Fault finding
Welcome to an introduction to fault finding
Today, we are going to learn the basics of boiler fault finding to give you a good platform to start from, now before we start I’m going to ask you 8 questions. So have a pen and paper ready to write down your answers, don’t worry we will go through these later on, so if you don’t know just leave it blank, these scenarios are all from real jobs.
I’ll now give you a minute to get your pens and paper.
Ready? now Let’s start.
1. Customer has explained to you that they have got heating but no hot water. When the hot tap is turned on the boiler does nothing.
2. Customer has called saying the boiler is not working. As you turn the hot tap on the pump runs and the fan runs but the boiler gets no further.
3. Landlord calls explaining that the boiler is not working. When a hot tap is turned on the light indicating demand lights up but the boiler will not run.
4. Customer has hot water but no heating. When you get there the boiler fires on hot water but not heating when the thermostat is at 30 degrees.
5. Customer has called as the boiler will not work on heating but is fine on hot water. They mentioned they have recently had a plumber out to fit a radiator. The boiler fires but will go off after a few seconds.
6. Tenant has reported that the boiler is not working on heating or hot water. When there is a demand the boiler pump runs, Fan runs, the Diverter valve switches and it sparks, but after 3 attempts locks out.
7. Customer has reported that the boiler is not working at all. When there is a demand the boiler pump runs, Fan runs, the Diverter valve switches and the boiler lights but goes out after a few seconds it will do this 3 times in succession then lock out.
8. Customer has reported that the boiler is not working. When you arrive there is a light on the boiler flashing you then reset the boiler and it is still flashing.
Ok so now we have gone through some scenarios, but without knowing they are not too easy to answer right?
Let’s get down to how a boiler works, so a boiler works with a sequence of operations 1 has to be ok before it can move to 2, and 2 needs to be ok before it can move to 3 and so on.
Knowing how a boiler works and knowing the sequence is the key to fault finding, all you have to do is work out where in the sequence it got to,
So a combi boiler has two sides a heating side and a hot water side, so let’s look at the heating side first and how it works, the sequence is based on an older boiler non-condensing type, we will go over new boilers later but they’re all fairly similar,
Boiler heating sequence
1. Demand called from timer + stat
So this is time on and the thermostat turned up, you will be surprised how many calls you will go to for no heating and the thermostat is down.
2. Pump runs – activating primary flow switch then micro switch
The pump starts moving the water around the boiler, some boilers have a pump-sensing device some don’t
3. Diverter valve remains in at “rest” position (central heating port open only)
So a diverter valve sends the water to either the radiators or the secondary heat exchanger for hot water, in this case, we want the water to go to the radiators, we have the pump moving the water and the diverter valve directing it.
4. Voltage is sent to the fan and the fan runs
so the fan is clearing anything in the combustion chamber and getting the environment ready for the burner to ignite,
5. Air pressure switch activates Sending a signal back to the Printed circuit board
The boiler needs to check if the environment is safe for the burner so boilers have an air pressure switch, this checks the fan is running,
6. Overheat cut-out thermostat is checked for closed circuit (continuous circuit) Primary thermistor is also checked for correct thermal resistance
These are checked by the PCB, the overheat stat is normally on the main heat exchanger and sometimes has a manual reset or reset by the reset button on the front of the boiler,
7. Spark generator and ignition electrode
Now we have the demand stat/ timer on calling for heat, the pump pumping the water, the pump flow switch checking the pump is running, the diverter valve diverting the water to the radiators, the fan is running, the air pressure switch checking the fan is running and overheat stats are ok, and we also have a spark all ready for ignition,
8. gas valve opens
the gas valve opens and allows gas to flow to the burner, as the gas flows and the ignition spark is already going it should light, and the fan is sucking the fumes up and pushing them out the flue,
9. Flame detection electrode – flame rectification and ignition
So now the boiler is a light there needs to be something that sensors that it’s a light, this is done by flame rectification, a dc current direct current is sent to a probe and when a flame is present this current changes to ac alternating current, the boiler PCB then knows that the boiler is a light and can stop the sparking,
10. Boiler control thermostat or room thermostat is satisfied or the timer shuts down the boiler
The boiler will continue to run until the demand, thermostat or timer, tells it to stop
11. Fan and pump overrun continue to remove heat
The fan and pump will run for a few mins to clear any heat built up
So this is the sequence for heating
- Demand
- Pump
- Diverter valve
- Fan
- Air pressure switch
- Overheat stat/ thermistors
- Spark and flame rectification
- Gas valve opens
- Flame detection and spark stops
- The boiler is now running
- The boiler is now running until the thermostat or programmer tells it to stop
- Pump and fan overrun
Now the PCB printed circuit board controls everything within this sequence, it’s the brain of the boiler.
