When providing outdoor combustion air through two openings, what must one opening begin with?

When providing outdoor combustion air through two openings, it's important to ensure that the openings are designed properly to facilitate efficient air intake for combustion processes. The requirement that one opening should start with 12 inches at the top and 12 inches at the bottom ensures that there is both a high and low-level intake for combustion air.

This design is beneficial because it allows for proper air mixing and circulation, minimizing the chances of negative pressure and ensuring that the combustion appliance receives adequate air. The top opening can draw in fresh air while the bottom opening allows for exhaust or displaced air to escape, creating a balanced airflow.

Moreover, having openings at different elevations helps accommodate variations in atmospheric conditions, such as wind direction and pressure changes, which can affect the performance of combustion appliances. This arrangement also reduces the risk of backdrafts that can occur when openings are not appropriately positioned.

In contexts where outdoor combustion air is needed, adhering to standards regarding placement and sizing of air intake openings is crucial for safe and efficient operation.