HVAC Return Air Filter Box

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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

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    19 Discussions

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    Kurokami

    4 days ago

    That is a wonderful plenum, I've seen "professionals" choke a three square foot duct to less than one.

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    DrummerD23

    23 days ago

    So great! Some would say it's over-engineered but when you have the math to back it up it's worth the extra effort for the peace of mind.
    A little more detail would help for an Instructible, but still an inspiring project.

    2 replies
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    helijocDrummerD23

    Reply 23 days ago

    I'm sorry for the lack of detail. I built this 5 years ago and only took minimal photos for my records. I had never considered sharing it here until recently.

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    The is an excellent tutorial. You should seriously consider entering it into one of the contests that are currently running on the site.

    2 replies
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    studleylee

    21 days ago

    Wow!!! Not over engineered: perfect!!!!!! Well Executed!!!!

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    Klendall

    23 days ago

    A bit over done, but hey it's in your own home and if you don't work in ventilation on a daily I guess it's better to be over done then done wrong. Good job.

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    helijocKlendall

    Reply 23 days ago

    With this return and a few other "upgrades" to the ducting I was able to use a 3.5 ton unit instead of a 4.0 that was the recommended size. Last summer with 115 degree days it had no problem keeping 72 degrees.

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    ShawnR77

    Question 23 days ago on Step 9

    Is that insulated flex duct you have connected to the return box?

    1 answer
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    charlessenf-gm

    23 days ago

    Excellent work! Some in 'the industry' might come up with the 'over done' remarks, but if a job is worth doing, it is worth doing the right way the first time. In my opinion, folks in 'the industry' are far more likely to cut corners than shoot for perfection.
    Yours is a perfect seamless construction as opposed to a box made of several sections with all manner of seams 'taped shut' and a 'mess' upon close inspection.
    I agree that additional detail on your fiberglassing and insulating materials would be appreciated - maybe you could add a bit. For instance - Six layers of 25# fiberglas cloth or 25 pounds of fiberglas cloth that weighed twenty-five pounds?
    What did the final piece weigh - or do you know? It looks rather thin in the pics, but six layers - how thick were the walls?
    One assumes you put your Air Handler in the Attic as opposed to mounting it in a 'closet' on the ground/main floor because there was no room for a 'closet' for the air handler.
    Did you ever consider designing a duct take off box for the other end of the equation?
    One question, if the area of the filter is 800 sq inches and the diameter of the exit is 18" or 254 SQ IN, does that 'choke point' negate the effort?

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    helijoccharlessenf-gm

    Reply 23 days ago

    The cloth I used was left over from a boat build, I don't know the weight but it is a thick coarse weave. The finished layup had 1/8 - 3/16 thick walls. This is not a structural part so it only needs to be strong enough to handle. Once installed the framing gives it support. I am guessing here on weight about 25 pounds.
    At the 18" duct the air flows faster than at the filter. At the filters the air is moving slower, the slower movement allows the dust particles to be caught rather than forced through, more efficient cleaning.

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    mf70

    23 days ago

    That;'s wonderful. Sadly, my app is a complex stove duct, and I'm afrfaid I'll have to set a tin-knocker loose to get what I want that won't melt. Perhaps your idea of a foam mock-up would still speed the process (and might help in validating installation steps as well.)

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    GTO3x2

    23 days ago

    Wow! That is some determination and good work. Even the insulation is a great job.

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    mr3d_guy

    23 days ago

    This was an excellent solution to tricky and tight geometric requirements. The positive plug method could be used to generate a mold for any project. Your use of common materials is a plus.

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    P DavidM

    23 days ago on Step 9

    I know you are being efficient with the shape, but dang, it's really beautiful. How much other sculpture are you hiding in your ceiling?

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    seamster

    24 days ago

    Darn - you skipped over the aspect I was most interested in! ; )

    If you're inclined, I think it would be awesome to see at least a couple steps on the fiberglass process, or even a whole separate instructable on it. Some info on what specific products you used, and any tips for how to make the process go smooth would really be nice to see included. Very good first instructable though!! Hope you'll share more!