Study guide
3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting
ACS CCN1 Core Domestic Natural Gas Safety Assessment Exam
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1 Chapter 0: Introduction & ACS CCN1 Overview
0.1 ACS CCN1 Assessment Format, Open-Book Strategy & Gas Safe Path 0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics
2 Chapter 1: Gas Safety Legislation & Regulations
1.1 Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — Operative Duties 1.2 Gas Safe Register Rules, Competence Categories & Operative ID Cards 1.3 Landlord Gas Safety Duties & CP12 / Safety Check Records (Regulation 36) 1.4 Building Regulations (Part J), HASWA & Health and Safety on Site
3 Chapter 2: Tightness Testing & Let-By Testing
2.1 Gas Meter Types (U6/G4/E6), Emergency Control Valves (ECVs) & MIVs 2.2 Let-By Testing Procedures & Electronic / U-Tube Manometer Setup 2.3 Tightness Testing Procedures for Natural Gas (IGEM/UP/1B) 2.4 Permissible Pressure Drops, Calculations & Tracing Gas Leaks
4 Chapter 3: Gas Rating, Pressure & Calorific Value
3.1 Gas Pressure Standards: Standing, Working & Meter Outlet Pressures 3.2 Gas Rating Domestic Appliances — Imperial & Metric Meter Calculations 3.3 Gross Calorific Value (39.5 MJ/m³), Gross/Net Heat Input & Efficiency 3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting
5 Chapter 4: Pipework Sizing, Installation & Purging
4.1 Gas Pipework Materials: Copper, Steel, PE & Corrugated Stainless Steel (CSST) 4.2 Pipe Sizing Calculations & The 1 mbar Maximum Pressure Drop Rule 4.3 Pipework Jointing, Sleeving, Corrosion Protection & Concealed Routes 4.4 Main Equipotential Bonding (within 600mm) & Direct Purging (IGEM/UP/1B)
6 Chapter 5: Ventilation Requirements for Domestic Gas
5.1 Ventilation Fundamentals & BS 5440-2 Standard Requirements 5.2 Ventilation Calculations for Open-Flued Appliances (5 cm²/kW Rule) 5.3 Ventilation for Flueless Appliances, Cookers & Appliance Compartments 5.4 Adventitious Air, Free Area Grille Sizing & Ventilation Defects
7 Chapter 6: Flue Types, Siting & Spillage Testing
6.1 Flue Classifications: Open-Flued (Type B), Room-Sealed (Type C) & Flueless (Type A) 6.2 Flue Terminal Siting, Clearance Distances & Protection (BS 5440-1) 6.3 Flue Flow Testing Procedures & Smoke Pellets Usage 6.4 Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions
8 Chapter 7: Combustion Analysis & Carbon Monoxide Safety
7.1 Complete vs Incomplete Combustion & Flame Picture Analysis 7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols 7.3 CO/CO₂ Ratio Action Levels (BS 7967) & GIUSP Actions 7.4 Carbon Monoxide Poisoning Symptoms, Causes & CO Alarm Rules (BS EN 50291)
9 Chapter 8: Emergency & Unsafe Situations (GIUSP / RIDDOR)
8.1 Gas Industry Unsafe Situations Procedure (GIUSP / IGEM/G/11) Overview 8.2 Immediately Dangerous (ID) Situations: Definition, Actions & Warning Labels 8.3 At Risk (AR) Situations & Advising Not to Current Standards (NCS) 8.4 Gas Escape Actions (0800 111 999), Isolation & RIDDOR Reporting Regulations
3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting
Key Takeaways Explain This Topic AI
Burner pressure is the regulated dynamic gas pressure measured downstream of the appliance gas valve governor at the burner test point during maximum heat demand.
Gas volumetric flow through an injector follows Q ∝ √P (doubling pressure raises flow by √2 ≈ 41.4%); injector size controls primary air entrainment—undersized injectors under-gas, oversized/reamed injectors over-gas and raise CO risk.
Adjusting multifunctional gas valve pressure governors modulates burner pressure to match data plate specifications, but all adjustments must be verified with meter gas rate checks.
A correct meter working pressure (20 mbar) combined with low burner pressure and low gas rate points to appliance governor misadjustment or internal blockage.
