AHU Fire Interlock Testing
Ensuring air handling units shut down correctly on fire alarm activation.
Why AHU Fire Interlocks Are Critical
Air handling units (AHUs) circulate vast volumes of air throughout buildings via shared ductwork. During a fire event, operating AHUs become smoke distribution systems, spreading toxic gases and combustion products to areas far from the fire's origin. This defeats compartmentation strategies and creates untenable conditions across multiple zones simultaneously.
Fire interlocks are control sequences integrated with the fire alarm system and building management system (BMS). When the fire alarm activates, interlocks command AHUs to shut down and close motorised fire dampers within ductwork. This prevents smoke from travelling through the HVAC system and maintains building compartmentation.
Annual testing verifies that shutdown sequences execute correctly, dampers close fully, and BMS logging functions capture fire alarm events. Failures can result in rapid fire spread, prosecution under the Regulatory Reform (Fire Safety) Order 2005, and insurance invalidation if investigations reveal inadequate maintenance.
Compliance Standards
BS 5588 / Approved Document B
Building Regulations guidance on fire safety, including requirements for fire dampers and smoke control in HVAC systems. Specifies interlock requirements to prevent smoke spread via ductwork.
BS EN 15650
Ventilation for buildings – Fire dampers. Defines performance requirements and testing protocols for fire dampers, including closure time, leakage rates, and periodic inspection intervals.
Regulatory Reform (Fire Safety) Order 2005
Requires the Responsible Person to maintain fire safety systems, including AHU interlocks and fire dampers. Non-compliance can result in prosecution and unlimited fines.
Testing Requirements
Damper Behaviour Testing
Simulating fire alarm activation to verify that motorised fire dampers close fully. Measuring closure times, checking for mechanical obstructions, and verifying damper blade seating. Dampers should close within specified time limits (typically 30-60 seconds).
Shutdown Sequencing
Verifying correct AHU shutdown sequences: fans stop, dampers close, and systems enter safe mode. Testing zone-specific interlocks to ensure only relevant AHUs shut down (not entire building-wide systems if fire is localised).
BMS Integration Checks
Testing communication between fire alarm panels, BMS controllers, and AHU control panels. Verifying alarm event logging, interlock status monitoring, and manual reset functions. Checking that BMS cannot override fire interlocks.
Manual Reset & Restart Procedures
Verifying that AHUs cannot automatically restart after fire alarm events. Testing manual reset procedures and ensuring only authorised personnel can reinitiate HVAC operation after fire investigations.
Risks of Non-Compliance
Rapid Fire & Smoke Spread
Failed interlocks allow HVAC systems to distribute smoke to areas far from the fire's origin. This defeats compartmentation, creates untenable escape conditions, and can result in multiple casualties.
Compromised Fire Compartmentation
Open fire dampers breach fire-rated compartment boundaries, allowing fire to spread through ductwork. This undermines the building's passive fire protection strategy and can invalidate fire engineering approvals.
Legal and Insurance Consequences
The Responsible Person faces prosecution if fire spread results from inadequately maintained interlocks. Insurance claims may be rejected if investigations reveal non-compliant HVAC fire protection.
Ensure Compliant AHU Fire Interlocks
Get a fixed-price proposal for annual testing, damper inspections, and BMS integration verification. Our engineers specialise in complex HVAC and fire safety systems.