Gas Cooker and Hob Installation: Bayonet Fittings, Proximity Rules, Category I/II/III Appliances

Summary

Gas cooker installation is high-volume bread-and-butter work for Gas Safe domestic engineers. Despite its apparent simplicity — connect the hose, check for leaks, light the burners — there are numerous technical and regulatory requirements that are frequently missed. The consequences of incorrect installation include gas leaks, carbon monoxide risk, fires, and potential prosecution under the Gas Safety (Installation and Use) Regulations 1998.

The most common installation errors are: using a flexible hose longer than 1.5m (maximum), failing to check that a second-hand appliance is category-matched to the gas supply, ignoring proximity requirements for combustible surfaces, and not conducting a tightness test after connection. Each of these creates a potential safety incident.

The appliance category system (I, II, III) is particularly important when dealing with second-hand or imported appliances. An appliance marked for category I (G20 natural gas at 20mbar) may physically connect to an LPG supply but will have completely incorrect gas rates and air/fuel ratios, leading to incomplete combustion and potentially lethal CO production.

Key Facts

• Gas Safe registration — Any gas work including cooker connection requires Gas Safe registration; no exemption for simple connections
• BS 669 bayonet fitting — Standard UK bayonet socket for freestanding cooker flexible connection; self-sealing when disconnected
• Maximum hose length — 1.5m (including any bends) per IGEM/UP/2i; do not extend with couplings
• Hose standard — BS 669 reinforced flexible hose; replacement interval typically 5 years per manufacturer instructions; check for kinks, damage before connection
• Tightness test — Mandatory after every connection; U-gauge test per IGEM/UP/1B
• Category I appliances — G20 (natural gas, 20mbar inlet pressure); cannot be used with LPG without conversion
• Category II appliances — Dual gas; designed for two gas types with conversion (usually natural gas and LPG)
• Category III appliances — G30 (butane, 28–30mbar) or G31 (propane, 37mbar); LPG only
• Proximity: side — Minimum 50mm from hob edge to adjacent vertical combustible surface (e.g. cupboard unit side panel)
• Proximity: above — Manufacturer specifies height above hob to combustible overhead surface; typically 600–760mm to extract canopy; check manufacturer data sheet
• Proximity: below — If oven below hob, it must be a matched/compatible appliance; not a storage drawer on some models
• Ignition cables — Check ignition cable routing; should not contact gas pipework or hot surfaces
• Earth bonding — Gas pipework bonding required per BS 7671; earth clamp within 600mm of meter ECV (main bonding) and supplementary bonding in kitchen if applicable
• Commissioning — Light each burner; check flame appearance; check ignition; check flame supervision device (FFD) on modern appliances

Quick Reference Table

Detailed Guidance

Freestanding Cooker Connection

The standard connection for a UK freestanding gas cooker uses a BS 669 bayonet fitting:

Supply side (wall-mounted bayonet socket):

• Female bayonet socket on a rigid tail protruding from wall or floor
• Self-sealing valve closes automatically when appliance connector is removed
• Must be within 1.5m of cooker connection point (measured along the flexible hose routing)

Appliance side:

• Male bayonet connector on the flexible hose attached to appliance
• Push and turn (similar to a bayonet light fitting) to connect

Flexible hose specification:

• Must comply with BS 669 Part 1 or Part 2
• Maximum 1.5m length
• Must not pass through walls, floors, or ceilings
• Must not be hidden behind panels or plinths (must be accessible for inspection)
• Check date stamp; replace if over 5 years old or if any visible damage, kinking or cracking
• Natural gas hoses are orange; LPG hoses may be orange or yellow (check marking)

After connection:

1. Turn on gas at ECV
2. Conduct tightness test using U-gauge manometer: apply 20mbar pressure, isolate supply, check for pressure drop over 1–2 minutes
3. If test passes, light all burners in turn
4. Check flame appearance: natural gas = blue with possible yellow tips; excessive yellow/orange indicates air starved or incorrect gas rate

Built-in Hob Connection

Built-in hobs do not use bayonet fittings — they connect via rigid pipework or approved flexible connectors:

Rigid pipework:

• Run 15mm copper or stainless steel pipe to the rear of hob position
• Fit an isolating valve (accessible, not hidden) within 1m of appliance
• Connect to hob inlet using compression or appropriate fitting per hob manufacturer specification
• No flexible hose in void unless manufacturer's specifically approved flexible connector used

Flexible connectors for built-in hobs:

• IGEM/UP/2i permits flexible metallic connections in cabinetry installations
• Must be manufacturer-approved type (check appliance installation manual)
• Maximum length typically 500mm for cabinetry installations
• Must be accessible for inspection — some installers fail this by permanently panelling over the connection

Cabinetry considerations:

• Under-hob cupboard: if closed unit with gas appliance, adequate ventilation must be present
• Ventilation: opening at front bottom of unit or duct to outside; typically 5,000mm² per kW gas rate [verify specific figure in manufacturer installation instructions]
• Never seal a gas appliance inside an airtight unit

Proximity Requirements for Combustible Surfaces

Adjacent vertical surfaces (cupboard sides, walls): Per the manufacturer's installation instructions (always takes precedence), and per IGEM/UP/2i general guidance:

• Minimum 50mm from hob edge to adjacent combustible side panel
• Some manufacturers specify 100–200mm for high-output burners
• If side panel is non-combustible (tiled, metal), clearance may be reduced — check manufacturer data

Above the hob:

• Minimum distance from hob top surface to overhead combustible surface (e.g. wooden extractor canopy): varies by manufacturer, typically 600–760mm
• Extract canopy must be manufacturer-approved gas-rated type
• Under-cabinet electric extractor over gas hob: minimum 650mm to hob in most cases
• A circulating (non-ducted) extractor must never be positioned at the minimums above — grease accumulation plus gas flame is a fire risk. Ducted extraction preferred.

