What is the diameter of the vertical vent pipes for the boilers?

The diameter of the vertical vent pipes for the boilers is critical for ensuring proper airflow and efficient operation of the heating system. An 18" diameter allows for sufficient exhaust flow, accommodating the volume of gases produced by the boilers without causing excessive back pressure. This helps in maintaining the efficiency of the boiler system and compliance with safety standards.

It’s important to note that vent pipe sizing is not arbitrary; it’s typically determined based on the output capacity of the equipment and the type of fuel being used. An improperly sized vent pipe could lead to various problems, such as inefficient energy use or hazardous exhaust conditions.

In this context, choosing the 18" diameter is the correct answer because it balances the need for adequate venting while aligning with standard practices and manufacturer specifications for boiler venting systems.