DIY Dictionary: Trunk Line

Family Handyman

What is a Trunk Line?

The word trunk often conjures up images of big things carrying lots of stuff: Car trunks crammed with suitcases, travel trunks packed with clothes, elephant trunks full of water. So, it comes as no surprise that the trunk line of a heating system is the large duct leading out of the furnace that delivers the massive amounts of air it takes to heat or cool a house.

This main trunk or distribution line often runs the length of a house and is punctuated by dozens of smaller feeder ducts that direct air into other areas of the home. The location of a trunk line depends on how and where the house was built. If a house has a basement, the trunk line is usually located there, but in warmer regions or areas where houses lack basements, trunk lines can run in the attic area. As trunk lines move further away from the furnace, they often taper in size for greater efficiency.

A trunk line must be sized properly for greatest efficiency—a calculation best left to the pros.