CCN1 competency 11 requires proving appliance gas safety devices: thermoelectric thermocouple FSDs, electronic flame ionisation, ODS/ASD on flueless heaters, and overheat/limit stats must shut gas off when tested.
Last updated: July 2026
3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting
In domestic gas appliances, achieving clean, complete combustion depends on precise regulation of burner pressure and correct injector orifice sizing . While meter working pressure (20 mbar) ensures adequate supply to the appliance inlet, the appliance's internal multifunctional control valve (governor) steps down this inlet pressure to a specified burner pressure (typically 3.0 mbar 3.0\text{ mbar} 3.0 mbar to 14.0 mbar 14.0\text{ mbar} 14.0 mbar depending on burner design).
Gas Safe engineers must understand how burner pressure adjustments and injector hydraulics affect volumetric flow rate, aeration, and flue gas emissions under BS 7967 and ACS CCN1 assessment criteria.
1. Multi-Functional Gas Control Valves & Governors
Domestic gas appliances utilize multi-functional gas control valves (e.g., Honeywell, SIT, 845 Sigma) that integrate multiple safety and control functions:
Thermo-electric flame supervision or electronic ionization sensing.
Twin solenoid safety shut-off valves.
An adjustable gas pressure governor (regulator).
The Role of the Pressure Governor
The internal governor maintains a constant burner pressure despite minor fluctuations in incoming meter working pressure (between 19 mbar and 23 mbar). Adjusting the governor screw alters the spring compression on the internal regulator diaphragm, raising or lowering the downstream burner pressure.
2. Injector Hydraulics & The Square Root Law
Gas flows from the burner manifold through precision-drilled brass injector orifices (jets) into the burner venturi tube, where high-velocity gas jets entrain primary air for pre-mixing before combustion.
According to hydraulic principles for fluid flow through an orifice (Bernoulli's equation): Q = C d × A × 2 Δ P ρ Q = C_d \times A \times \sqrt{\frac{2 \Delta P}{\rho}} Q = C d × A × ρ 2Δ P
Where Q Q Q = volumetric gas flow rate, A A A = injector orifice cross-sectional area, Δ P \Delta P Δ P = differential burner pressure, and ρ \rho ρ = gas density.
Practical Rule: The Square Root Pressure Law
For a fixed injector orifice size, volumetric gas flow rate ( Q Q Q ) is directly proportional to the square root of burner pressure ( P P P ) : Q 2 Q 1 = P 2 P 1 ⟹ P 2 = P 1 × ( Q 2 Q 1 ) 2 \frac{Q_2}{Q_1} = \sqrt{\frac{P_2}{P_1}} \quad \implies \quad P_2 = P_1 \times \left(\frac{Q_2}{Q_1}\right)^2 Q 1 Q 2 = P 1 P 2 ⟹ P 2 = P 1 × ( Q 1 Q 2 ) 2
Impact of Doubling Burner Pressure: If burner pressure is increased from 4 mbar 4\text{ mbar} 4 mbar to 16 mbar 16\text{ mbar} 16 mbar (a 4-fold increase), the gas rate increases by 4 = 2 \sqrt{4} = 2 4 = 2 (doubles).
Impact of Small Pressure Changes: To achieve a 10 % 10\% 10% increase in gas flow rate ( Q 2 / Q 1 = 1.10 Q_2 / Q_1 = 1.10 Q 2 / Q 1 = 1.10 ), burner pressure must be increased by 1.10 2 = 1.21 1.10^2 = 1.21 1.1 0 2 = 1.21 (a 21 % 21\% 21% increase in pressure).
Worked Example: Calculating Governor Pressure Adjustment
Scenario: During commissioning of a conventional atmospheric gas boiler, an engineer measures a burner pressure ( P 1 P_1 P 1 ) of 8.0 mbar 8.0\text{ mbar} 8.0 mbar and a metric gas rate ( Q 1 Q_1 Q 1 ) of 1.80 m3/h 1.80\text{ m3/h} 1.80 m3/h . The boiler data plate specifies a nominal required gas rate ( Q 2 Q_2 Q 2 ) of 2.05 m3/h 2.05\text{ m3/h} 2.05 m3/h . Calculate the target burner pressure ( P 2 P_2 P 2 ) required on the governor adjustment screw.
