6.3 Flue Flow Testing Procedures & Smoke Pellets Usage
Key Takeaways
- A flue flow test must be carried out on open-flued chimney systems prior to installing or commissioning an appliance to verify adequate thermal updraught and freedom from blockages.
- Before applying a smoke pellet, the chimney flue must be pre-heated for 5 to 10 minutes using a suitable heat source (e.g., blowtorch) to establish an initial thermal updraught and overcome cold air stasis.
- Standard flue flow testing requires an approved smoke pellet capable of producing a minimum of 15m³ of dense smoke over a 2.5-minute burn duration.
- During the flue flow test, an engineer must observe both the internal flue inlet to ensure complete smoke evacuation and the external flue terminal to confirm dense smoke exit.
- If smoke issues from loft spaces, adjacent flues, or neighboring properties during a flue flow test, the chimney is leaking and must be classified as Immediately Dangerous (ID) under the GIUSP.
Flue Flow Testing Procedures & Smoke Pellets Usage
Before connecting or commissioning any open-flued gas appliance (Type B), gas engineers must establish that the chimney flue system is completely clear of obstructions, structurally sound, and capable of creating sufficient thermal updraught to safely evacuate products of combustion to the external atmosphere. The mandatory diagnostic procedure for evaluating chimney pull and structural containment is the Flue Flow Test, executed in strict compliance with BS 5440-1 and ACS CCN1 core gas safety procedures.
1. Purpose & Pre-Test Physical Inspections
A flue flow test must be performed whenever:
- A new open-flued appliance is installed or commissioned.
- An existing chimney is re-connected after period of non-use.
- An annual safety inspection or appliance service is carried out.
- Structural alterations have occurred on the property or roof.
Pre-Test Visual Checks
Before introducing any smoke or heat into the flue duct, the engineer must carry out thorough visual examinations:
- Chimney Route & Construction: Inspect the entire visible length of the chimney or flue pipe for cracks, loose brickwork, missing mortar, or illegal unsealed openings.
- Catchment Space Depth: For open-flued fires fitted into masonry chimneys, verify that the catchment space beneath the flue socket is clean of debris, soot, and fallen mortar, maintaining a minimum depth of 250 mm below the flue entry point.
- Flue Terminal Inspection: Verify that the external chimney stack is fitted with an approved gas flue terminal or cowl (e.g., GC1 cowl or approved rain cap). Ensure that restrictive solid fuel cowls, dampers, soot doors, or restrictive bird guards are removed.
2. Step-by-Step Flue Flow Test Procedure
The flue flow test follows a mandatory, standardized sequence to prevent false readings caused by cold air columns within the chimney flue.
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| STEP 1: PRE-HEAT CHIMNEY FLUE |
| Apply blowtorch or heat gun to flue entry for 5 to 10 minutes to |
| break cold air column and establish positive thermal updraught. |
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| STEP 2: POSITION APPROVED SMOKE PELLET |
| Ignite smoke pellet (min 15m³ smoke over 2.5 min) and place in |
| flue socket, draught diverter entry, or catchment space base. |
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| STEP 3: OBSERVE INTERNAL FLUE ENTRY |
| Confirm all smoke is drawn smoothly into flue without spilling |
| back into the internal room. |
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| STEP 4: OBSERVE EXTERNAL FLUE TERMINAL |
| Verify dense smoke issuing freely from the correct flue terminal |
| with strong discharge velocity outdoors. |
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| STEP 5: CHECK ADJACENT SPACES & NEIGHBORING PROPERTY |
| Check loft spaces, cavity walls, and adjacent flues for cross- |
| contamination or smoke leakage. |
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Detailed Execution Protocol
Phase A: Pre-Warming the Chimney (5 to 10 Minutes)
Cold masonry chimneys contain a dense column of stagnant cold air that acts as a physical plug, resisting upward flue flow.
- The engineer must apply a direct heat source (a gas blowtorch, high-output heat gun, or flaming warm-up burner) directly into the flue socket or base of the chimney for 5 to 10 minutes.
- Pre-warming creates warm, buoyant air that initiates a positive thermal updraught up the flue duct.
Phase B: Smoke Pellet Ignition & Placement
- Use an approved flue testing smoke pellet. Standard specification requires a pellet capable of generating a minimum of 15 m³ of dense smoke over a 2.5-minute burn period (for small single flues, an approved smoke match generating sustained smoke may be used).
- Light the pellet and place it securely inside the flue entry, draught diverter socket, or catchment space opening.
Phase C: Internal & External Observation
- Internal Observation: Watch the smoke pattern at the flue entry. All smoke generated must be drawn rapidly and completely into the chimney. There must be zero smoke spillage back into the room.
- External Observation: The engineer (or an assistant) must observe the external roof level. Dense, thick smoke must issue freely and forcefully from the designated flue terminal.
3. Interpreting Flue Flow Test Results & Diagnosing Faults
| Observed Result | Diagnosis / Cause | Required Action / GIUSP Classification |
|---|---|---|
| All smoke drawn into flue; dense smoke issuing freely from correct terminal | Flue flow test PASSED. Chimney is clear and drawing properly. | Proceed to appliance spillage testing. |
| Smoke spills back into room; little or no smoke at external terminal | Blocked or restricted flue (bird nest, soot collapse, cap restriction, cold plug). | TEST FAILED. Classify as Immediately Dangerous (ID) if appliance connected; clear blockage. |
| Smoke issues from adjacent chimney pot or shared stack | Cross-contamination / Broken mid-feather wall between flues in shared stack. | TEST FAILED. Classify as Immediately Dangerous (ID); isolate flue access. |
| Smoke visible in loft space, bedroom void, or neighboring property | Leaking flue / Broken masonry joint / Split liner. High carbon monoxide risk! | TEST FAILED. Classify as Immediately Dangerous (ID); seal/re-line chimney before use. |
4. Safety Considerations & Environmental Factors
Atmospheric Down-Draught & Wind Effects
Extremely cold ambient temperatures, heavy rain, or high local wind pressure over nearby roof ridges can induce temporary downdraught. If downdraught persists despite thorough pre-heating for 10 minutes, an approved anti-downdraught flue cowl (such as an anti-downdraught gas cowl complying with BS EN 1856) may need to be specified.
Record Keeping
All flue flow test results must be documented on the commissioning certificate or gas safety record (CD/11 or equivalent CP12 digital safety record), stating the date, test outcome, smoke pellet batch, and any corrective actions taken.
What is the primary purpose of pre-heating an open-flue chimney for 5 to 10 minutes before conducting a flue flow test?
What volume of smoke must an approved smoke pellet produce during a standard flue flow test on a domestic chimney?
If smoke is observed escaping into a loft space or an adjacent property during a flue flow test, how must the gas engineer classify the installation under the GIUSP?