Commercial Boiler Fault Library
Engineering diagnosis of commercial boiler faults: ignition and flame failure, lockouts, overheating, sensors, fans, gas valves, flues, condensate, pressure and combustion.
Fault codes differ between Ideal, Hamworthy, Remeha, Viessmann, Baxi, Potterton, Hoval and the rest, but the underlying faults do not. An ignition lockout on an Evomax and on a Quinta is the same physics: no spark, no gas, or no flame signal. This library is organised by fault, not by code, so the diagnosis applies whichever boiler is on the wall.
Each guide sets out what the fault means, the common causes in order of likelihood, the checks a commercial engineer carries out, and how to stop it coming back. Use the manufacturer pages for brand-specific support, and call us when plant is down.
Guides in this section
- Commercial Boiler Ignition Failure: Causes & Diagnosis
Why a commercial boiler fails to ignite: no spark, no gas or no flame proving — and the checks that separate them.
- Commercial Boiler Flame Failure Explained
What a flame failure lockout on a commercial boiler means, and how engineers trace flame detection, gas supply and combustion faults.
- Boiler Locks Out After Ignition: Why It Happens
Why a commercial boiler lights then locks out seconds later, and the usual causes from flame sensing to gas pressure.
- Flame Loss During Operation on Commercial Boilers
Why a commercial boiler loses its flame mid-run and how engineers diagnose intermittent flame loss.
- Commercial Boiler Overheating: Causes & Fixes
Why commercial boilers overheat or trip on high temperature, and how engineers trace flow, control and pump faults.
- Blocked Commercial Boiler Heat Exchanger
Signs, causes and remedies for blocked commercial boiler heat exchangers on the water and combustion sides.
- High Differential Temperature (ΔT) on a Commercial Boiler
What a high flow-return temperature difference means and why commercial boilers lock out on high ΔT.
- Low Differential Temperature on a Commercial Boiler
Why a low flow-return ΔT causes short cycling, poor condensing efficiency and wasted energy in commercial plant.
- Low System Pressure on Commercial Boilers
Why sealed commercial heating systems lose pressure and how engineers find leaks, expansion vessel and pressurisation faults.
- High System Pressure on Commercial Boilers
Causes of high pressure on sealed commercial heating systems, from expansion vessels to passing filling loops.
- Commercial Boiler Short Cycling
Why commercial boilers fire and stop repeatedly, the costs, and how to fix short cycling with controls and hydraulics.
- Commercial Boiler Fan Fault
How commercial boiler fan faults present and how engineers test fan speed, wiring and air proving.
- Commercial Boiler Gas Valve Fault
Symptoms of a failing commercial boiler gas valve, how to test it and why combustion must be checked afterwards.
- Boiler Ignition Electrode Fault
How worn or misaligned ignition electrodes cause commercial boiler ignition failures, and correct replacement.
- Boiler Ionisation (Flame Sensing) Fault
How flame ionisation works on commercial boilers and why a weak ionisation current causes lockouts.
- Commercial Boiler Flue Fault
Causes of flue faults on commercial boilers — blockages, leaks, terminal problems — and the safety implications.
- Commercial Boiler Condensate Fault
Why condensate blockages and frozen pipes cause commercial boiler lockouts, and how to fix them permanently.
- Commercial Boiler PCB Fault
How to confirm a genuine PCB fault on a commercial boiler before replacing an expensive control board.
- Commercial Boiler Pump Fault
Diagnosing commercial heating pump faults: seized pumps, air locks, wiring and control signals.
- Boiler Flow Sensor Fault
How flow temperature sensor faults affect commercial boilers and how to test NTC sensors.
- Boiler Return Sensor Fault
What a return temperature sensor fault means on commercial boilers and how it affects ΔT protection.
- Boiler High-Limit Thermostat Fault
Why a commercial boiler's high-limit thermostat trips and why it must never be bypassed.
- Poor Combustion Readings on Commercial Boilers
What poor flue gas analyser readings indicate on commercial boilers and how combustion is corrected.
- High CO Readings on Commercial Boilers
What high carbon monoxide readings mean on commercial boilers and the action engineers must take.
- Incorrect CO/CO₂ Ratio: What It Means
How the CO/CO₂ ratio is used to assess commercial boiler combustion and what a high ratio indicates.