Below the hob (underhob space):

• Manufacturer determines whether a storage drawer (cold) is acceptable directly below
• Many hobs specify a minimum clearance between burner grate and item in drawer
• Ovens: only a compatible matched combination oven can be installed in the below-hob cabinet without specific clearance tests

Appliance Category Identification and Conversion

Reading the data plate: Every gas appliance has a data plate (usually on the back, inside the door, or on the back of a drawer) showing:

• Appliance category: I, II, II+ or III
• Gas type: G20, G25, G30, G31
• Inlet pressure (mbar)
• Heat input/output (kW or BTU/hr)
• Country(ies) of sale (e.g. GB = Great Britain)

Identifying a second-hand appliance:

1. Find and read the data plate
2. Confirm category against supply type (G20 for natural gas, G30/G31 for LPG)
3. If category does not match supply: do not connect until converted

Conversion procedure:

• Obtain conversion kit from manufacturer (specific to model)
• Kit contains replacement injectors for each burner in the correct size for the new gas type
• For oven burners: replacement injector and possible pressure adjustment
• Retain conversion label; affix to appliance after conversion
• Re-test all burners after conversion

Imported appliances: Appliances bought in other European countries may be correctly CE-marked but configured for different supply pressures or gas categories common in those countries. An Italian-market appliance may be G20 at 20mbar (compatible) or G25 (Dutch network gas, different Wobbe index). Always verify before connection.

Tightness Testing After Installation

A tightness test is mandatory after every gas connection, regardless of how confident the engineer is in the quality of the joints.

Procedure (IGEM/UP/1B domestic tightness test):

1. Close all appliance cocks
2. Connect U-gauge manometer to test point (usually on meter or ECV outlet)
3. Pressurise installation to working pressure (approximately 20–25mbar)
4. Observe pressure over 2 minutes
5. Pressure must remain stable (no drop)
6. If pressure drops: there is a leak; do not complete the installation until found and repaired

Locating leaks:

• Use gas leak detector fluid (Snoop or equivalent) on all joints
• Never use a naked flame to detect leaks
• Electronic gas detectors can identify leak location in accessible areas

Record keeping:

• Record tightness test in Gas Safe notification
• Landlords require Gas Safety Certificate (CP12) for each gas appliance annually — includes tightness test

Frequently Asked Questions

Can a customer connect their own gas cooker?

No. Under the Gas Safety (Installation and Use) Regulations 1998, any work on a gas fitting — including connecting a gas cooker to an existing bayonet socket — is classed as gas work and must be carried out by a Gas Safe registered engineer. There is no DIY exemption for domestic gas connections.

The existing flexible hose looks fine — does it need to be replaced?

The flexible hose should be replaced if: it is more than 5 years old (check date stamp on hose); it shows any kinking, cracking, discolouration, or physical damage; or the existing hose is not to BS 669. The age of the hose is hard to assess visually. If date stamp is absent, replace it. A new hose costs £15–£30 and takes 5 minutes to fit — the risk of a faulty hose is not worth the saving.

What is the maximum number of gas appliances that can be connected to a standard domestic supply?

There is no fixed maximum by count, but total gas demand must not exceed the capacity of the meter, supply pipe, and regulator. Use the calculation: sum all appliance inputs in m³/hour (divide kW input by 10.8 for natural gas); if total is under 6 m³/hour, a standard U6 meter is adequate. For larger combined loads, a meter upgrade is required (see gas meter upgrade).

What is an FFD and do I need to test it?

An FFD (Flame Failure Device) is a thermocouple or thermopile that cuts off gas if the flame goes out. Modern hobs and most modern ovens have FFDs on all burners. Testing is simple: light the burner, hold the ignition button for 15 seconds to heat the thermocouple, then release — the flame should stay on. If it goes out, the FFD may be defective or the thermocouple/seat may need cleaning. A non-functioning FFD means gas continues to flow if the flame blows out — this is a serious safety hazard and must be repaired or the appliance condemned.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — primary regulation for all domestic gas work

• IGEM/UP/2i — Installation pipework on industrial and commercial premises; referenced for domestic connections also

• IGEM/UP/1B — Tightness testing and purging of domestic natural gas installations

• BS 669 Part 1 and Part 2 — Bayonet fittings and flexible hoses for gas appliances

• BS EN 30-1-1 — Domestic cooking appliances burning gas (safety requirements)

• European Gas Appliance Regulation (EU/2016/426) — applies to all appliances sold in UK/EU; category and marking requirements

• BS 7671:2018+A2:2022 — Earth bonding requirements for gas pipework

• HSE Gas Safety Installation and Use Regulations — Overview of GSIUR requirements

• Gas Safe Register Technical Bulletins — Technical bulletins on specific installation topics

• IGEM Publications — IGEM/UP/2i and IGEM/UP/1B

• gas meter upgrade — Meter capacity and upgrade when demand exceeds U6 rating

• gas smell — Emergency procedure and tightness test if gas smell present after installation