How to Replace the Box That an Air Handler Sits On

Look carefully at the installation to determine the most efficient methods of entry and removal of the current plenum. Hang a lamp or two over the workspace, if necessary, to provide adequate light.

Detach any duct work feeding into the box. Use the reciprocating saw to cut into the plane the plenum box feeds into and trace around the air canal. Cut shallow as to avoid damaging anything on the inside. Remove all ducts from the plenum area.

Dismantle the plenum by wedging a crowbar into the joints or cutting through the join lines of the box. Remove the sections of the old box one at a time.

Measure the dimensions of the air handler unit to determine the size of the new box. Sketch the design of the new box to scale on a piece of graph paper. For example, if you require a box that is 3-by-3 feet, determine what a foot measures on the graph paper, i.e., two grids, and sketch the shape appropriately.

Draw cut lines on the sheet metal according to your dimensions. Make lines to accommodate each of the six planes necessary to build the box. Use a straight edge to make the lines.

Secure the sheet metal to be cut to your workstation and use the cutting torch to make the panels of the plenum box.

Determine the proper HVAC insulation to use for the climate where the plenum resides. This is determined by the R-value of the insulation. Cut five sheets of insulation to match the dimensions of the planes of sheet metal. Attach the insulation to the metal panels with foil tape.

Clear your work space and prep your welding equipment and don your safety gear, most importantly eye protection and gloves. Work with two panels at a time, welding the corners together at right angles. Weld the shapes together until the box is complete.

Take measurements of any ducts that must feed into the plenum, if necessary. Mark the space on the face of the box and then make your cuts with sheet metal snips.

Insert the start collar into the hole you cut in the box, then bend the tabs on the edge of the collar inside the box until they are perpendicular to the body of the box and sit in place. Next attach install the box in place on the air handler and attach the duct work to the start collar.

Wrap the edges of the plenum box and any tubes feeding into it with foil tape.