Oil Boiler Fault Finding: No Ignition, Lockout Causes, Smoke, Sooting and Pressure Gauge Readings

Summary

Oil boiler fault finding requires a methodical approach because the oil combustion system has more mechanical components than a gas boiler — pump, nozzle, photocell, ignition electrodes, solenoid valve — and each can fail independently. The burner control box (often a Danfoss or Riello unit) generates a lockout condition if the firing sequence does not complete successfully, but the lockout is a symptom, not a root cause.

OFTEC-registered engineers are the only qualified persons for oil boiler work in the UK. This guide is written for registered OFTEC engineers and those working under their supervision. Attempting oil boiler repairs without appropriate competence and gas-free verification procedures is dangerous.

Understanding the firing sequence helps enormously with fault diagnosis. A typical pressure jet burner fires in this sequence: thermostat calls for heat → burner motor starts → air purge (pre-purge) → ignition electrodes fire → solenoid valve opens (fuel to nozzle) → photocell detects flame → control box locks to running state. Any break in this chain causes lockout.

Key Facts

• Lockout reset — press the reset button on the control box; only reset once without diagnosis — repeated resetting can flood the combustion chamber with unburnt oil, creating an explosion risk
• Pre-purge period — most control boxes purge for 10–15 seconds before opening the fuel solenoid; if no flame is detected within 5–10 seconds of solenoid opening, the box locks out
• Photocell (cadmium sulphide cell) — detects the infrared radiation from the oil flame; if dirty, cracked, or failed, it cannot detect flame and triggers lockout; also triggers lockout if the photocell "sees" daylight in certain installations (check for stray light paths)
• Nozzle wear — nozzles should be replaced annually at service; a worn nozzle changes spray pattern and droplet size, causing poor combustion and potential lockout
• Nozzle blockage — fuel line contamination (diesel bug, wax) blocks the nozzle orifice (0.4–0.75 gallons/hour); causes no-start or lockout after brief flame
• Ignition electrode gap — must be set to manufacturer's spec (typically 2.5–3.5mm between electrodes, 1.5–2mm from nozzle tip); incorrect gap causes weak or absent spark
• Oil pump pressure — typically 100–120 psi for domestic pressure jet burners; test with a pressure gauge at the pump outlet port; low pressure = worn pump or internal bypass valve failure
• Fuel starvation — blocked filter, waxed fuel in cold weather, empty tank (check the gauge — float may be stuck), or failed oil pump
• Flue gas analysis targets — CO₂: 12–13% for kerosene; O₂: 3–5%; CO: <100 ppm; Bacharach smoke number: 0–1 maximum
• Yellow/smoky flame — high CO, high smoke number; causes: excess fuel (nozzle too large), insufficient air, wrong fuel type, or nozzle defect
• Sooting — indicates persistent combustion problem; soot on the heat exchanger reduces efficiency by ~10% per mm of deposit
• Overheat lockout — high limit thermostat trips at approximately 100–110°C; causes: failed pump, blocked primary circuit, airlock, or too high a flow temperature setting

Quick Reference Table

Detailed Guidance

Fault Finding Decision Tree

BURNER FAILS TO START / LOCKS OUT
│
├─ Does the burner motor run?
│   │
│   ├─ NO → Check control box output to motor
│   │          Check motor windings (resistance)
│   │          Check control box fuse
│   │          Check thermostat call signal to control box
│   │
│   └─ YES → Does the burner spark?
│              │
│              ├─ NO → Check ignition transformer output (>15,000V)
│              │        Check electrode gap (2.5–3.5mm)
│              │        Check electrode insulator for cracks
│              │        Check HT lead connections
│              │
│              └─ YES → Is there flame?
│                         │
│                         ├─ NO (short attempt, no flame) →
│                         │   Check fuel supply:
│                         │   - Pump pressure (target 100–120 psi)
│                         │   - Filter (blocked?)
│                         │   - Fire valve (open?)
│                         │   - Manual shut-off valve (open?)
│                         │   - Tank level (is there fuel?)
│                         │   Check nozzle (blocked or worn)
│                         │
│                         └─ YES but LOCKS OUT →
│                             Photocell issue:
│                             - Clean photocell lens
│                             - Test: resistance in light ~1kΩ, dark >500kΩ
│                             - Check for stray light path
│                             - Replace photocell if failed

Testing the Oil Pump

The oil pump is one of the most frequently overlooked components. A worn pump will operate but deliver insufficient pressure to the nozzle, resulting in a weak flame and intermittent lockout.

