Introduction: HVAC Return Air Filter Box

Custom A/C return air duct

The project started when I decided to install central air in my house. I solicited the help of a friend who was an HVAC contractor. He gave me a lot of very helpful information and advice.

I did a lot of research on the internet and found that a common problem lacking in most residential HVAC installations was having enough return air filter area. I wanted to be sure that my installation would not suffer from that. I did the load calculations and I needed a 3.5 ton package unit.

I designed to have (2) 20 x 20 return filters giving me 800 sq in of filter area , (700 sq in is the recommended). Because my roof was a 3-12 pitch the attic space is real cramped and the supply ducting filled it up leaving little room for the return ducting. The best solution was to make a custom return duct / boot/ filter holder.

The following pictures show the process. I made a foam mock up of the inside of the duct starting with a base and 19” round to represent the “in” and “out” of the duct. I used blue non–beaded foam.

After the air handler was mounted on the roof with the plenum in place these first two pieces of foam were placed in position in the attic space and joined. Then back in the shop I added foam to fill in the perimeter. The surface imperfections were filled with drywall mud. The whole thing was then painted 2 coats with latex paint to protect it from the vinyl ester resin used in the fiberglass process.

I used fiberglass mat cloth for building boats. I used 6 layers of cloth approximately 25 pounds, and 2 gallons of vinyl ester resin. Once the resin dried I chipped the foam out and painted the inside to have a smooth surface for airflow.

Step 1: Modeling the Important Dimensions

The rectangle part represents where the air filters will be and the round part represents where the metal start collar (added later) will fit. I used Styrofoam to make a plug that is the exact measurements of the inside of the duct I want to create. The Styrofoam I used is available at Home Depot, called blue foam board.

Step 2: Filling in the Mold, Starting to Take Shape

Thin foam is added to create a smooth transition. I used gorilla glue that foams up to assemble, it fills and sands easily.

Step 3: Smoothing the Contours

I used drywall mud to fill in all the imperfections and create smooth contours for the airflow. The easier the air flows the less energy is used.

Step 4: A Protection Layer

Latex paint makes a good barrier for the resin used in the fiberglass process. The resin will dissolve the Styrofoam plug quickly without a barrier. Use as many layers (drying in between) of paint as you can. A small pinhole in the barrier will create a very large void in the foam plug ruining hours of work. I stagger paint coats with contrasting colors to be sure each coat covers the previous one.

Step 5: After the Resin Has Cured

I did not explain how to fiberglass the plug, that is another topic in itself. After the fiberglass has cured the foam is chipped out and cleaned up with acetone. Then the entire interior is sanded, remember airflow likes a smooth surface. The 18" metal start collar is now added and a few layers of fiberglass to secure it.

Step 6: Paint the Inside

Use a good quality automotive paint for the inside to get a smooth finish.

Step 7: Insulate the Outside

I did not spend much time on the outside fiberglass since it was going to be insulated.

Step 8: Testing the Fit

The ceiling joists needed a bit of adjusting to make a snug fit.

Step 9: The Finished Product