Herz
Herz Boilers Error Codes
Complete list of fault codes and troubleshooting guides. Use the search below to find your specific error.
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A01
Boiler locks out, burner does not ignite, red fault LED is on, display shows A01.
Ignition Failure / No Flame Detected
A02
Boiler starts, flame appears briefly, then shuts down and locks out with code A02.
Flame Loss During Operation
A03
Boiler locks out shortly after start, display shows A03, overheat LED may be lit.
Safety Temperature Limiter (STB) Tripped / Overheat
A04
Boiler does not start, display shows A04, no fan operation.
Fan / Air Pressure Fault
A05
Boiler locks out with A05, fan runs but no ignition sequence starts.
Air Pressure Switch Not Proving
A06
Boiler shows A06, intermittent ignition, sometimes a noisy start.
Ignition Spark Fault
A07
Boiler display shows A07, no heating or hot water, pump may run briefly then stop.
Flow Temperature Sensor Fault (NTC Flow Open/Short)
A08
Boiler shows A08, temperature reading is fixed or unrealistic, boiler may cycle rapidly.
Return Temperature Sensor Fault (NTC Return Open/Short)
A09
Boiler locks out with A09, DHW temperature unstable or not reaching setpoint.
DHW Temperature Sensor Fault (NTC DHW)
A10
Boiler shows A10, no heating, pump may run continuously.
Flow Switch / Flow Sensor Fault
A11
Boiler display shows A11, burner stops, fan may continue to run.
Flue Gas Temperature Too High
A12
Boiler shows A12, will not start, sometimes after a power surge.
Control Board Internal Fault
A13
Boiler shows A13, external safety circuit open, no operation.
External Safety Chain Open (e.g., limit stats, external interlocks)
A14
Boiler shows A14, burner output limited, may not reach set temperature.
Outdoor Sensor Fault / Weather Compensation Error
A15
Boiler shows A15, no response to room thermostat, heating stays off.
Room Thermostat / Control Signal Fault
A16
Boiler shows A16, frequent short cycling, limited modulation.
Modulation / Gas Valve Control Fault
A17
Boiler shows A17, pump runs but burner does not start, system may be cold.
Low Water Pressure / Water Shortage
A18
Boiler shows A18, pressure rises rapidly during heating, safety valve may discharge.
Overpressure / Expansion Fault
A19
Boiler shows A19, condensate pump runs frequently or boiler shuts down.
Condensate Drain / Pump Fault
A20
Boiler shows A20, burner stops unexpectedly, flue gas safety device activated.
Flue Gas Safety Thermostat Trip
A21
Boiler shows A21, burner output limited, fan speed abnormal.
Fan Speed / Tachometer Fault
A22
Boiler shows A22, DHW priority fault; heating may be disabled.
DHW Flow Switch / Turbine Fault
A23
Boiler shows A23, communication error with remote controller or cascade manager.
BUS Communication Fault
A24
Boiler shows A24, burner stops, system temperature rises unexpectedly.
Pump Fault / No Circulation
A25
Boiler shows A25, burner locked out; gas supply appears normal.
Gas Valve Safety Shutoff Fault
A26
Boiler shows A26, system temperature sensor mismatch, erratic operation.
Sensor Calibration / Mismatch Fault
A27
Boiler shows A27, burner stops during DHW draw, temperature overshoots.
Plate Heat Exchanger Blockage / DHW Overheat
A28
Boiler shows A28, cascade system not responding correctly.
Cascade Control / Master-Slave Communication Fault
A29
Boiler shows A29, burner output limited, oxygen level control fault (if equipped).
Lambda / O2 Sensor Fault
A30
Boiler shows A30, internal clock or scheduling not working.
Internal Clock / Backup Battery Fault
Symptom
Boiler cycles on and off rapidly without reaching room temperature
Short Cycling / Oversized Boiler or Control Issue
Symptom
Boiler does not respond to any buttons on the front panel
User Interface / Keypad Failure
Symptom
Boiler emits a burning smell or plastic smell during operation
Overheating Electrical Component / Insulation
Symptom
Boiler fan runs continuously even when there is no heat demand
Post-Purge / Control Stuck
Symptom
Boiler flue terminal shows white staining and excessive condensate dripping
Condensate Management / Flue Slope Issue
Symptom
Boiler frequently loses pressure and needs topping up
Chronic Pressure Loss
Symptom
Boiler leaking water from underneath the casing
General Water Leak in Hydraulic Circuit
Symptom
Boiler makes loud banging or knocking noises when starting or stopping
Kettling / Hydraulic Shock
Symptom
Boiler produces a whistling or high-pitched noise during firing
Combustion Resonance / Gas Flow Noise
Symptom
Boiler runs but radiators remain cold or only lukewarm
Poor Circulation / Balancing Issue
Symptom
Boiler starts normally but shuts down after a few minutes without error code
Intermittent Overheat or Control Interruption
Symptom
Hot water temperature fluctuates between hot and cold during use
DHW Temperature Instability
Symptom
Radiators get hot even when there is no call for heating
Unwanted Heating / Valve Bypass
| Error Code | Symptom | Fault Type | Guide |
|---|---|---|---|
| A01 |
Boiler locks out, burner does not ignite, red fault LED is on, display shows A01.
|
Ignition Failure / No Flame Detected | View Details |
| A02 |
Boiler starts, flame appears briefly, then shuts down and locks out with code A02.
|
Flame Loss During Operation | View Details |
| A03 |
Boiler locks out shortly after start, display shows A03, overheat LED may be lit.