Testing procedure:

1. Fit a pressure gauge to the pump outlet port (most Danfoss and Suntec pumps have a standard gauge port)
2. Start the burner and observe pressure at steady state
3. Target: 100–120 psi (6.9–8.3 bar); manufacturers will specify their pump's target pressure
4. If pressure is low: check for air in the fuel line (bleed the pump using the bleed screw); if pressure remains low after bleeding, the pump internal bypass valve may be worn
5. Check vacuum on the inlet side with a vacuum gauge: should not exceed 0.4 bar; high vacuum indicates a restriction (blocked filter, collapsed flexible hose)

Pump replacement is straightforward on most domestic burners — standard flange mounting with a flex coupling. Confirm the new pump's rotation direction (clockwise or anti-clockwise when viewed from drive end) matches the original.

Photocell Diagnosis

The cadmium sulphide (CdS) photocell is a light-sensitive resistor. In the dark, its resistance is high (>500kΩ on a good cell). When exposed to the light of the oil flame, resistance drops dramatically (typically 1–10kΩ). The control box monitors this resistance change to confirm flame presence.

Testing without a flame:

• Remove the photocell from the burner
• Measure resistance with a multimeter in a darkened room: should be >500kΩ (open circuit effectively)
• Expose to bright light (phone torch): resistance should drop to ~1kΩ or below
• A cell that does not change resistance is faulty and must be replaced

Common photocell issues:

• Soot coating: accumulation of soot on the lens reduces light transmission; wipe with a clean dry cloth (do NOT use solvents — they leave a residue that attracts more soot)
• Cracked lens: physical damage; replace
• Stray daylight: if a light path exists from outside the burner into the photocell (loose tube, cracked burner housing), the cell "sees" daylight even when the burner is off, and will not signal "no flame" at the end of a firing cycle; this causes the control box to lose track of its sequence
• Aged cell: CdS cells degrade over time; replace as part of every annual service

Ignition Electrode Inspection and Setting

The ignition electrodes must be precisely set. Specifications vary by burner manufacturer but typical domestic values are:

Electrodes must be clean and free from carbon deposits. Carbon bridging across the electrode tip causes a weak or absent spark. Clean with fine abrasive paper; replace if the ceramic insulator is cracked.

The ignition transformer produces high voltage (15,000–20,000V) AC at the electrode tips. Do NOT touch the HT leads when the burner is running. Test the transformer output with an approved HT tester or by observation of spark quality.

Combustion Analysis and Smoke Testing

Every oil boiler service must include a flue gas analysis and Bacharach smoke test. These are not optional — OFTEC OFT105 requires recorded flue gas readings.

Target readings for kerosene (Class C2):

• CO₂: 12–13% (at nozzle size and pump pressure specified by manufacturer)
• O₂: 3–5% (inverse relationship with CO₂ — excess air dilutes CO₂)
• CO: <100 ppm; values >100 ppm indicate incomplete combustion
• Flue temperature: typically 200–280°C; very high temperature indicates scale on heat exchanger
• Bacharach smoke number: 0 to 1 maximum; 0 = clean, 4+ = visibly sooty, unacceptable

Common combustion faults:

• Low CO₂ + low CO = excess air (combustion air damper too open or leak in combustion chamber)
• High CO₂ + high CO = insufficient air (damper too closed, air inlet blocked)
• High smoke number = incomplete combustion (nozzle fault, wrong fuel, air problem)
• High flue temperature = scale buildup on heat exchanger; schedule descale or heat exchanger replacement

Sooting: Causes and Remediation

Soot on the combustion chamber walls and heat exchanger is the most common consequence of chronic combustion problems. A 1mm layer of soot reduces heat transfer efficiency by approximately 10%, meaning a 92%-efficient boiler effectively drops to ~82%.