This is how or close to most boilers work some are slightly different but this will point you in the right direction. Just follow the sequence and see where it’s getting to.
Let’s look at some common boiler components in relation to the sequence.
1. Demand
Demand is from on the heating side a timer and or thermostat and on the hot water side a diaphragm/flow switch these tell the boiler to come on, to test a thermostat or timer you need to check if the switch live is getting to the boiler this can be checked using a multimeter, but keep it simple is there a demand light or indicator on the boiler first if so you know its getting demand. For hot water it’s the same is there a demand light or indicator on the boiler? If not what sensors the water flow? it will be on the cold supply into the boiler, the diaphragm is there a micro switch can you see it move?
2. Pump
Is the pump working, you can check by the noise/ vibration it’s making,
You can check if it has power using a volt stick or multimeter,
To check a pump is running you can unscrew the cap at the front and insert a screwdriver if you can stop it or if it’s not spinning it needs replacing, if it’s not spinning you can sometimes start it spinning using the screwdriver and turning it the way of rotation indicated on the front. Another indication of a faulty pump is if it’s red hot and you can’t keep your hand on it. the pump is required for circulation to move the water around the boiler and system.
3. Diverter valve
This moves the water direction from heating to hot water, this is sometimes combined with the diaphragm section or sometimes separate to check this is the water flowing where it should be, when the hot water is on does the radiators get hot? Can you see the spindle move if it has one
4. Fan
This clears the air in the combustion chamber and draws in fresh air from outside, is it spinning? Can you hear it or see it move, does it have power you can check with a volt stick or multimeter
5. air pressure switch
This device checks the fan is running
The fan has a venturi where the air passes through and creates a suction / blow to move the diaphragm in the aps to prove its on, to test this you can remove the pipe and suck on it to hear it click
6. overheat stats and thermistors
Overheat stats checks an upper-temperature limit, if the boiler temperature exceeds this it will cut the boiler off, this can be reset by either a manual reset on the stat or reset on the front of the boiler.
Thermistors check the running temperature of the boiler and are on the heating and hot water side these can be tested using resistance ohms reading on a multimeter against the MI’s
7. spark and flame rectification
The spark is there to light the gas
To test it – can you see the spark, can you hear the spark? is it in the correct position, is the gap as per mi’s
Flame rectification is to sense the flame is present it sends an ac/dc signal between two probes and checks it’s alight and not just pumping out unlight gas.
8. gas valve this lets gas into the burner
To test it check it has power using a volt stick or multimeter, check the inlet pressure, check the inlet working pressure, check the output/burner pressure this will show if it’s open or not
Typical Sequence:
As the system water increases in temperature, the resistance of the NTC thermistor decreases, and the potentiometer at the p.c.b then varies the voltage to the module on the gas valve, which in turn, modulates the burner accordingly, to match the demand for the heat load.
To Test a Thermistor:
- Electrically isolate boiler/system controls
- Ascertain system water temperature (near to thermistor) by using a thermal laser or clamp on the thermal sensor
- Remove thermistor wires (at the p.c.b end) to prevent stray resistances from the p.c.b itself
- Set multimeter on ohms scale for resistance readings
Note: (PTC thermistors only: the black lead from the multimeter must go on the metal casing of the sensor and the red lead from the multimeter must go on the terminal of the sensor. Resistance readings must be cross-referenced to that of the manufacturer specification, to prove correct manufacturer calibration.
