charge produced by the flame is sensed by the electronics on the PCB and allows the ignition sequence to continue by stopping the production of the spark and energizing the main gas valve of the boiler to allow heating demand to be met.

The drawings below show the different stages of the ignition process and the importance of the correct positioning of the ignition electrode should be apparent.

Initial demand spark commences:-

Stage 2

Burner ignited, low rate flame rectification occurs, stops, spark moves to stage 3.

Stage 3

Ignition proved the main gas supply solenoid energised to match heating.

Potential Flame Rectification Problems

  1. Position the electrode too close to the burner and there would be insufficient or lack of spark to ignite the burner.

  2. Position the electrode too far from the burner and no spark would be produced. (Spark would not be able to jump the gap)

  3. An electrode not positioned correctly within the flame area would lead to the ignited burner failing to be sensed by the electrode and the ignition sequence breaking down.

Alternative method using separate rectification/flame sensing probe.

Certain manufacturers prefer to use a second electrode to sense the presence of a flame and in this case the second sensor is known as a 'flame sensing or rectification probe'.

The positioning of the probes Fuses

Electronic circuits are susceptible to damage if the conditions drift outside certain limits. They carry small currents for the most part so the mains fuse offers little protection. This makes it more difficult to protect circuits from mains fuse excess current, high voltages, and overheating.

A fuse will offer some protection against overcorrect conditions but a transistor can be destroyed faster than a fuse wire will melt! Fuse holders near the PCB may be chassis mounted or soldered to the board itself. They are readily accessible and are a likely source of failure.

When checking the fuse look for signs of overheating, such as charring of the area around the holder. PCB's require close inspection since fuse clips may not be located in an obvious position. Also, the fuse wire may be too fine to see within the fuse itself It is worth checking the fuse with a multimeter for continuity.

It is important to choose the correct type of fuse. They may be fast-blow types or possibly, semi- delay. Choose one that has the specified fast-blow fuse current value. Fast blow types are usually fine wires found in a clear glass package but may have a ceramic case. Common sizes are 20 mm ceramic fuse and 11/4 inch.

Fuses will not protect equipment that is getting overheated. The problem may be a lack of ventilation, poor design or components malfunctioning. A thermal fuse is designed to operate when the temperature exceeds a preset value. Some types look like electronic components and may be impossible to identify by inspection. Other types can be set in a similar way to a miniature circuit breaker. These types react quite slowly but they offer protection from excess temperatures so this is not a practical difficulty.

Faster action may be provided using miniature circuit breakers operating in a similar manner to an MCR However, quite sophisticated electronic devices and circuits may be employed to protect sensitive electronic equipment. This is not likely to be serviceable without specialist knowledge.

Around the mains fuse there may be a number of other components, such as chokes and capacitors, which protect the circuit against mains surges and suppress interference.

will vary from manufacturer to manufacturer but the basic principles remain the same as at the end of the day the process is designed to produce and sense safe and satisfactory ignition of the burner.

Gas Boiler Diagnosis Checklist

No Fan , No power indication (screen or power light) on boiler

  1. check rocker switches on and pump position on

  2. check 240 volts across terminals

  3. check the boiler fuses

  4. check the 3A fused spur

If power is available and fuses are OK then assume motherboard is defective.

does the fan run? If not check Fan Faults.

does pump run? If not see central heating pump diagnosis.

does boiler stay on low fire rate? if yes see boiler stays on low fire rate

does the boiler modulate? if not you should check the NTC thermistor and also the check gas valve

Does the boiler go to lockout

does the boiler lockout

  1. check there is gas supply to boiler live and purged of air

  2. check water deficiency valve open

  3. check C/H temp monitor - should be a normally closed circuit

  4. check spark generator, leads and electrodes

  5. check ignition rate gas burner pressure

  6. check flame stability - usually indicates flue or gas burner pressure problems

does the fan pressure switch operate?

  1. check flue joints

  2. check combustion chamber seals

  3. check for obstructions in the flue

  4. check for obstruction at the flue terminal

  5. check air pressure switch tubes are nt blocked or kinked and correctly fitted

  6. check fan is working properly

if fan not working after checks replace fan pressure switch.

Boiler stays on low fire rate

  1. check standing and working gas pressure to the boiler

  2. check diaphragm pump operating and tubes clear

  3. check voltage at diaphragm pump

  4. check gas valve adjustment


Boiler fan not running

  1. check voltage across fan connections

  2. check/reset main heat exchanger overheat ( which voltage is OK?)

  3. check fan with demand for heating- external controls on.

  4. check DHW temperature monitor - should be open circuit

  5. check NTC thermistors - pull off lead, if boiler fires change NTC

  6. check anti-surge fuse (make sure value is correct)

  7. check transformer

  8. check variable potentiometer (boiler stat) setting

  9. check boiler not in time dela

Electrical Checks

  1. Make sure there is AC voltage between the flame diode and the base of the flame. Measure AC voltage from the flame-sensing rod to the base of the flame. If there is no voltage, check to see that the wires from the module to the flame-sensing rod are correctly connected, or if the flame sensing rod or the wire connected to it is grounded.

  2. If voltage is present, make sure the flame rod and burner are clean and free of dust, dirt or debris. If they are not, take them out clean them and reinstall. Cleaning the flame rod and burner will remove impedance from the flame current circuit and thus should increase flame current.

  3. If the current is still too low, check to make sure the connections from the controller to the flame diode and the connection from the controller ground to the flame rod ground is low (it should be well under 100 ohms).

  4. If all else fails, replace the flame sensing rod. Now you should be able to measure the correct DC offset current.

  5. If the heater is still not operating correctly then the problem may lie in the controller.


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