Oil Boiler Service Procedure: Nozzle Replacement, Combustion Analysis Targets, Flue Draught and Fire Valve Check

Quick Answer: An annual oil boiler service includes: replace the nozzle (always — nozzles degrade over 12 months); clean the heat exchanger, combustion chamber, and burner head; check and clean the oil filter; set electrode gap; fire up and perform combustion analysis (target CO₂ 10–13%, CO <100ppm, smoke ≤1); check flue draught (-0.1 to -0.2 mbar); test the remote fire valve; and issue an OFTEC service record. The service must be performed by an OFTEC-registered engineer.

Summary

Annual servicing is a condition of most oil boiler manufacturer warranties and is strongly recommended for safety, efficiency, and reliability. Unlike gas boilers, oil appliances require replacement of consumable components (nozzle, oil filter cartridge) at every service — these items have a finite service life and their condition directly affects combustion quality and safety.

For OFTEC-registered engineers, the annual service procedure is defined in OFTEC OFT600. The service must be documented and an OFTEC service record issued.

Key Facts

• Annual service interval — oil boilers should be serviced annually; some manufacturers require it as a warranty condition; OFTEC recommends annual service regardless of warranty
• Nozzle replacement — the nozzle atomises the oil into a fine spray for combustion; it degrades over time (wear, partial blockage, distortion) causing poor spray pattern and reduced combustion quality; always replace at every service — do not clean and reuse
• Nozzle specification — nozzles are rated by flow rate (gallons per hour, USgph), spray angle (°), and pattern (S=solid, H=hollow, B=semi-hollow); always replace with the same specification as the boiler/burner manufacturer's data plate; the nozzle determines the heat input — fitting the wrong nozzle is a serious safety and efficiency error
• Oil filter — the oil supply line filter catches contaminants before they reach the burner pump and nozzle; a blocked filter causes oil starvation and lockout; replace the filter cartridge at every service; inspect and clean the filter body
• Heat exchanger cleaning — combustion deposits (soot, unburned carbon) accumulate on the heat exchanger surfaces over 12 months; clean with a brush and vacuum to restore heat transfer efficiency; excessive soot indicates combustion problems — investigate before cleaning
• Combustion chamber inspection — inspect the ceramic combustion chamber/fire pot for cracks or deterioration; a cracked combustion chamber allows combustion gases to bypass the heat exchanger and may cause CO ingress into the building; replace if cracked
• Electrodes — the electrodes ignite the oil spray; check gap and position against the burner manufacturer's setting data; clean carbon deposits; replace if cracked or heavily worn
• Burner head (diffuser) — the air diffuser (stabiliser) behind the nozzle creates turbulence to mix air and oil; clean of carbon deposits; check for distortion
• Photocell (flame sensor) — the photocell detects the flame and signals the burner controller; clean the glass face with a dry cloth; a dirty photocell causes nuisance lockouts or failure to detect the flame
• Remote fire valve (RFV) test — the fire valve in the oil supply line must be tested at each service to confirm it operates freely; do not test by triggering the heat sensor (single-use); test by manually pressing the test button (if fitted) or visually confirming the valve is not seized; record that the test was performed
• Flue inspection — inspect the flue for blockage, bird nests (seasonal), deterioration of the liner or joints; measure flue draught; clean the flue if required
• Oil tank condition — visually inspect the tank for leaks, corrosion, or damage; check oil level; check the tank filter at the outlet; check the vent pipe is unobstructed

Service Procedure Sequence

1. Preparation and Shutdown

• Confirm oil supply valve is operational; check approximate oil level in tank
• Shut down the boiler; allow to cool (minimum 30 minutes after shutdown before opening burner)
• Isolate electrical supply at the local isolation switch

2. Burner Removal and Inspection

• Remove the burner from the boiler; place on a clean surface
• Remove the nozzle assembly: unscrew the nozzle holder; withdraw electrode assembly
• Replace the nozzle with a new unit of the same specification (flow rate, angle, pattern)
• Set electrode gap per the burner manufacturer's setting card (typically 3–4mm gap, 1–2mm above and 2–3mm forward of nozzle)
• Clean the electrodes and burner head with a wire brush; check for cracks
• Clean the photocell glass face
• Check the oil pump pressure (measure with a pressure gauge at the pump test port); set to the manufacturer's specification (typically 7–10 bar)
• Replace the burner head gasket if deteriorated

