Baxi Duo Tec Fault Finding Guide
Domestic Hot Water Diagnostics
This guide provides a structured fault-finding process for diagnosing issues associated with the domestic hot water (DHW) service on the Baxi Duo-tec Combi boiler range. It covers electrical faults, error codes, and component-level diagnostics for parts like the pump, PCB, gas valve, and various sensors.
Initial Electrical and Display Checks
When starting diagnostics, activate a demand for both central heating and hot water. The first check is to see if the boiler’s user display is illuminated.
If the display is not illuminated, the fault is related to the electrical supply. A gas engineer should check the mains electrical supply to the boiler, including any external switches or fused spurs. Test the fuses within the boiler and the plug. If all external and internal electrical checks are satisfactory, the fault likely lies with the main printed circuit board (PCB), which may require replacement.
Error E110, E130, E133: Overheat Fault
The flashing error codes E110, E130, or E133 can indicate an overheat condition. The first action is to press the reset button for at least 5 seconds.
If the error persists after a reset, it suggests the overheat thermostat has activated or is faulty. To diagnose this, a multimeter should be used to test for continuity across the overheat thermostat’s terminals. If the boiler is hot, allow it to cool down completely before testing. If there is no continuity or the resistance is greater than 1.5 ohms, the overheat thermostat is faulty and must be replaced. If continuity is proven and the fault remains, the issue may be with the main PCB.
Error E20, E28, E50: Temperature Sensor Faults
Error codes E20, E28, or E50 indicate a potential fault with one of the boiler’s NTC temperature sensors (thermistors). The resistance of these sensors changes with temperature, and this can be measured with a multimeter to verify correct operation.
The resistance of the domestic hot water (DHW) and central heating (CH) sensors should be approximately $10k\Omega$ at $25°C$. The flue gas temperature sensor should have a resistance of approximately $20k\Omega$ at $25°C$. If the measured resistance values are significantly different from these benchmarks, the relevant sensor is faulty and should be replaced.
Error E119: Low Water Pressure
The error code E119 signifies that the central heating system water pressure is too low, typically below 0.5 bar. The first step is to visually check the system pressure gauge.
If the pressure is below 0.5 bar, the system needs to be repressurised using the filling loop until the pressure is within the recommended range, usually between 1.0 and 1.5 bar. If the system pressure is confirmed to be above 0.5 bar but the E119 error persists, the fault may lie with a faulty water pressure switch or its associated wiring, which should be tested for continuity and correct operation. Additionally, ensure the cap on the automatic air vent (AAV) is open to allow air to be purged from the system.
Pump and Circulation Faults
If there are no specific error codes but the boiler is not providing heat, check the circulation pump. With a demand for central heating active (thermostat set to maximum), verify if the pump is running.
If the pump is not running, confirm there is a legitimate demand for heat from the boiler’s controls. If demand is confirmed, check for 240V at the pump’s electrical connections. If 240V is present but the pump is not running, it may be seized or electrically faulty and will require replacement.
If 240V is not present at the pump, the next step is to test the output from the main PCB. Check for 230V at the PCB’s X3 connector, across terminals 3 and 4. If 230V is present at the PCB but not at the pump, the pump’s supply wiring harness is faulty. If there is no 230V output from the PCB terminals, the PCB itself is faulty and needs to be replaced.
Error E125, E193: Primary Circulation Fault
The error codes E125 or E193 indicate a problem with the circulation of water in the primary heating circuit. This can be caused by low water pressure (see Error E119), a pump failure, or a blockage within the system pipework or primary heat exchanger. If pump operation is confirmed, investigate for potential blockages restricting water flow.
Three-Way Valve Faults
A faulty three-way valve (also known as a diverter valve) can prevent the boiler from operating correctly for heating or hot water. Check that the valve is correctly positioned for the active demand (heating or hot water).
