CCN1 Study Guide 2026

Where a gas flue pipe passes through combustible material (such as a timber floor, ceiling, or wall), a non-combustible fire-resistant sleeve must be fitted. There must be a minimum 25 mm air gap between the flue pipe and the combustible material, filled with non-combustible material if necessary. This prevents heat transfer from the flue igniting the surrounding structure.

Section 2 of the Health and Safety at Work etc. Act 1974 places a general duty on employers to ensure, so far as is reasonably practicable, the health, safety, and welfare at work of all employees. Section 3 extends this duty to non-employees who may be affected by the undertaking.

CO alarms provide an audible warning when CO concentrations rise, giving occupants time to evacuate before reaching dangerous levels. They are the only way to detect CO, which has no smell or colour.

Vitiation occurs when an open-flued or flueless appliance consumes oxygen from the room for combustion, progressively reducing the oxygen level in the space. This can lead to incomplete combustion, increased carbon monoxide production, and ultimately a dangerous situation if adequate ventilation is not provided.

The standard working pressure for natural gas in UK domestic properties is 21 mbar (approximately 8.4 inches water gauge). This is the nominal pressure maintained by the gas distribution network at the meter outlet. Engineers must verify this pressure when commissioning or servicing appliances.

A let-by test checks whether the meter's internal valve (ECV closed, downstream open) is passing gas. The test is performed with the ECV closed and the pressure monitored. Any rise in pressure on the installation side indicates the ECV is letting by, which constitutes a gas escape that must be reported to the gas transporter.