Closed circuit of less than 1 ohm = faulty
No resistance/infinity = faulty.
An example of NTC resistance values:
Temperature (degrees C) Resistance (ohms)
0 35400
10 22500
20 14700
25 12000
30 9835
40 6712
50 4672
60 3311
70 2388
80 1749
90 1300
100 980
110 749
Now to test different components you will need to use a multimeter so let’s go over how to use one
Safe use of multimeter functions:
- AC – (Alternate current) will show a wavy line with dots diagonally opposite or ACV.
- DC – (Direct current) will show a straight line with dots below or DCV. (measures a steady reading).
- Typical ranges for digital multimeters are:
- DC Voltage: 200mV, 2000mV, 20V, 200V, 600V
- Ac Voltage: 200V, 600V
- Dc Current:200μA, 2000 μA, 20mA, 10mA
- OHMS scale will show an uppercase omega symbol of Ω. (Resistant of movement).
- Resistance: 200Ω, 2000Ω, 20KΩ, 200KΩ
- Continuity buzzer will show the symbol of ⁾⁾⁾.
- (buzz = closed circuit / continuous circuit / zero resistance)
- (no buzz = open circuit / no continuous circuit/infinity / OL).
The black test lead always goes into the COM terminal on the multimeter (first).
The red test lead always goes into VΩ or ACV or V~ terminal on the multimeter (second).
Always select a voltage range above 240 as less than this will read off the scale when measuring for mains voltage and never rotate the dial while the meter is connected to a circuit – you may rotate it through a current range, short-circuiting the probes and damaging either the meter or the circuit.
When measuring across L and N parts of a circuit (motor windings), a reading of zero resistance indicates a fault (short circuit).
The resistance readings in motor windings will vary (pumps, fans, solenoids, transformers).
When measuring closed switches (micro switches, overheat cut-out stats) a reading of zero resistance (closed circuit/continuity) = ok.
When measuring open switches (micro switches, overheat cut-out stats) a reading of infinity / OL (open circuit / no continuity) = ok.
When measuring in volts AC the black test lead must first be connected to E or N connections and then secondly the red test lead be connected to the L connection, which then completes the circuit.
All electrical test equipment used should be suitable for the electrical tests being carried out and any leads/probes to be inspected before use to ensure it is safe.
Respect the circuitry you are working on – If you touch exposed metal, or cause a short circuit, or connect to high voltage, you may expose yourself to a serious hazard.
Although more expensive, an auto-ranging multimeter is far easier as it automatically adjusts its range to give a meaningful reading. Some of the ranges typically displayed on a multimeter will rarely be used, if at all by gas engineers.
The main ranges typically used are AC, DC and continuity – keep it simple rule!
240v test
- Connect the boiler to the power supply via 3 pin plug top or insert a 3 amp fuse into the switch
- Ensure the boiler is on but with all controls off, so there is no demand
- Set the multimeter to the correct voltage scale on V AC
- Firstly put the black lead from the multimeter to the E terminal block on the boiler then secondly the red lead from the multimeter to the L terminal block on the boiler – this should read approx. 240V AC
240 V TEST
- Ensure the boiler is on but with all controls off, so there is no demand
- Set the multimeter to the correct voltage scale on V AC
- Firstly put the black lead from the multimeter to the N terminal block on the boiler then secondly the red lead from the multimeter to the L terminal block on the boiler – this should also read approx. 240V AC
Mains voltage and polarity tests:
This test will ascertain the correct polarity between L and N although, some boilers will operate unaffected by reversed polarity.
- Connect the boiler to the power supply via 3 pin plug top or inserting a 3 amp fuse into the switch
- Ensure the boiler is on but with all controls off, so there is no demand
- Set the multimeter to the correct voltage scale on V AC
- Firstly put the black lead from the multimeter to the E terminal block on the boiler and secondly the red lead from the multimeter to the N terminal block on the boiler – this should read 0V AC, but up to 15V AC is ok
Note: Should a reading be greater than 15V AC, this indicates a fault is present within the house wiring, but the polarity at the fused switch should first be checked, before calling an (NICEIC) approved electrician.
