6.4 Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions

Spillage Testing Procedures under Normal & Adverse/Worst-Case Conditions

While a flue flow test confirms that an unlit chimney duct is clear and capable of drawing air, a spillage test evaluates the actual operating safety of an open-flued gas appliance (Type B) under live burning conditions. The spillage test determines whether all products of combustion are successfully drawn into the draught diverter and evacuated outdoors, or whether lethal flue gases (including carbon monoxide) are spilling back into the room. Gas engineers must perform spillage testing under both normal operating conditions and adverse / worst-case room depressurisation conditions as specified in BS 5440-1 and ACS CCN1 assessment standards.

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1. Appliance Preparation & Pre-Test Setups

Before starting the spillage test, the engineer must set up the room environment to reflect standard baseline conditions:

1. Close all external doors and windows in the room where the appliance is installed.
2. Ensure all dedicated combustion air vents supplying the appliance room are fully open, clean, and unobstructed.
3. Close all internal doors communicating between the appliance room and other internal spaces.
4. Ensure all mechanical extraction fans, cooker hoods, and tumble dryers are turned OFF for the initial baseline test.

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2. Phase 1: Spillage Testing under Normal Operating Conditions

+-------------------------------------------------------------------+
| STEP 1: FIRE APPLIANCE AT MAXIMUM RATE                            |
| Light the appliance burner and operate at full heat input for     |
| exactly 5 minutes to allow flue circuit to reach thermal equilibrium.|
+-------------------------------------------------------------------+
                                  |
                                  v
+-------------------------------------------------------------------+
| STEP 2: PREPARE SMOKE TESTING DEVICE                              |
| Light a standard smoke match or approved smoke match holder.      |
+-------------------------------------------------------------------+
                                  |
                                  v
+-------------------------------------------------------------------+
| STEP 3: SAMPLE ALONG DRAUGHT DIVERTER LIP                         |
| Hold smoke match 10 mm below/around the lower lip of the draught   |
| diverter relief opening along its entire perimeter.               |
+-------------------------------------------------------------------+
                                  |
                                  v
+-------------------------------------------------------------------+
| STEP 4: EVALUATE SMOKE DYNAMICS                                   |
| All smoke must be drawn smoothly and completely into the diverter.|
| Zero smoke must spill back into the room air space.               |
+-------------------------------------------------------------------+

Critical Timing & Cold-Start Clearance Rules

• 5-Minute Burn Rule: The appliance MUST be operated at maximum heat input rate for 5 minutes continuous running before performing the smoke match check. This allows the heat exchanger and chimney column to warm up fully.
• Initial Ignition Spillage Allowance: When lighting a cold appliance, temporary spillage at the draught diverter is permitted for a brief period as thermal draught establishes. However:
  • Spillage must clear completely within 3 minutes of ignition for standard metal or prefabricated flues.
  • Spillage must clear completely within 5 minutes for heavy masonry chimneys.
  • After this initial warm-up clearance window, continuous clearance must be maintained.

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3. Phase 2: Spillage Testing under Adverse / Worst-Case Conditions

In modern homes, mechanical extraction equipment can create severe negative room pressures (depressurisation) that overcome natural thermal flue updraught, pulling dangerous flue gases directly out of draught diverters into living areas.

Establishing the "Worst-Case" Room Scenario

To test appliance resilience against room depressurisation, the gas engineer must configure the property into its most severe negative pressure state:

1. Turn ON all mechanical extraction fans in the property to their maximum speed setting (kitchen extractors, bathroom extract fans, utility room fans).
2. Turn ON all mechanical equipment that exhausts air outdoors (such as vented tumble dryers).
3. Open all internal communicating doors between the appliance room and the rooms containing extraction fans or appliances.
4. Close all external doors and windows throughout the property.
5. Light any open solid fuel fires in adjacent connected rooms (or simulate their natural stack draw).

[ External Window CLOSED ]                  [ External Window CLOSED ]
+-------------------------+                 +-------------------------+
|    APPLIANCE ROOM       |                 |       KITCHEN           |
|                         |  Internal Door  |                         |
|   +-----------------+   |    OPENED       |   +-----------------+   |
|   |  Type B Boiler  |   | =============== |   | Extractor Fan   |===> (Max Speed
|   | (Draught Div.)  |   |                 |   | (Cooker Hood)   |     Exhaust)
|   +-----------------+   |                 |   +-----------------+   |
|            |            |                 |                         |
|     [Smoke Match]       |                 |   +-----------------+   |
|   (Testing Spillage)    |                 |   | Tumble Dryer    |===> (Exhausting)
+-------------------------+                 +---+-+---------------+---+

Executing the Adverse Spillage Check

• With the worst-case extraction setup active, operate the gas appliance at maximum heat input rate for a further 5 minutes.
• Pass the lighted smoke match along the entire length of the draught diverter lip, holding the smoke source approximately 10 mm below the relief edge.
• Observe the smoke behaviour carefully.

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4. Spillage Pass/Fail Criteria & GIUSP Classifications

Pass Criteria

All smoke from the smoke match is drawn cleanly, rapidly, and continuously into the draught diverter relief opening under both normal and worst-case extraction setups.

Fail Criteria & Risk Classifications

If smoke spills back into the room, is blown away from the diverter lip, or fails to enter the flue during the test:

Diagnostic Checklist for Spillage Failure

If spillage occurs, engineers must investigate four primary root causes:

1. Inadequate Combustion Ventilation: Blocked or undersized permanent room air vent.
2. Chimney Draught Failure: Flue blockage, severe downdraught, low flue height, or cold air stagnation.
3. Fan Depressurisation: High-output kitchen extractor fan pulling air back down the chimney.
4. Appliance Fault: Heat exchanger blockage, distorted draught diverter, or over-firing gas rate.