Ventilation. Open-flued appliance installed in a compartment requiring purpose-provided high and low-level permanent air supply, providing less than 90% of the requirement for each ventilator position.
Both high and low ventilation are required. Correct sizing, position and configuration of air vents is essential to prevent vitiation occurring within the compartment.
An air supply that is less than 10% undersized is unlikely to have a detrimental effect on the safe operation of the gas appliance.