If the valve is not moving, test for 230V at the motorised valve’s electrical connections. If 230V is present, the valve motor is likely faulty, and the valve assembly should be replaced. If there is no voltage at the valve, check the PCB’s output at the X3 connector. There should be 230V across terminals 5 & 6 for a central heating demand, and across terminals 5 & 7 for a domestic hot water demand. If voltage is present at the PCB but not the valve, the wiring harness is faulty. If there is no voltage output from the PCB, the PCB requires replacement.
Error E160: Fan and Air Flow Faults
The error code E160 points to a fault with the fan. If the fan is not running at all, the first check is the wiring between the fan and the PCB. Test for a 230V supply from the PCB on the X2 connector at terminals 3 and 4.
If 230V is present at the PCB connector but the fan is not running, the fan is likely jammed or electrically faulty and needs to be tested and potentially replaced. If there is no 230V output from the PCB, the PCB is faulty. If the fan is running but the E160 fault persists, it may be running at an incorrect speed. In this case, test the PCB’s X401 connector at terminals 5, 6, 7, and 8. Any issues found here typically indicate a faulty fan unit.
Error E133: Ignition Failure
An E133 error code can mean a failure to ignite. If the boiler does not make a sparking sound during its ignition sequence (which involves three attempts of at least 5 seconds each), an ignition fault is present.
First, perform a boiler reset. If the fault continues, inspect the ignition system. Check the ignition electrode and its lead for damage or degradation. Ensure the electrical connection is secure. Verify that the spark gap and the position of the electrode relative to the burner are correct as per the manufacturer’s instructions.
If the boiler still fails to spark, use a multimeter to test for less than 100V DC across the PCB’s X2 connector, between terminals 2 (common) and 5. If the voltage is correct, the ignition electrode assembly is likely faulty. If the voltage is incorrect, the fault lies with the PCB.
Error E133: Flame Rectification Failure
A flame rectification fault also presents as error E133, but this occurs when the burner lights successfully but then extinguishes after approximately 5 seconds. This indicates the boiler has failed to sense a stable flame.
Check that the incoming dynamic gas supply pressure is correct, typically between 10-11 mbar. Inspect the flame sensing electrode and its lead for damage, secure connections, and correct positioning. The flame rectification current must be greater than $0.5\mu A$; if it is below this value, clean or replace the flame sensing electrode. If all these checks are satisfactory and the fault persists, the PCB is likely faulty.
Gas Valve and Burner Faults
If the boiler completes its spark ignition sequence but the burner does not light, the issue may be with the gas supply. Ensure the main gas supply is turned on and the pipework is purged of air.
Inspect the gas valve and the venturi for blockages or damage, and clean them if necessary. At the PCB’s X3 connector, check for a 230V DC supply to the gas valve during the ignition sequence. If this voltage is present but no gas is being delivered to the burner, the gas valve is faulty and must be replaced. If no voltage is being supplied from the PCB, then the PCB needs to be replaced.
Error E130: Modulation and Temperature Control Faults
If the boiler fires but fails to modulate its output to maintain the set temperature, and an E130 error is displayed, this indicates a temperature control issue. This can also manifest as poor hot water performance.
First, check the domestic hot water flow rate and reduce it if it is excessive. If the boiler still fails to modulate correctly, the issue could be caused by a scaled-up or dirty DHW NTC sensor, which will not read the temperature accurately. The sensor should be cleaned or replaced. A scaled-up or blocked domestic hot water plate heat exchanger can also cause poor modulation and temperature control, and it may need to be cleaned or replaced.
DHW Sequence of Operation
The following describes the correct sequence of operation for a successful domestic hot water cycle on the Baxi Duo-tec combi boiler.
- A hot water tap is opened, and the boiler’s flow sensor detects water movement.
- The boiler initiates its ignition sequence, the burner lights, and heats the water.
- The hot water tap is closed, and the flow sensor detects that water movement has stopped.
- The burner immediately extinguishes.
- The fan continues to run for a post-purge period of 10 seconds.
- The pump continues to run for a pump overrun period of 30 seconds to dissipate residual heat from the heat exchanger.
- The boiler returns to a standby state, having completed a successful DHW sequence.
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