Causes of sooting:

• Wrong fuel (gas oil in a kerosene boiler)
• Oversized nozzle (too much fuel)
• Insufficient combustion air
• Failed nozzle (damaged spray pattern)
• Blocked air tube (lint, debris)
• Low pump pressure

Remediation:

• Identify and fix the root combustion problem first — brushing soot out without fixing the cause means the soot returns immediately
• Brush heat exchanger with purpose-made brushes
• Vacuum soot from combustion chamber and heat exchanger surfaces
• Check flue for soot accumulation — if significant, clean and inspect for structural integrity
• Re-commission with flue gas analysis to confirm problem resolved

Water-Side Faults

Oil boilers have the same water-side issues as any other boiler:

Low system pressure: Check for leaks (visual inspection at radiator valves, connections, pump). Check expansion vessel pre-charge (should match static head of system, typically 0.5–1.5 bar). PRV set pressure typically 3 bar — if weeping, either the system overpressures or the PRV is at end of life.

No circulation: Check pump operation (listen for hum/vibration; check capacitor; check for seized shaft). Check for airlocks (bleed radiators; check pump has prime). Check motorised valves (mid-position valve can fail and block flow to zones).

Boiler overheating / high limit trips: If the high limit thermostat is tripping, the primary issue is usually a circulation failure — check the pump, zone valves, and look for a blocked primary circuit before assuming the high limit thermostat itself is faulty.

Frequently Asked Questions

Is it safe to reset the lockout button more than once?

No. Reset once, observe the firing sequence, and diagnose if it locks out again. Repeated resets can pump unburnt fuel oil into the combustion chamber — if the ignition then fires, the accumulated fuel can ignite explosively. If the boiler locks out a second time, leave it in lockout and diagnose the cause before attempting another reset.

The boiler worked fine this morning and now it's locked out — what has changed?

Intermittent lockouts are often caused by a component at the end of its reliable life — the photocell, the oil pump pressure dropping as it warms up, or an electrode that works when cold but fails when hot. Carry out a full service including nozzle replacement, photocell replacement, and combustion analysis before concluding it's an intermittent fault requiring further investigation.

My customer says there is an oil smell — is this dangerous?

An oil smell in a boiler room can indicate: a fuel line leak at a compression or olive joint; an over-fired burner (too much fuel causing partial quenching); or a cracked combustion chamber allowing flue gases with unburnt fuel vapour to escape into the room. This is a serious concern. Shut the boiler down, isolate the oil supply, and inspect all fuel connections and the combustion chamber before re-commissioning.

How do I know if the nozzle size is correct?

The nozzle size is stamped on the nozzle body (e.g., "0.55 80° H" = 0.55 gallons/hour at 80° spray angle, hollow cone). Compare this to the boiler manufacturer's specification for the installed heat output. If the nozzle has been changed to a non-standard size, the combustion analysis will reveal it — CO₂ out of range is the main indicator.

Regulations & Standards

• OFTEC OFT105 — commissioning certificate; combustion analysis results must be recorded

• Building Regulations Approved Document J — all oil heating work by OFTEC-registered engineers

• BS 5410-1:2019 — code of practice for oil-fired heating equipment for domestic use; covers installation, servicing, and commissioning requirements

• OFTEC Technical Book 3 — comprehensive servicing and fault finding guidance for oil heating engineers

• Health and Safety at Work Act 1974 — general duty of care applicable to oil heating engineering

• OFTEC: Technical guidance for oil heating engineers — primary reference for fault finding procedures

• Danfoss: Burner control box manual (OBC) — control box sequence and fault codes

• Riello: Burner installation and service manuals — burner-specific fault finding

• GOV.UK: Approved Document J — building regulations reference

• oil boiler service procedure — full annual service checklist including combustion analysis

• kerosene vs gas oil — fuel type diagnosis when wrong fuel is suspected

• oftec competent person scheme — OFTEC registration requirements for oil heating work

• condensate pipe oil boiler — condensate faults specific to oil condensing boilers