3. Heat Exchanger and Combustion Chamber Cleaning

• Remove access covers on the boiler; vacuum out loose soot and debris
• Brush the heat exchanger flueways with a suitable brush; vacuum again
• Inspect the combustion chamber/fire pot for cracks; clean carbon deposits
• Check baffle plates or turbulators (if fitted); clean or replace if deteriorated
• Refit all access covers and seals securely

4. Oil Filter Replacement

• Isolate the oil supply to the boiler (oil supply valve at tank or in-line valve)
• Unscrew the filter bowl; discard the old cartridge
• Clean the filter bowl; fit a new cartridge
• Refit the bowl; open the oil supply valve; check for leaks at the filter housing

5. Flue Inspection

• Inspect the flue system (accessible portions): flexible liner, flue pipe, terminal
• Check for blockage, bird nests, loose joints, or deterioration
• Measure flue draught at the sampling point (after burner refit and firing)

6. Burner Refit and Firing

• Refit the burner to the boiler; secure the fixing bolts; connect the oil supply tube
• Restore electrical supply; open oil supply valve
• Start the boiler; observe ignition; confirm flame established
• Allow to run for 10 minutes to reach stable operating temperature

7. Combustion Analysis

• Insert the flue gas analyser probe into the flue sampling point
• Allow analyser to stabilise; record CO₂%, CO (ppm), flue gas temperature, efficiency
• Perform smoke test (Bacharach); record smoke number
• Measure flue draught; record
• Adjust air settings if CO₂% or smoke number outside target range; re-measure and record final values
• Target values:

8. Safety Checks

• Remote fire valve: test that it moves freely; record check performed
• Overheat thermostat: confirm functional (check reset button status)
• System pressure (for wet heating systems): check and record
• Carbon monoxide alarm: check is present in the boiler room; test alarm function if accessible

9. Documentation and Customer Handover

• Complete the OFTEC service record via the OFTEC portal
• Record: appliance details, components replaced (nozzle spec, filter), combustion analysis results, any defects found and rectified, any advisory items
• Issue the service record to the customer (copy retained by OFTEC and installer)
• Advise the customer of any safety concerns (if a defect cannot be immediately rectified, issue an OFTEC advisory notice and advise on urgency)

Frequently Asked Questions

Can I clean and reuse the nozzle instead of replacing it?

No. Oil nozzles are precision-machined components rated for single-season use. Cleaning a nozzle with a wire or pipe cleaner will damage the precision orifice, altering the spray pattern and flow rate. A modified spray pattern causes poor combustion, sooting, and increased CO. Always replace the nozzle at every service with a new unit of the correct specification.

The customer says their boiler was serviced last year but CO₂ is only 8%. What does this mean?

CO₂ of 8% indicates excess air — the burner is running too lean (too much air relative to oil). This wastes heat (excess air exits hot up the flue) and increases running costs. The air band on the burner should be adjusted to increase CO₂ to the 10–13% target range. If the previous service didn't correct this, it was not serviced to OFTEC standards.

My customer has a condensing oil boiler. Is the service procedure different?

The core service steps are the same. Additional steps for condensing oil boilers: inspect and clean the condensate trap; confirm the condensate drain pipe is clear and draining freely; check the secondary heat exchanger (condensing section) for scale or fouling; measure flue gas temperature (should be 50–80°C in condensing mode).

Regulations & Standards

• OFTEC OFT600 — OFTEC technical standard for oil-fired appliance installation and servicing; the reference document for annual service procedures

• Building Regulations Approved Document J — fire valve requirement; combustion air; flue inspection

• BS 5410-1:2014 — Code of practice for oil firing; servicing requirements and recommendations

• HSE COSHH — combustion product exposure; relevant to flue gas analyser use in enclosed spaces

• OFTEC OFT600 — oftec.org — servicing procedure standard

• Kane Instruments — kane.co.uk — flue gas analyser guidance

• Danfoss nozzle guide — danfoss.com — nozzle specification and selection data

• oftec registration oft101 — OFTEC registration required to perform and certify oil boiler services

• oftec oft105 commissioning — combustion analysis targets and procedure

• building regs part j oil — Part J requirements including fire valve and flue

• oil storage tank regulations — tank inspection as part of annual service visit