Gas Interlock Systems for Commercial Kitchens: Solenoid Valves, Extract Proving and BS 6173

Summary

Commercial kitchen gas interlock systems are a statutory safety requirement under BS 6173:2009 (now largely superseded by updates). They prevent gas appliances from being used when the ventilation system is not operating. Without interlock systems, the accumulation of combustion products and cooking fumes in an inadequately ventilated kitchen is a serious risk of asphyxiation, fire, and explosion.

The interlock system creates a supervised logic chain: the extract system must prove adequate airflow before the solenoid valve opens to allow gas flow. If the fan fails, the filter blocks, or the extract duct is obstructed, the system shuts off the gas supply and requires a manual reset to confirm the fault has been investigated.

For Gas Safe engineers working in the commercial catering sector, understanding interlock system design and commissioning is essential. Incorrect installation — wrong solenoid sizing, poorly positioned airflow proving switch, or incorrect manual reset logic — renders the system legally non-compliant and potentially dangerous.

Key Facts

• BS 6173:2009 — the primary UK standard for commercial kitchen ventilation and gas interlock; specifies requirements for extract proving and gas shut-off
• Solenoid valve — normally closed (NC) 24V DC (or 240V AC) electrically operated valve in the gas supply; opens only when energised by the controller
• Normally closed — the fail-safe position; gas supply is interrupted when power is lost or when extract is inadequate
• Airflow proving switch — a differential pressure switch or paddle-type flow switch installed on the extract duct or canopy; confirms adequate airflow before enabling the solenoid
• Manual reset — required after any fault condition; the operator must physically reset the controller to confirm the fault has been investigated; automatic restart is not permitted
• COSHH — Control of Substances Hazardous to Health; kitchen gases and combustion products are substances hazardous to health; interlock systems form part of COSHH risk management
• Gas Safe notification — all interlock system installation and commissioning on commercial gas systems requires a Gas Safe registered engineer
• IGE/UP/1B — Institute of Gas Engineers and Managers document for commercial gas installations; covers emergency control valves and interlock requirements
• ECV — Emergency Control Valve; manually operated valve in a prominent accessible location; required in all commercial gas premises
• Annual service — interlock systems require an annual service and test as part of the gas safety inspection programme
• CO alarm integration — CO alarms should be integrated into the interlock logic in smaller enclosed kitchens; CO concentration above a threshold should trigger gas shut-off

Quick Reference Table

Detailed Guidance

Regulatory Basis and BS 6173

BS 6173:2009 (Specification for Installation of Gas Cooking Appliances in Commercial Catering Establishments) was the principal standard requiring gas interlock systems. It has been updated and the current applicable guidance integrates with IGEM/UP/1B and Building Regulations Part F (ventilation):

When an interlock is required:

• All new commercial kitchen installations with gas appliances
• Refurbishment of commercial kitchens where the ventilation system is replaced or significantly modified
• Any kitchen in England and Wales where ventilation is provided by mechanical extract (which is essentially all commercial kitchens)
• Where an environmental health officer or building control officer requires one as a condition of approval

Legal status: BS 6173 is a British Standard, not a statutory instrument. However, Building Regulations Part F and the Health and Safety at Work Act 1974 (general duty of care) make it effectively mandatory for commercial operators. An interlock is expected practice; its absence in a commercial kitchen will likely be flagged by environmental health, Gas Safe, or insurance inspections.

Solenoid Valve Selection and Installation

The solenoid valve is the critical safety component. Its selection must be correct:

Sizing: The solenoid valve must be sized for the total gas demand of all appliances it controls. Use the same diversity-applied demand figure as for pipe sizing (IGEM/UP/2 method). An undersized solenoid valve causes pressure drop and appliance performance issues.

Voltage: 24V DC is preferred for new installations (safer, allows battery backup during power cuts). 240V AC solenoids are legacy; some older controllers use 240V AC output. Confirm the controller output voltage matches the solenoid specification.

