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Baxi Combi 80e & 105e

Models Covered:

Baxi Combi 80e – GC: 47 075 06

Baxi Combi 105e – GC: 47 075 08

Baxi Combi 80e & 105e of 9


The following is a step by step guide on the testing of the following components

Pumps Fans Gas Valves Temperature Sensors Transformers

Safe working practises must be carried out when carrying out any electrical work. All these tests were performed on a Baxi 105e boiler.

Expected readings table:

Fan 240vAC... approx 32ohmsGas Valve (Honeywell) Coil 1 approx. 2900 ohms

Coil 2 approx. 1400 ohmsModureg approx. 45 ohms

Temperature sensor 15Kohms @ 25CTransformer Primary winding Secondary winding (low voltage)

240vAC...approx.24ohms240vAC...approx.5ohms

Pump Testing

The pump is used to circulate water through the boiler or system. On certain boilers or systems the pump may have to run before the boiler will start its operation. Within the pump an electric motor rotates, causing the impeller in the waterways of the pump to spin. This, in turn, causes water to be drawn in and pushed out of the unit. Because it is an electric motor, it requires power - and a resistance (Ohms) through its winding - for the pump to operate satisfactorily a standard pump requires a 230V AC power supply - and the resistance will vary, depending on the pump speed setting. So, the faster the pumps speed, the lower the resistance.

lower the resistance. Approximate readings across the windings should be:

• Pump Setting 1 =approx. 350 Ohms

• Pump Setting 2 =approx. 250 Ohms

• Pump Setting 3 =approx. 150 Ohms.



Pump resistance testing

1. Carry out all preliminary safety checks and positively isolate power.2. Remove pump electrical box cover.3. Identify and disconnect the Live and Neutral wiring from the pumps electrical

terminals.4. Set Fluke Multimeter to the Ohms scale.5. Place probes onto the vacated Live and Neutral terminals at the pump and note the

reading-6. If the reading is O.L. this indicates a breakdown in the pump windings. Therefore, the

pump or pump head will have to be renewed.7. Other readings will have to be checked, depending on the pump setting, as set out in

the table (above). These are approximate readings taken from the most commonly used pumps you wall come across.

8. If the readings are correct, re-connect the disconnected wires to the correct terminals of the pump and replace the electrical box cover.

9. If resistance is correct, check the power supply

Fans

Combustion in a boiler produces waste products which need to be removed. The combustion fan is used to extract these products from the boiler whilst also introducing fresh air so that the boiler can operate satisfactorily. The combustion fan is similar in


The resistance has been taken across the pump L-N and a satisfactory reading has been obtained (see table) a reading of 0L would indicate a faulty pump. A higher than expected reading could indicate a slow running pump


operation to the pump, but the motor runs at a higher speed so the resistance readings could be considerably lower. A normal 230V AC fan will have a resistance reading of around 90 Ohms - but fans do not always have a common resistance reading, so readings may differ from boiler to boiler. The resistance could be especially low (as little as 30 Ohms) if fed via a PCB. So, always check with the relevant manufacturer’s instructions,

Fan resistance testing

1. Carry out all preliminary safety checks and positively isolate power.2. Identify and disconnect the Live and Neutral wires to the fan motor (see the

manufacturer's wiring diagram if required).3. Set Fluke Multimeter to the Ohms scale.4. Place Fluke Multimeter probes on the now-vacated fan motor terminals — one to

where the Live wire was disconnected, and the other to where the Neutral wire was disconnected.

5. Note the reading.6. If the reading is O.L. this indicates a breakdown in the fan's windings, so the fan will

have to be renewed.7. If the reading is correct (as per the manufacturer's information), the fan's windings

are functioning satisfactorily.8. Re-connect the disconnected fan wires to their relevant fan terminals.9. If resistance is correct, check the power supply.


The reading on this fan is 0L.This indicates that the fan is faulty and should be replaced (Note the wires have been disconnected)


Gas Valve

There are many types of gas valve, but all work on a similar basis. The gas valve has an inlet and outlet gas port; to make the gas valve operate, an electrically operated solenoid is required, electrically opening and closing the gas valve on demand. Some boilers have a permanently lit pilot light - and their gas valves have a pilot take-off point fed through a thermoelectric unit. The most common thermo-electric gas valves incorporate only one main electric solenoid on the outlet of the valve. The voltage required to operate the solenoid may vary from 240V AC to 24V AC (check manufacturer's data or the information on the gas valve). Since it has no permanent pilot, an electronic ignition boiler does not require a thermoelectric unit. However, its gas valve usually has two main solenoids, for safety reasons: if one solenoid should fail in the Open position, or should it not seat correctly, the other solenoid will stop un-burnt gas from entering the burner. The voltage required to operate these solenoids is usually 240V AC (check manufacturers data or the information on the gas valve). On newer modulating boilers, the gas valve will also incorporate another solenoid called the Modureg (modulating valve). Instead of the boiler coming on or off at a pre-set temperature, the electronics are designed to allow the boiler to modulate the burner from maximum to minimum gas pressure, in relation to the temperature of the boiler, operating via a temperature sensor. This eliminates the need for the boiler to constantly cycle on or off at high temperatures, thus reducing noise from the gas to water heat exchanger. This solenoid does not just open or close - it gradually decreases or increases from high fire to low fire. Accordingly, a DC voltage is required to operate it. On the most common modureg valve, this will operate between 0V DC low fire to 28V DC high fire.

