7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols
Key Takeaways
- Electronic Flue Gas Analysers (FGAs) used for domestic gas safety testing in the UK must comply with BS EN 50379-1 & 2 and hold a valid annual calibration certificate.
- FGAs must always undergo a fresh air zeroing routine outside the building or in a clean environment prior to taking combustion samples.
- The water trap and inline particulate filter must be inspected, emptied, and dry before every combustion test to protect internal electrochemical sensors from damage.
- Gas appliances must operate for a minimum stabilization period of 10 to 15 minutes at maximum thermal input before recording official combustion measurements.
- BS 7967 Parts 1–4 define the statutory code of practice for testing combustion performance on room-sealed, open-flued, and flueless domestic appliances.
7.2 Electronic Flue Gas Analysers (FGA) & BS 7967 Testing Protocols
ACS CCN1 Exam Notice: Modern UK gas safety regulations require gas engineers to be fully competent in the operation of Electronic Flue Gas Analysers (FGAs). ACS candidates must master pre-use checks, calibration standards, fresh air zeroing protocols, and sampling methods specified under British Standard BS 7967.
Electronic Flue Gas Analysers (FGAs) have replaced manual chemical sampling tubes as the definitive diagnostic tool for evaluating domestic combustion performance. FGA readings provide quantitative data on carbon monoxide (CO), carbon dioxide (CO₂), oxygen (O₂), flue gas temperature, and ambient air quality. To ensure accuracy and legal defensibility, all FGA testing must strictly follow BS 7967 (Code of practice for the use of electronic flue gas analysers in the verification of the performance of gas-fired appliances).
Standards & Calibration Requirements (BS EN 50379)
FGAs used for safety assessment and commissioning of domestic gas appliances in the UK must comply with BS EN 50379 Parts 1 and 2 (Specification for portable electrical apparatus designed to measure combustion flue gas parameters of heating appliances).
Sensor Technologies & Limitations
- Carbon Monoxide Sensor: Electrochemical cell designed to measure CO concentrations in parts per million (ppm). Standard range: 0 to 2,000 ppm (with high-range overload protection up to 10,000 ppm).
- Oxygen Sensor: Electrochemical cell measuring O₂ percentage (0% to 21%).
- Carbon Dioxide Calculation: Most domestic FGAs directly measure O₂ and calculate CO₂ percentage based on the fuel type selected (e.g., Natural Gas vs LPG).
- Direct Infrared Sensors: Advanced FGAs utilize Non-Dispersive Infrared (NDIR) sensors to measure CO₂ directly.
Mandatory Calibration Schedule
FGAs must undergo annual calibration and recertification by a manufacturer-approved UKAS-accredited calibration laboratory. A valid calibration certificate must be maintained. Operating an FGA past its calibration expiration date invalidates any safety reports, commissioning records, or landlord gas safety certificates issued.
Mandatory Pre-Use Instrument Safety Checks
Before initiating combustion sampling, the gas engineer must perform a five-point physical and operational check of the analyser system:
+-----------------------------------------------------------------------+
| FGA PRE-USE PHYSICAL INSPECTION |
| |
| [Sampling Probe] ---> [Inline Filter] ---> [Water Trap] ---> [FGA] |
| (Inspect hose for (Must be clean (Must be empty (Verify|
| cracks & kinks) & bone dry) & sealed tight) calib.)|
+-----------------------------------------------------------------------+
- Calibration Status: Verify the instrument calibration sticker and confirm the certificate is within its 12-month validity window.
- Sampling Probe & Hose Integrity: Inspect the flexible neoprene hose and stainless steel probe for cracks, splits, kinks, or blockages that could draw in ambient dilution air.
- Water Trap Check: Ensure the inline condensation water trap is completely empty, clean, and securely fitted with intact rubber seals. Moisture entering the FGA unit will permanently poison electrochemical sensors.
- Particulate Dust Filter: Check the inline paper or PTFE filter disc. Replace immediately if stained yellow, brown, or black with soot deposits.
- Battery & Fuel Type Selection: Ensure sufficient battery charge for the full testing sequence and select Natural Gas (NG) as the active fuel profile on the instrument menu.
Fresh Air Zeroing Protocol
Fresh air zeroing is the single most critical pre-test operational step. The electrochemical sensors inside an FGA require a clean baseline measurement to calibrate zero point for CO and 20.9% for O₂.
