Replacement Packaged Unit Sizing with some caveats

[rbelisle1]

"Something else to remember is that by going downshot, the static pressure will run higher on some units. At least with an elbow, the air can come straight out the back of the unit and make a gradual downturn through the roof. Downshot, it smacks the back of the unit and has to force its way down. "

Yep, thats a fact. take my Rheem rjnla060 (package unit) for example. The manual shows that you should calculate between 0.05@1400 cfm and 0.1 @2400cfm additional static pressures for downflow applications.

Of course, a really good tin bender can fix that with by fab'ing up a internal elbow with turning vanes. I was able to remove almost 0.15 inwc of static pressure by having an internal elbow w/ turning vanes made and installed.

[ACFIXR]

"Something else to remember is that by going downshot, the static pressure will run higher on some units. At least with an elbow, the air can come straight out the back of the unit and make a gradual downturn through the roof. Downshot, it smacks the back of the unit and has to force its way down. "

Yep, thats a fact. take my Rheem rjnla060 (package unit) for example. The manual shows that you should calculate between 0.05@1400 cfm and 0.1 @2400cfm additional static pressures for downflow applications.

Of course, a really good tin bender can fix that with by fab'ing up a internal elbow with turning vanes. I was able to remove almost 0.15 inwc of static pressure by having an internal elbow w/ turning vanes made and installed.

It's the configuration of the unit openings that cause a static difference, not because it's a downflow. The air coming off of the coil is at about 500 fpm, then it fills the cavity.The pressure then forces it out of the unit openings and gives it velocity. I would not put anything inside the cabinet to "help" the air flow. The only way to "help" is to make the unit openings larger and/ or improve the design of the first fittings.

[rbelisle1]

Dont know what to tell you. Static pressure went down, CFM delivered via the supply ducts when up. This along with a raft of other changes lowered my bills by 25 percent. It may not be for everyone, but sure it works for me.