Gas approval: The solenoid valve must be approved for gas service (BS EN 161 for automatic shut-off valves or equivalent). A water solenoid valve must not be used on gas.

Installation position:

• Downstream of the gas meter, upstream of all gas appliances
• On a horizontal pipe run where possible (some valves are orientation-sensitive)
• Accessible for maintenance without dismantling other equipment
• Clearly labelled "GAS SOLENOID VALVE"

Manual bypass: Some designs include a manual bypass for maintenance. A bypass valve must be locked in the closed position during normal operation and can only be opened by an engineer with a key. The bypass must not be operable by kitchen staff.

Airflow Proving Switch: Selection and Setting

The airflow proving switch confirms that the extract fan is running and the ventilation canopy is achieving adequate airflow:

Types:

• Differential pressure switch: measures the pressure difference between the extract duct and the kitchen atmosphere; closes its contact when pressure difference exceeds the setpoint
• Paddle-type flow switch: a vane inserted into the duct deflects when airflow is present; triggers a micro-switch
• Fan current monitor: monitors the extract fan motor current; confirms fan is running but does not confirm adequate airflow through the duct (filter blockage would not be detected)

Differential pressure switches are preferred as they detect both fan failure and duct blockage (blocked filter reduces airflow and pressure differential).

Setting the airflow switch:

• Commission the extract system fully (filters fitted, access panels closed, registers in design position)
• Measure the differential pressure across the canopy or at the duct probe position with the fan at design speed
• Set the airflow switch setpoint to approximately 70–80% of the measured operating pressure differential
• This ensures the switch trips before the airflow drops below a safe minimum, while allowing for filter loading over time

Controller Logic and Manual Reset

The interlock controller manages the complete safety logic:

Normal operating sequence:

1. Kitchen staff arrive; fan controller starts extract fan
2. After a time delay (typically 30–60 seconds for fan to reach full speed), airflow switch closes
3. Airflow switch signal reaches the interlock controller
4. Controller energises solenoid valve — gas supply opens
5. Indicator: green "ventilation OK" LED illuminates; amber "gas enabled" LED illuminates

Fault sequence:

1. Extract fan fails (power loss, thermal trip, belt failure, etc.)
2. Airflow switch opens (no airflow, below setpoint)
3. Controller de-energises solenoid valve — gas supply closes immediately
4. Red fault LED illuminates; audible alarm sounds
5. System locks out — will not reset automatically
6. Manual investigation required; fault identified and rectified
7. Engineer (or authorised person) presses manual reset button to clear fault
8. System re-enables after fan restart and airflow confirmation

Manual reset requirement (BS 6173): Manual reset is mandatory; an automatic restart would defeat the safety purpose of the system. The reset button is typically accessible to kitchen management but not to all kitchen staff — some installations require a key or PIN for reset to ensure a responsible person has confirmed the fault investigation.

CO Alarm Integration

Carbon monoxide is produced by incomplete combustion. In a commercial kitchen with multiple gas appliances, CO accumulation can reach dangerous levels if combustion is poor:

Integration options:

1. CO alarm with relay output connected to interlock controller's fault input — CO alarm above threshold triggers gas shut-off and requires manual reset
2. CO alarm as a standalone warning device (not integrated) — kitchen staff evacuate; CO alarm does not automatically shut off gas
3. CO alarm integrated into Building Management System (BMS) which controls the interlock

BS 6173 and IGEM/UP/1B guidance supports CO integration for smaller, enclosed kitchens. In large open-plan kitchens with multiple extract systems, CO integration may generate false trips; risk assessment guides the decision.