Gas valve resistance testing

1. Carry out all preliminary safety checks and positively power.2. Remove any gas valve electrical box cover, where applicable.3. Identify the gas valve solenoid to be tested and disconnect the solenoid wires.4. Set the Fluke Multimeter to the Ohms scale.5. Place probes on the now vacated terminals of the gas valve solenoid and note the

reading.


A new fan has been fitted and a correct reading has been achieved (see table)


6. If the reading is O.L. this indicates a breakdown of the solenoid windings and will require renewal of the solenoid or gas valve.

7. If the reading is correct, as per the manufacturer's information, the disconnected wires can be refitted to their correct terminals on the solenoid.

8. If resistance is correct, check the power supply.9. Replace any electrical box cover that was removed earlier

Temperature Sensors

Temperature sensors are also known as thermistors. The sensors are either screwed into a wet pocket or strapped to the pipe work of the boiler - and a sensor will always give a resistance reading across its terminals. As the boiler is heated, the sensor temperature changes - and the resistance of the sensor either increases or decreases, depending on the type of sensor fitted by the manufacturer. Boiler designs differ widely - and whilst some manufacturers may use only one primary sensor, others may use two (the second being a hot water sensor). On recent condensing boilers there may up to three sensors: a flow sensor, a return sensor and a hot water sensor


Note the position of the probe’s on the Gas valve there are two coils on a gas valve both must be checked to ensure correct operation of the valve.

This picture shows the main gas solenoid and the modureg


Sensor types

There are three different types of sensors - and each has different testing methods.

Type 1: These are used on the older type Worcester boilers. They are tested by removing the wires from the sensor. The boiler should then fire.

Type 2: These are used on the older Saunier Duvals. They are tested by removing the wires from the sensor. Join them together and the boiler should fire.

Type 3: The most common sensor fitted today is the NTC (Negative Temperature Co-efficient). This requires a specific resistance reading, compared to the temperature where the sensor is fitted. If readings fall outside the stated parameters, the boiler will fail to operate. As the sensors of the NTC get hotter, the resistance reading of the sensor reduces. So, for example, at 25°C the sensor reading will be approximately 15,000 Ohms (15 kOhm). At 80°C the sensor reading will be approximately 1,500 Ohms (1.5 kOhm). The sensor is wired back to the PCB in the control box; the PCB's electronic circuitry adjusts the gas valve (Modureg) voltages, in turn reducing or increasing burner gas pressures.

Temperature sensor operation

Temperature sensors are a more effective way of controlling boiler temperatures in comparison to the older on/off-type boiler thermostats, because the boiler can be kept at a constant temperature.

As the boiler temperature increases, modulation of the burner pressure stops the burner from constantly going on/off. If hot water is run on a combination boiler with a standard on/off thermostat, the increase in water temperature will cause the burner to shut off. Then it will re-light as the boiler temperature reduces, resulting in slugs of cool water at the taps. With the thermistor, the burner reduces or increases as the temperature rises or cools. This allows for the maintenance of a steady temperature at the boiler, and a constant outlet temperature at the taps.

Thermistor testing

1. Carry out preliminary safety checks and positively isolate power.2. Identify and locate temperature sensor to be tested.3. Remove wires from the sensor.4. Set the Fluke Multimeter to the Ohms scale.5. Place probes onto the vacated terminals of the sensor.6. Note reading and check in relation to temperature at where the sensor is fitted

(using the manufacturer's data).7. If the sensor reading is O.L. or outside sensors parameters, clean or renew the

sensor.8. If the sensor reading is correct, replace wires back onto sensor.



Transformers

Transformers are used to reduce the voltage to various circuits and components contained in a boiler. Possible output voltages could be:

• 200-150V AC• 24V AC• 3VAC• 1.5V AC.

There are many uses for low voltages, such as:

• Variable fan speeds for fanned warm-air units• supplying low voltage for main circuits (feeding clock, boiler stat, gas valve)• Glow coil ignitions.

The transformer is made up of two different windings: the primary (230V AC) and the secondary windings (low voltage). The output voltage is taken off the secondary winding, through the component and back to the transformer. All transformer output voltage


The thermistor has been disconnected and upon testing was found to be faulty 0.L

A new sensor has been fitted and a satisfactory reading has been achieved


readings are volts AC. To check a transformer, it is necessary to check the resistance across both the primary and the secondary windings.

Transformer testing

1. Carry out all preliminary safety checks and positively isolate power.2. Identify and gain access to the transformer.3. Set the Fluke Multimeter to the Ohms scale.4. Remove wiring from the primary transformer windings (230V AC input).5. Place probes on the vacated terminals of the transformer and take a reading.6. If the reading is O.L. then the primary transformer's windings are faulty and the

transformer will need to be replaced.7. If the reading is satisfactory, refit the transformer wiring back onto the correct

terminals.8. Check the secondary windings in the same manner, rising the common Neutral of

the secondary windings for each output winding.9. If the readings are correct, check the power supplies


Transformer showing primary winding (240V AC - black and red wires) and secondary winding (24V AC -yellow wires). Opposite the coloured wires are the internal wires from the transformer. These also give alternative output voltages, from 100V AC to 210V AC

The boiler has been isolated. The main primary wiring has been removed, allowing for resistance testing of the primary windings of the transformer.

The secondary (yellow) wiring can also be removed, allowing for resistance testing of the low-voltage windings.

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