Step 1: Apply the Square Root Law Formula P 2 = P 1 × ( Q 2 Q 1 ) 2 P_2 = P_1 \times \left(\frac{Q_2}{Q_1}\right)^2 P 2 = P 1 × ( Q 1 Q 2 ) 2
Step 2: Substitute Known Values P 2 = 8.0 mbar × ( 2.05 1.80 ) 2 = 8.0 × ( 1.1389 ) 2 = 8.0 × 1.297 = 10.38 mbar P_2 = 8.0\text{ mbar} \times \left(\frac{2.05}{1.80}\right)^2 = 8.0 \times (1.1389)^2 = 8.0 \times 1.297 = 10.38\text{ mbar} P 2 = 8.0 mbar × ( 1.80 2.05 ) 2 = 8.0 × ( 1.1389 ) 2 = 8.0 × 1.297 = 10.38 mbar
Step 3: Action The engineer adjusts the governor spring screw until burner pressure reads 10.4 mbar 10.4\text{ mbar} 10.4 mbar on the manometer, then re-runs the metric meter gas rate test to confirm the actual input matches the 2.05 m 3 / h 2.05\text{ m}^3/\text{h} 2.05 m 3 / h target within ± 5 % \pm 5\% ± 5% .
3. Step-by-Step Burner Pressure Measurement Procedure
Isolate Appliance : Turn off electrical supply and isolate gas supply at the appliance isolation valve.
Connect Manometer : Locate the burner pressure test point (downstream of gas valve seats on manifold). Slacken sealing screw by 1 full turn. Connect gauge tube from a zeroed manometer.
Ignition & Maximum Firing : Restore gas and electrical supplies. Light the appliance and force it to operate on maximum output .
Record & Compare : Allow reading to stabilize for 2 minutes. Record static and dynamic burner pressure. Compare against data plate figures (e.g., 11.2 mbar 11.2\text{ mbar} 11.2 mbar ).
Adjust Governor : If burner pressure is out of specification, remove protective cap on gas valve and adjust governor screw clockwise to increase pressure, or counter-clockwise to decrease pressure.
Re-seal & Leak Test : Turn off appliance, remove manometer, tighten test point screw, restore gas pressure, and test test-point seal for tightness using approved leak detection fluid (LDF).
4. Comprehensive Gas Rate & Pressure Troubleshooting Matrix
When gas rate or pressure tests fail to meet specification, engineers use the diagnostic matrix below:
Observed Fault Condition Meter Working Pressure Burner Pressure Gas Rate Reading Root Cause Diagnosis Corrective Action
Symptom A: Under-gassing Normal ( 20 mbar 20\text{ mbar} 20 mbar ) Low (e.g. 4 mbar 4\text{ mbar} 4 mbar vs 10 mbar 10\text{ mbar} 10 mbar ) Low ( > 5 % >5\% > 5% under data plate) Misadjusted or faulty gas valve governor; stuck regulator spring. Adjust governor screw; if non-responsive, replace multi-functional gas valve.
Symptom B: Under-gassing with correct burner pressure Normal ( 20 mbar 20\text{ mbar} 20 mbar ) Correct per data plate Low ( > 5 % >5\% > 5% under data plate) Undersized or partially blocked burner injector orifices; incorrect jet size fitted. Clean injectors with solvent (never wire/reamers); check part numbers against manual.
Symptom C: Systemic Under-gassing Low ( < 19 mbar <19\text{ mbar} < 19 mbar ) Low Low Undersized installation pipework; restricted meter regulator; partial pipe blockage. Upgrade pipe sizes per BS 6891; report low meter pressure to ESP/Transporter.
Symptom D: Over-gassing Normal ( 20 mbar 20\text{ mbar} 20 mbar ) High (e.g. 16 mbar 16\text{ mbar} 16 mbar vs 10 mbar 10\text{ mbar} 10 mbar ) High ( > 5 % >5\% > 5% over data plate) Misadjusted governor; locked governor diaphragm; wrong gas valve model fitted. Adjust governor down; check for high CO levels; replace faulty gas valve immediately.
Symptom E: Severe Over-gassing with normal pressure Normal ( 20 mbar 20\text{ mbar} 20 mbar ) Correct High Oversized injectors; injectors drilled out by uncertified person; LPG jets fitted on Natural Gas. Replace all burner injectors with genuine manufacturer specification jets.
