The BS 6173 Gas Interlock Failure: Why Plant Room Compliance Stops at the Kitchen Door

Most commercial gas compliance failures do not occur where operators expect them. They occur at the interface between two systems that are typically managed by different contractors, certified under different regimes, and inspected on different cycles: the plant room gas installation and the commercial kitchen extraction system.

BS 6173:2020 is explicit. Any commercial catering establishment with a gas appliance under a mechanical extraction canopy must have a gas interlock system that prevents gas flow unless the ventilation is operating at the designed air volume. The standard specifies air pressure switches or air flow sensors, a current monitoring relay on the extract fan, and an emergency knock-off valve on the incoming gas supply. The interlock must be commissioned on installation and tested at defined intervals.

Three failure modes recur across commercial portfolios:

First, split responsibility. The gas installation is certified by a COMCAT3-qualified engineer. The extraction system is installed and maintained by a ventilation contractor, often under a separate PPM. Neither party holds responsibility for the interlock as a system. When the extract fan is replaced or rebalanced, the air pressure switch set point drifts out of calibration. The interlock still closes the valve in a full failure, but no longer trips at the designed minimum flow rate. The installation is non-compliant the day the fan is replaced. Nobody records this.

Second, bypassed interlocks. Kitchen staff encounter an interlock that trips during service and escalate to the FM help desk. A reactive contractor attends, identifies a faulty pressure switch, and "temporarily" bridges it to restore gas. The temporary bypass becomes permanent. This is not rare. It is recurrent across hospitality portfolios, particularly in 24-hour operations.

Third, documentation gaps. The original commissioning certificate exists. The annual gas safety inspection is current. The extraction PPM is current. But no document records interlock integrity as a tested item. Under HSE scrutiny following an incident, the absence of this record is itself the finding.

The implication for FM directors and hotel operations leads is straightforward. Gas safety inspection records and extraction PPM records, held separately, do not constitute compliance evidence for the interlock. The interlock is a cross-system control and must be tested, documented, and auditable as one.

Portfolio operators managing more than a handful of commercial kitchens should audit interlock commissioning records against current extract fan specifications. The gap, where it exists, is the liability.