The Mandatory Procedure
- Location: The analyser MUST be turned on and zeroed outside the premises in clean ambient air, or in an unheated room known to be entirely free from combustion products, vehicle exhaust, or tobacco smoke.
- Prohibition: NEVER turn on or zero an FGA inside a room containing an operating gas appliance, inside an enclosed boiler room, or near a flue terminal discharge.
- Zeroing Countdown: Allow the instrument to complete its automated purge cycle (typically 60 to 180 seconds). Confirm the initial baseline display reads: CO = 0 ppm and O₂ = 20.9%.
Appliance Stabilization & Sampling Positioning
Combustion analysis recorded on a cold appliance will yield false high CO readings because cold heat exchanger surfaces temporarily induce flame quenching.
Stabilization Period
- The appliance must be fired and run at maximum thermal rate for a minimum stabilization period of 10 to 15 minutes prior to taking official combustion measurements.
- For modulating condensing boilers, ensure the boiler is locked into high-fire service mode via installer menu options.
Probe Insertion & Sampling Point Selection
- Room-Sealed Fan-Assisted Boilers: Insert the stainless steel sampling probe through the dedicated flue sampling port (usually situated on the boiler flue elbow). Position the probe tip in the center stream of the flue gas flow.
- Open-Flued Appliances: Insert the probe into the primary flue draft diverter canopy or secondary flue pipe at a point at least 150mm above the draft diverter, ensuring the probe tip is centered to sample undiluted combustion gases.
- Flueless Appliances (Cookers & Space Heaters): Position the sampling probe directly over the burner flame boundary or appliance discharge grill in accordance with BS 7967-2.
BS 7967 Parts 1–4 Testing Framework
BS 7967 is structured into four specific operational parts:
- BS 7967-1: Procedure for deciding whether an appliance is operating safely when no specific manufacturer instructions are available.
- BS 7967-2: Protocol for investigating carbon monoxide spills and ambient room atmosphere testing.
- BS 7967-3: Action criteria and procedures for dealing with unsafe combustion readings.
- BS 7967-4: Operational guidance on FGA selection, maintenance, and calibration.
Summary Table: FGA Pre-Check & BS 7967 Test Workflow
| Workflow Stage | Action Required | Technical Standard / Mandatory Limit | Consequence of Non-Compliance |
|---|---|---|---|
| 1. Calibration Check | Verify calibration label date | Valid within 12 months (BS EN 50379) | Void test results; illegal gas safety report |
| 2. Physical Check | Inspect hose, filter, water trap | Filter clean; water trap empty & sealed | Sensor contamination & erroneous readings |
| 3. Fresh Air Zero | Purge FGA in outside air | Baseline CO = 0 ppm; O₂ = 20.9% | False zero offset; incorrect CO calculation |
| 4. Appliance Warm-Up | Fire appliance at maximum rate | Run continuously for 10–15 minutes | Premature sampling during flame quenching |
| 5. Probe Placement | Center probe tip in flue gas stream | Dedicated port / draft diverter centerline | Air dilution causing falsely low CO/CO₂ ratio |
| 6. Data Recording | Record CO, CO₂, O₂, Temp, Ratio | Stable reading held for at least 60 seconds | Unstable transient readings recorded |
Flueless Appliance & Ambient Room Atmosphere Sampling (BS 7967-2)
When investigating suspected carbon monoxide incidents or testing flueless gas appliances (such as gas hobs or flueless space heaters), engineers must conduct ambient room air testing under BS 7967-2:
- Take an initial ambient CO reading in the room prior to lighting the appliance (baseline must be 0 ppm).
- Operate the flueless appliance for 15 minutes with doors and windows closed.
- Take FGA ambient room readings at breathing height (1.5 metres above floor) and 1 metre away from the appliance.
- If ambient CO exceeds 9 ppm above baseline, or if CO₂ exceeds 1,000 ppm, turn off the appliance immediately and investigate combustion ventilation faults.
Where MUST an Electronic Flue Gas Analyser (FGA) be switched on and zeroed prior to testing a gas appliance?
Under BS EN 50379, what is the maximum permissible calibration interval for an FGA used in domestic gas safety checks?
What is the minimum recommended warm-up stabilization period for a gas appliance at maximum thermal rate before recording official FGA combustion readings?