What is the most common connection size for automatic air vents in residential applications?

In residential applications, the most common connection size for automatic air vents is indeed 1/8" NPT. This size is typically used for smaller valves and fittings, which is appropriate for the compact nature of air vents. Automatic air vents are designed to remove air from hydronic heating systems, and due to their size and function, a 1/8" NPT connection is often sufficient to allow for the necessary flow while keeping the fixture lightweight and unobtrusive.

The other sizes mentioned are generally used for different applications or components. The 1/4" NPT connection is often found in applications that require slightly larger flow rates, such as in some valves or fittings, while 1/2" NPT is more commonly used in larger piping systems where increased capacity is needed. The 3/8" NPT connection can also serve in various plumbing applications, but it is less typical for air vents than the 1/8" size. Understanding these typical applications helps in selecting the right size for efficient installation and operation.