CO alarm placement in kitchens:

• CO mixes relatively uniformly with air (close to neutral buoyancy)
• Install approximately 1m below the ceiling level
• Not directly above or adjacent to gas appliances (risk of false activation from transient rich combustion during ignition)
• Not in the extract airstream (dilution effect reduces sensitivity)

Emergency Control Valve (ECV)

Under IGE/UP/1B, commercial gas premises must have a manually operated emergency control valve:

• Located in a prominent, accessible position — typically adjacent to the main entrance/exit to the kitchen
• Clearly labelled "EMERGENCY GAS SHUT-OFF — TURN TO CLOSE"
• Must be operable without special tools
• Staff must be trained in its use; location and operation should be included in staff induction
• ECV is separate from and upstream of the interlock solenoid

The ECV enables rapid manual gas isolation in an emergency without requiring the gas engineer's service isolator (which may be locked or difficult to access).

Annual Service Requirements

Gas interlock systems must be serviced annually as part of the commercial gas safety programme:

Annual service checklist:

• Test solenoid valve operation (energise and de-energise; confirm valve closes fully)
• Test airflow switch: reduce airflow (partially block canopy) and confirm interlock trips
• Test manual reset: confirm system holds in fault position until manually reset
• Test CO alarm (if integrated): use CO test gas; confirm interlock trips
• Inspect and clean airflow proving switch probe
• Check solenoid valve for gas tightness when closed
• Confirm all indicator LEDs functional
• Test audible alarm
• Record all test results; issue service report

Gas Safe engineers must record the interlock system test results alongside the Landlord Gas Safety Record or commercial gas safety inspection report.

Frequently Asked Questions

What happens if the interlock fails during service?

If the interlock system fails during operation — typically the solenoid valve fails to open, or the airflow switch trips due to a blocked filter — the gas supply is interrupted. Kitchen operations must stop. The system must not be bypassed to restore gas supply; the fault must be investigated and rectified by a Gas Safe engineer. Bypassing the interlock is a serious safety violation and renders the installation non-compliant.

Does a small coffee shop need a gas interlock?

If the coffee shop uses gas appliances (gas espresso machine, gas range, etc.) and has mechanical extract ventilation, a gas interlock is required under BS 6173 and Building Regulations Part F. The scale of the kitchen does not exempt the operator from the requirement. A simple single-zone interlock system for a small kitchen can be cost-effectively installed for £500–1,000.

Can the interlock cover multiple independent extract zones?

Yes. More complex interlock systems cover multiple extract zones, each with its own airflow proving switch. The logic is typically configured so that gas is available only when the extract zone covering the gas appliances is confirmed running. If a separate extract zone serves a different area (e.g. a dishwasher or prep area), the interlock logic can be configured to allow gas independently of that zone.

How long does gas interlock commissioning take?

A straightforward single-zone commercial kitchen interlock takes 1–2 hours for a Gas Safe engineer to commission from a pre-wired installation. This includes: setting the airflow switch, testing the solenoid valve, confirming the trip sequence, testing manual reset, and completing the commissioning documentation. Gas Safe notification must be submitted within 28 days.

Regulations & Standards

• BS 6173:2009 — specification for installation of gas cooking appliances in commercial catering establishments; gas interlock requirements

• IGEM/UP/1B — Institute of Gas Engineers and Managers; guidance on commercial gas installation including ECVs and interlock systems

• Building Regulations Part F (2021) — ventilation for buildings; commercial kitchen extract requirements

• Gas Safety (Installation and Use) Regulations 1998 — requires all commercial gas work to be carried out by Gas Safe registered engineers

• COSHH Regulations 2002 — risk assessment for combustion products in commercial kitchens; interlock systems as a COSHH control measure

• Gas Safe Register: Commercial Kitchen Safety — Gas Safe Register

• IGEM/UP/1B: Commercial Gas Installations — Institute of Gas Engineers and Managers

• Building Regulations Approved Document F — HM Government

• CHAS: Commercial Kitchen Compliance — Contractors Health and Safety Assessment Scheme

• Catering Equipment Suppliers Association (CESA) — guidance on commercial kitchen gas and ventilation

• gas pipework installation — gas pipe sizing, materials, and pressure testing

• flue liner installation — solid fuel flue liner installation requirements

• coshh — COSHH risk assessment principles