8 Plenums, Trunks and Branch lines

Now that we have our unit placed and have we have located all of the supply and return outlets and inlets, we need to connect them together. We need to install the duct work from the unit to the space. We do this with plenums, trunk lines and branch lines.

Plenums-The unit will first be connect to a plenum, which is basically a box which acts as a pressurizing chamber. There will be both a supply and return plenum.

Plenum Take Off (PTO) A PTO is the connection between the plenum and trunk line. Usually with a flex connection at this point, the PTO Is usually a transition fitting.

Supply trunk-The supply trunk typically runs from one end of the house to the other and is suspended from the floor joists. They must be size according to the size of the unit. The supply trunk also has acts like a pressure chamber in a way. Although it does have airflow within it, it works with static pressure to balance the air being delivered to each room. For this reason, we must follow guidelines on where and when to attach a branch line. The trunk is where all of the supply branch lines will originate from.

Return trunk-For the return, the trunk line is much shorter because it does not need to go all the way across the home. The return grills were located on the interior of the home, so the return trunk only needs to extend far enough to reach them. And in some case, we don’t even have the trunk go that far. We utilize the space between floor joists as the trunk. We use joist liner to encase the bottom of the joists, creating a passageway for the return air, without even using ductwork. Any cavity within the home can be used for return trunks. This saves materials used for ductwork and saves on installation time.

Branch lines-Starting with what is called a take-off, the branch lines come off the trunk line and go to each supply register.

The branch line has a damper to fine tune each rooms airflow needs. This damper is located in the take-off or as close to the trunk as possible. There is no reason to send conditioned air down the branch line if it will not be used.

The branch may have to contend with different obstacles. We have to be very mindful of this. It is easy to start grabbing elbows and flexible pipe to accomplish the run, but the over use of these items can quickly ruin a system. Both elbows and flex have their purpose, but we need to remember that both restrict airflow 3-4 times that of straight pipe. The proper use of these items must always be considered.