|
Safety Temperature Limiter (STB) Tripped / Overheat | View Details |
| A04 |
Boiler does not start, display shows A04, no fan operation.
|
Fan / Air Pressure Fault | View Details |
| A05 |
Boiler locks out with A05, fan runs but no ignition sequence starts.
|
Air Pressure Switch Not Proving | View Details |
| A06 |
Boiler shows A06, intermittent ignition, sometimes a noisy start.
|
Ignition Spark Fault | View Details |
| A07 |
Boiler display shows A07, no heating or hot water, pump may run briefly then stop.
|
Flow Temperature Sensor Fault (NTC Flow Open/Short) | View Details |
| A08 |
Boiler shows A08, temperature reading is fixed or unrealistic, boiler may cycle rapidly.
|
Return Temperature Sensor Fault (NTC Return Open/Short) | View Details |
| A09 |
Boiler locks out with A09, DHW temperature unstable or not reaching setpoint.
|
DHW Temperature Sensor Fault (NTC DHW) | View Details |
| A10 |
Boiler shows A10, no heating, pump may run continuously.
|
Flow Switch / Flow Sensor Fault | View Details |
| A11 |
Boiler display shows A11, burner stops, fan may continue to run.
|
Flue Gas Temperature Too High | View Details |
| A12 |
Boiler shows A12, will not start, sometimes after a power surge.
|
Control Board Internal Fault | View Details |
| A13 |
Boiler shows A13, external safety circuit open, no operation.
|
External Safety Chain Open (e.g., limit stats, external interlocks) | View Details |
| A14 |
Boiler shows A14, burner output limited, may not reach set temperature.
|
Outdoor Sensor Fault / Weather Compensation Error | View Details |
| A15 |
Boiler shows A15, no response to room thermostat, heating stays off.
|
Room Thermostat / Control Signal Fault | View Details |
| A16 |
Boiler shows A16, frequent short cycling, limited modulation.
|
Modulation / Gas Valve Control Fault | View Details |
| A17 |
Boiler shows A17, pump runs but burner does not start, system may be cold.
|
Low Water Pressure / Water Shortage | View Details |
| A18 |
Boiler shows A18, pressure rises rapidly during heating, safety valve may discharge.
|
Overpressure / Expansion Fault | View Details |
| A19 |
Boiler shows A19, condensate pump runs frequently or boiler shuts down.
|
Condensate Drain / Pump Fault | View Details |
| A20 |
Boiler shows A20, burner stops unexpectedly, flue gas safety device activated.
|
Flue Gas Safety Thermostat Trip | View Details |
| A21 |
Boiler shows A21, burner output limited, fan speed abnormal.
|
Fan Speed / Tachometer Fault | View Details |
| A22 |
Boiler shows A22, DHW priority fault; heating may be disabled.
|
DHW Flow Switch / Turbine Fault | View Details |
| A23 |
Boiler shows A23, communication error with remote controller or cascade manager.
|
BUS Communication Fault | View Details |
| A24 |
Boiler shows A24, burner stops, system temperature rises unexpectedly.
|
Pump Fault / No Circulation | View Details |
| A25 |
Boiler shows A25, burner locked out; gas supply appears normal.
|
Gas Valve Safety Shutoff Fault | View Details |
| A26 |
Boiler shows A26, system temperature sensor mismatch, erratic operation.
|
Sensor Calibration / Mismatch Fault | View Details |
| A27 |
Boiler shows A27, burner stops during DHW draw, temperature overshoots.
|
Plate Heat Exchanger Blockage / DHW Overheat | View Details |
| A28 |
Boiler shows A28, cascade system not responding correctly.
|
Cascade Control / Master-Slave Communication Fault | View Details |
| A29 |
Boiler shows A29, burner output limited, oxygen level control fault (if equipped).
|
Lambda / O2 Sensor Fault | View Details |
| A30 |
Boiler shows A30, internal clock or scheduling not working.
|
Internal Clock / Backup Battery Fault | View Details |
| – |
Boiler cycles on and off rapidly without reaching room temperature
|
Short Cycling / Oversized Boiler or Control Issue | View Details |
| – |
Boiler does not respond to any buttons on the front panel
|
User Interface / Keypad Failure | View Details |
| – |
Boiler emits a burning smell or plastic smell during operation
|
Overheating Electrical Component / Insulation | View Details |
| – |
Boiler fan runs continuously even when there is no heat demand
|
Post-Purge / Control Stuck | View Details |
| – |
Boiler flue terminal shows white staining and excessive condensate dripping
|
Condensate Management / Flue Slope Issue | View Details |
| – |
Boiler frequently loses pressure and needs topping up
|
Chronic Pressure Loss | View Details |
| – |
Boiler leaking water from underneath the casing
|
General Water Leak in Hydraulic Circuit | View Details |
| – |
Boiler makes loud banging or knocking noises when starting or stopping
|
Kettling / Hydraulic Shock | View Details |
| – |
Boiler produces a whistling or high-pitched noise during firing
|
Combustion Resonance / Gas Flow Noise | View Details |
| – |
Boiler runs but radiators remain cold or only lukewarm
|
Poor Circulation / Balancing Issue | View Details |
| – |
Boiler starts normally but shuts down after a few minutes without error code
|
Intermittent Overheat or Control Interruption | View Details |
| – |
Hot water temperature fluctuates between hot and cold during use
|
DHW Temperature Instability | View Details |
| – |
Radiators get hot even when there is no call for heating
|
Unwanted Heating / Valve Bypass | View Details |
No matching codes found
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