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I did my homework here the best I could and I hope someone can help by checking my work.

In an attempt to make things as clear as possible, I made the following diagram. This is my existing system except for the planned return vents which are in red.

enter image description here

The entirety of the duct work plus the air handler are in the attic, hence why everything is in the ceiling. The air handler is a Rheem RH1T3617STANJA.

There are two main issues I'm trying to solve for:

  1. A single, undersized 10x30 filtered return air grille for the whole 2.5 ton rheem heat pump
  2. The return air grille is very noisy

By adding a return air vent to each room, I believe I can effectively add return air capacity and reduce noise from the existing return grille. In trying to figure out a solution, I learned about the concept of face velocity (ex: my undersized return has a high face velocity, thus it is noisy), so I accounted for that in my plans.

To calculate the size of the return ducts, I simply took the total CFMs that the current supply ducts in each room could handle according to the chart to the right of my diagram. Where there was no exact match, I errored on the larger side for the return air duct and plan to include a damper to adjust airflow accordingly.

For example, since the Guest bed and bathroom have a 7" and 4" duct, the total supply capacity is 140 cfm, the return duct will be an 8" duct with a capacity of 170 cfm. Because of the difference, a damper on the return duct will let me adjust to find more balanced airflow.

To keep the noise down from the new return air vents, I'm using the estimated return air CFM and cross referencing it with the spec sheets of the manufacturers of filtered return air grilles to keep the face velocity of the filtered grille between 200-300 cfm.

Because of the location of the rooms relative to the air handler and trying to keep the duct runs as short as possible, the guest room return will connect directly to the ductboard box on the return side of the air handler while the master bedroom return duct will connect to the ductboard box containing the existing return.

If it means anything, all the interior doors have return air vents in the bottom of the doors to help keep things balanced.

I'm pretty green when it comes to planning out a system this way, so any feedback or constructive criticism is welcome. However, I've installed ductwork in the past.

I'm trying to get the most out of my AC in this very old Florida home. Called a local AC company and their suggestion was to add another 10x30 grille next to the existing one and tie it to the existing ductboard box for expanded capacity and keeping the returns central. I started considering the return vents in each room instead of a larger central return because any larger of a central return would require cutting ceiling joists that are right next to a load bearing wall and I don't want to deal with that.

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  • All these return ducts inthe attic are a route for heat gain in the a/c mode. What is the temperature of the attic? Our 2000 sqft tract house has no return ductwork. The return is under the doors of the four bedrooms and the two bathrooms. The air handler is in a closet at the entrance of a central hall and has two grilled return openings. It is pretty noisy in the a/c mode but our electric usage has been less than if we had return ducts. As built there was one return grill and our hvac serivce added one to the plenum at a right angle to the original. Commented Jan 29 at 12:00
  • It's an unvented attic, so it can get up to ~120 at its worst in there. However, very large shade trees do help mitigate some of the worst temps. I get the duct work being in the attic isn't ideal, but its what I have to work with. I will be installing a radiant barrier along the rafters and a powered exhaust fan to further mitigate high temps.
    – deefofum
    Commented Jan 29 at 15:24
  • If you are installing a power exhaust, are you also putting in soffit vents? What kind of roof is this: hip roof or gable? Where is the insulation: on the attic side of the ceiling or on the underside of the roof decking? What are the roof penetrations: sewer vents, furnace flue, water heater flue? How is the house heated and the water heated? Commented Jan 29 at 16:47
  • It's an old craftsmen bungalow, so a typical gable roof. There are no soffit intakes, intake air would be entirely from the vents on each gable. I already did some math and found that there's enough intake area for the fan I want based on the attic square footage. Insulation is blown in cellulose on the attic floor. The only roof penetration is the sewer vent that's pretty much in the center of the home. House is heated by the same HVAC system in question--it's a heat pump. Water is heated by an outdoor tankless gas heater.
    – deefofum
    Commented Jan 29 at 17:38
  • Are gable vents already there or are you planning on adding them? Adding soffit vents at the eaves could be a problem if insulation is piled up. If gable vents are already there, would the flow through the attic been driven by a prevailing wind? Would the power exhaust go out one gable vent so that incoming air would come in the other on? What is the type and pitch of the roof? Commented Jan 29 at 19:23

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