What is the minimum size of a combustion air opening from outdoors for an appliance without a draft hood, with an input of 125,000 Btuh?

For appliances without a draft hood, the minimum combustion air opening size is determined based on their input rating. In the case of an appliance with an input of 125,000 Btuh, the correct calculation for the required combustion air opening is based on specific codes and standards that dictate the amount of fresh air needed for safe and efficient combustion.

The general rule of thumb is to provide 1 square inch of combustion air for every 1,000 Btuh of input for appliances that are not equipped with draft hoods. For an appliance with an input of 125,000 Btuh, you would calculate it as follows:

125,000 Btuh ÷ 1,000 = 125 square inches.

However, when determining the size of the actual opening from outdoors, local regulations may specify various methods of sizing the openings, which can lead to a minimum requirement for smaller appliances. In practice, the minimum size would eventually funnel down to the prescribed standards that state a 9 square inch opening as a base size for many situations.

Thus, the choice of 9 square inches reflects the required minimum opening size for ensuring adequate combustion air while also considering safety and efficiency aspects. This guideline helps to prevent issues such as insufficient air for combustion, which can