Resistance to earth: (dead test)
This test confirms the integrity (resistance) of the insulation between L and E on the boiler terminal strip or plug top pins (Black lead firstly to the E pin and red lead second to the L pin). Where the boiler supply is connected to a fused double pole switch, it should be isolated at the ring main supply end, to eliminate any possible, further faults present on the circuit (This is why a 3-pin plug is a much better option as a double pole spur can cause further problems).
With the boiler switches, time switches and thermostats on and the multimeter set on the ohms scale, the resistance reading between L and E should read no other than O L ( infinity ) and if so, there is a fault.
A detailed continuity check is required to trace the faulty component before proceeding any further
Short circuit: (dead test)
This test will ascertain the integrity of the internal resistance of the boiler and should read no resistance/infinity between L and N on the 3 amp fused plug top pins.
- Set multimeter on ohms scale for resistance readings
- First, put black lead from the multimeter to the N pin on the boiler terminal strip and secondly put the red lead from the multimeter to the L pin on the boiler terminal strip
Should the resistance reading move, there is a dead short and a fault is present. Where the boiler supply is connected to a fused double pole switch, it should be isolated at the ring main supply end, to eliminate any possible, further faults present on the circuit. (This is why a 3-pin plug is a much better option as a double pole spur can cause further problems).
There are six internal boiler components to check and the L and N connections for each component must be removed individually then the L and N at the 3-pin plug top or the boiler terminal block must be measured again.
Components to check (in this sequence):
- Pump
- Fan
- Gas valve
- Diverter valve
- Circuit board
- Internal wiring loom (visual observation only)
Should the internal resistance measure infinity/no resistance when the component is disconnected, then the faulty component has been traced.
Should the resistance reading maintain moving when the component is disconnected, the dead short is elsewhere (further investigation is needed in the above sequence).
Now we have covered the sequence
Boiler heating sequence
- Demand called from timer + stat
- Pump runs – activating primary flow switch then micro switch.
- The Diverter valve remains in at “rest” position (central heating port open only)
- The printed circuit board sends voltage to the fan
- Fan runs ok
- The air pressure switch activates Sending a signal back to the Printed circuit board
- Overheat cut-out thermostat is checked for closed circuit (continuous circuit) Primary thermistor is also checked for correct thermal resistance
- Spark generator and ignition electrode(s) ok
- The gas valve is energized and ignites ok (low rate)
- Flame detection electrode ok (sparking finishes – ionization proved)
- The gas valve ramps up to range-rated set pressure (Controlled via the thermistor and potentiometer at p.c.b)
- The boiler control thermostat or room thermostat is satisfied or the timer shuts down the boiler,
- Fan and pump overrun continue to remove heat
We can understand how to identify faults so for example if a boiler starts the sequence the pump starts and the fan starts but gets no further what are we going to look at, so the next but in the sequence is the air pressure switch so we know it’s a fan fault or APS fault,
Example 2, hot water, you turn the tap on to create demand but nothing happens what could it be? Looking at the sequence the first thing is demand, so flow switch or diaphragm so we know the fault is probably the flow switch or diagram
Example 3 the heating demand is on, the thermostat is up, the boilers pump runs, and the fan on APS switches, and starts to spark, boiler lights but it continues to spark and then locks out, looking at the sequence this points to the flame rectification so we know the issue is probably flame rectification
Now as promised let’s look at a modern boiler, condensing boiler works in much the same way but there is no air pressure switch, they have a zero guvnor, so the fan runs and sucks the gas and air mixes it and pushes it into the burner, the rest is pretty much the same.
This is an interactive boiler image. Click on the boiler to explore the parts
So now we have covered all the different elements let’s try the questions again, please complete the knowledge assessment next.