3.4 Burner Pressure Adjustments, Injector Sizing & Gas Rate Troubleshooting

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 $3.0\text{ mbar}$ to $14.0\text{ mbar}$ 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.

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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.

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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): $Q = C_d \times A \times \sqrt{\frac{2 \Delta P}{\rho}}$

Where $Q$ = volumetric gas flow rate, $A$ = injector orifice cross-sectional area, $\Delta P$ = differential burner pressure, and $\rho$ = gas density.

Practical Rule: The Square Root Pressure Law

For a fixed injector orifice size, volumetric gas flow rate ($Q$) is directly proportional to the square root of burner pressure ($P$): $\frac{Q_2}{Q_1} = \sqrt{\frac{P_2}{P_1}} \quad \implies \quad P_2 = P_1 \times \left(\frac{Q_2}{Q_1}\right)^2$

• Impact of Doubling Burner Pressure: If burner pressure is increased from $4\text{ mbar}$ to $16\text{ mbar}$ (a 4-fold increase), the gas rate increases by $\sqrt{4} = 2$ (doubles).
• Impact of Small Pressure Changes: To achieve a $10%$ increase in gas flow rate ($Q_2 / Q_1 = 1.10$), burner pressure must be increased by $1.10^2 = 1.21$ (a $21%$ increase in pressure).

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Worked Example: Calculating Governor Pressure Adjustment

Scenario: During commissioning of a conventional atmospheric gas boiler, an engineer measures a burner pressure ($P_1$) of $8.0\text{ mbar}$ and a metric gas rate ($Q_1$) of $1.80\text{ m3/h}$. The boiler data plate specifies a nominal required gas rate ($Q_2$) of $2.05\text{ m3/h}$. Calculate the target burner pressure ($P_2$) required on the governor adjustment screw.

• Step 1: Apply the Square Root Law Formula $P_2 = P_1 \times \left(\frac{Q_2}{Q_1}\right)^2$

• Step 2: Substitute Known Values $P_2 = 8.0\text{ mbar} \times \left(\frac{2.05}{1.80}\right)^2 = 8.0 \times (1.1389)^2 = 8.0 \times 1.297 = 10.38\text{ mbar}$

• Step 3: Action The engineer adjusts the governor spring screw until burner pressure reads $10.4\text{ mbar}$ on the manometer, then re-runs the metric meter gas rate test to confirm the actual input matches the $2.05\text{ m}^3/\text{h}$ target within $\pm 5%$.

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3. Step-by-Step Burner Pressure Measurement Procedure

1. Isolate Appliance: Turn off electrical supply and isolate gas supply at the appliance isolation valve.
2. 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.
3. Ignition & Maximum Firing: Restore gas and electrical supplies. Light the appliance and force it to operate on maximum output.
4. Record & Compare: Allow reading to stabilize for 2 minutes. Record static and dynamic burner pressure. Compare against data plate figures (e.g., $11.2\text{ mbar}$).
5. 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.
6. 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).

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4. Comprehensive Gas Rate & Pressure Troubleshooting Matrix

When gas rate or pressure tests fail to meet specification, engineers use the diagnostic matrix below:

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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:

1. 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.
2. 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.
3. Thermal Shock: Over-gassing exceeds the heat exchanger design limits, leading to metal fatigue, cracking, and flue gas leakage into dwelling spaces.

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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.