5. Hazards of Injector Tampering & Over-Gassing
Reaming out or hand-drilling injector orifices is strictly prohibited under Gas Safety (Installation and Use) Regulations. Altering injector geometry produces severe safety hazards:
Over-gassing & Incomplete Combustion : Excess fuel injection starves the flame of required secondary air, causing incomplete combustion, heavy soot deposition, and lethal Carbon Monoxide (CO) emissions.
Flame Lifting & Flame Rollout : High orifice velocity causes burner flames to lift off burner ports or roll out of combustion chambers, damaging internal wiring, flame sensors, and heat exchangers.
Thermal Shock : Over-gassing exceeds the heat exchanger design limits, leading to metal fatigue, cracking, and flue gas leakage into dwelling spaces.
6. Appliance Gas Safety Devices (CCN1 Competency 11)
Burner pressure and injector sizing only keep combustion correct when the appliance gas safety train is intact. ACS CCN1 competency 11 requires operatives to recognise the main flame and temperature safety devices on domestic Natural Gas appliances and to prove they shut gas off — not merely that a flame is present during a happy-path light-up.
Thermoelectric Flame Supervision Devices (Thermocouple FSD)
Many atmospheric boilers, cookers, and older water heaters use a thermoelectric flame supervision device :
A thermocouple sits in the pilot or main flame. Heat generates a small millivoltage (typically of order 10–30 mV) that energises an electromagnet holding the gas valve seat open.
If the flame fails, the thermocouple cools, millivoltage collapses, the electromagnet releases, and the gas valve closes .
Prove shut-off : after a successful light, extinguish the pilot/main flame (manufacturer method — often by carefully interrupting the pilot or using the appliance control sequence). Time how long the FSD takes to drop out. Gas must stop; a stuck open thermoelectric valve is Immediately Dangerous. Always follow manufacturer instructions (MIs) for dropout timing expectations and never bridge or short a thermocouple to “hold” a flame on during diagnosis.
Electronic Flame Ionisation (Flame Sensing Electrode)
Modern fan-assisted boilers and many combi appliances use electronic flame ionisation :
A flame sensing electrode in the burner flame measures a micro-amp ionisation current through the ionised gas.
Loss of flame (or a sooted/insulated electrode reading near zero) causes the PCB to de-energise the gas valve solenoids within the manufacturer’s flame-failure response time.
Prove shut-off : simulate flame failure per MIs (for example interrupt the sensing lead with the appliance firing, or use the manufacturer’s diagnostic test). Confirm the gas valve closes and lockout/lockout LED behaviour matches the handbook. Do not permanently override flame-failure inputs with jumpers for “testing.”
Oxygen Depletion Systems (ODS / ASD) on Flueless Heaters
Flueless space heaters and some decorative fuel-effect appliances rely on an oxygen depletion system (ODS), sometimes described as an atmosphere-sensing device (ASD):
A specially positioned ODS pilot assembly is designed so that falling room oxygen / rising products of combustion distort the pilot flame away from the thermocouple, shutting gas before the room atmosphere becomes unsafe.
Prove shut-off : follow MIs — typically verify correct ODS pilot assembly, check that the pilot flame envelope sits correctly on the thermocouple, and confirm that deliberate disturbance or manufacturer test procedure closes the valve. Never bypass an ODS on a flueless appliance; a defeated ODS is an Immediately Dangerous fault under GIUSP thinking.
Overheat and Limit Thermostats
Appliances also use overheat / limit stats (and often flue or heat-exchanger limit sensors) that open when water or metal temperatures exceed design limits:
On activation they interrupt the gas valve or boiler enable circuit so fuel cannot continue to fire into an already overheated heat exchanger.
Prove shut-off : where MIs provide a safe test method (for example a limit-stat open-circuit simulation or service mode), confirm gas shuts and the appliance locks out or refuses to fire until reset. A welded-closed limit thermostat that never opens is a latent overheat risk.
Exam-Focused Proof Mindset
For CCN1, the examiner wants to see that you treat safety devices as active gas shut-off components , not decorative sensors. Always: (1) identify the device type from the appliance and MIs; (2) confirm it is correctly located and connected; (3) prove that loss of flame, oxygen depletion, or over-temperature closes gas ; (4) restore the appliance to a safe commissioned state and re-check flame picture, burner pressure, and gas rate after any disturbance of the gas train.