Guide
Common Boiler Faults
A quick-reference guide to the most common boiler faults, their likely causes, and what to check. From low water and high pressure to flame failure and nuisance trips.
7 min readUpdated July 2026
Low Water Level
- Likely causes: Feed pump failure, makeup water supply failure, level control valve stuck closed, blowdown valve leaking, gauge glass reading incorrectly (actual level is fine but glass reads low).
- What to check: Feed pump operation, makeup water supply, level controller output, blowdown valve seal, gauge glass accuracy (blow down to verify).
- Action: If the low-water cutoff has tripped, do NOT restart the boiler until the cause is found and fixed. If the boiler has run dry, allow it to cool completely before adding water.
High Pressure Trip
- Likely causes: Pressure controller failure, modulating valve stuck, burner stuck on high fire, sudden reduction in steam demand, safety valve set too close to operating pressure.
- What to check: Pressure controller output, burner firing rate, modulating valve position, steam demand vs supply balance, safety valve set pressure.
- Action: Reduce firing rate manually if possible. Investigate the control system. If the safety valve lifted, investigate why the controls failed to maintain pressure.
Flame Failure
- Likely causes: Fuel supply interruption, contaminated fuel, dirty burner nozzle, flame scanner dirty or misaligned, combustion air supply problem, unstable flame.
- What to check: Fuel pressure and quality, burner nozzle condition, flame scanner lens, combustion air damper position, draft conditions.
- Action: Do NOT attempt to relight without purging the furnace. Follow the manufacturer's startup sequence. If flame failure recurs, call a burner technician.
Nuisance Trips
- Likely causes: Flame scanner dirty or misaligned, loose electrical connections, faulty sensors, control system glitch, unstable flame at low fire.
- What to check: Clean flame scanner lens. Check all electrical connections. Verify sensor calibration. Observe flame stability at all firing rates.
- Action: Recurrent nuisance trips should be investigated by a qualified technician. Do not bypass safety interlocks.
Boiler Not Reaching Pressure
- Likely causes: Excess steam demand (load too high), burner not firing at full capacity, scaled tubes reducing heat transfer, excess combustion air, feedwater too cold.
- What to check: Compare steam demand to boiler capacity. Check burner firing rate and combustion. Inspect for scale. Verify feedwater temperature.
- Action: Reduce demand or increase capacity. Clean tubes if scaled. Tune combustion. Improve feedwater heating.
Smoky Flue Gas
- Likely causes: Insufficient combustion air, dirty burner nozzle (oil), incorrect fuel/air ratio, contaminated fuel.
- What to check: Flue gas O2 and CO. Burner nozzle. Combustion air supply. Fuel quality.
- Action: Increase combustion air. Clean or replace burner nozzle. Retune combustion. Have fuel analysed if contamination suspected.
Never bypass safety interlocks
When a boiler trips, the temptation is to bypass the interlock and keep running. This is extremely dangerous. Every interlock exists because someone died when it was absent. Find and fix the root cause before restarting.
Tip: Keep a fault log. Record every trip, the suspected cause, and the corrective action. Over time, patterns emerge that reveal underlying issues. PlantLogica's maintenance logging system can track this automatically.