Return size/Addition of a remote return

[Isailer]

I've recently realized that my return is under sized. It's a 24" x 18" filter grill for a 3 1/2 ton unit. It's not possible to expand the existing return to the 24" x 24" required without some major changes. Therefore, I've decided to add a remote return. I think this approach will be more beneficial anyways since it's a split design house with a single return.

I wanted to connect the remote return using 14" insulated flex duct but the A/C equipment supplier freaked when I told him the length of this run will be 60'. He stated I couldn't get enough CFM to make it a viable option. I disagree since my figures show I only need an additional 600 CFM through the 14" flex. My calculator shows this can be accomplished and end up with a duct velocity of 562 FPM.

What do you guys say?

[beenthere]

Nothing wrong with it.

How much you will actually get. Will depend on bends, take off, start collar, and grille restriction.

[Isailer]

True, there will be two bends and two 90's, although, I'll make the two 90's out of duct board to get it into the attic from the unit. Do you think 14" would be appropriate or should I increase it to 16" flex?

[udarrell]

I ran it with one 90, & it still gets that exact velocity & CFM at 0.02" FR rate with that 60 foot run. It is 0.04" FR for a 100' run.

A 16" 60 foot run RA metal duct could deliver 800-CFM @ a velocity of 573-FPM, a 0.02" FR at 60-ft, @ a 100' of run it's a 0.03" FR.

As beenthere says, consider all the factors.

Make sure the filter racks are oversized, & maybe even go to a larger diameter duct. - Darrell

[captube]

You will pick up some heat from that run in the attic, depending where your located it could be a lot.
You can't find a suitable location closer?
Use r-8 at the minimum, or should i say have your installer use r-8.

[Isailer]

I could technically put another filter grill on the other side of the wall in the laundry room but I'm assuming I would benefit more by installing it on the other side of the house. I'll briefly describe the layout of my house.

On the West side I have the laundry room (where the A/C closet is) and the kitchen. In the center, I have the Dining room (where the return is), living room, and study. On the East side of the house all four bedrooms are connected by a long hallway. There is only one entry way leading from this hallway to the rest of the house. The house has an open floor plan until you get to hallway where the bedrooms are located. Because of this separation, I figured it would be better to install an additional return on this side of the house.

Before I go to the trouble, in your collective opinion, will it make a significant (noticeable) difference to do this? Keeping in mind of course that the current return is under sized?

Also, I'll check on the R-8. I know they had R-6 in stock but I'll see if they have R-8 and if it's not too much more money I'll purchase it.

[dash]

We use R6 in Florida attics,not that R8 isn't beter,but not a big issue.


Adding the return will lower the static,deliver more air ,and maybe more btus,problem is we don't know where you are at,btu wise,or where you will be going,as no one has calculated that.

[Isailer]

I checked and all of my existing duct work is rated R-6 and I'm located in north Florida.

I'm going to start this project this weekend and I had a couple of more questions. What filter grill size do I need for the 14" flex to obtain at least 600 cfm? I haven't measured my ceiling joists yet but I'd like the grill to fit between it to prevent complicating the install. Whatever size you recommend I can compensate by lengthening it if necessary to make it fit.

Also, I forgot to ask if the outside of my air handler sweating is because there is not enough airflow due to the under sized return?

[beenthere]

Low air flow can cause that.

You want around 400 sq in face.

[dash]

Go with Beens suggestion,sweating can be caused by low airflow.

Where's the indoor unit?

[Isailer]

Dash,
Read my post #6 and I'm not being rude by not repeating the location but rather I'd like your opinion if I'm making the right decision by adding a remote return on that side of the house.

Today I've pretty much confirmed (at least for myself) that adding the additional return in the hallway would be beneficial. I reached that conclusion by taking temperature readings in every room of the house today. The entire house aside from that separate hallway and 4 bedrooms was about the same temp, however, the hallway and all the bedrooms were 3-4 degrees warmer then the rest of the house. I'm thinking this is due to poor air circulation.

Also, I wanted to thank whomever suggested that I could be pulling in hot air from the attic in another thread. I inspected the air handler closet and used a burning incense to look for drafts. Well my closet wasn't sealed well and it was in fact pulling air through the seams in the wall and from the attic. I resealed the walls and the bottom plate where the suction was coming from. Now when I open the closet it's cool instead of hot!

[thatheatguy]

its been my experience that if theres one re occuring problem with any system, undersized returns, the best system is one you cant hear running.you can never have to much return while R-8 is required up here in washington for flex runnin thru an unconditioned space you need to check with your local county for proper code, the only thing worse than draggin a long piece of flex thru a hot attic is replacing that piece of flex when an inspector tags your job

[dash]

If the return is undersized,adding more will improve the system.

If the bedrooms cool well with the doors open,they a return or return path from each room,if they don't they could have undersized supplies.This assumes that area is not separated from the existing return grille.

Severely undersized retun,will reduce airflow to the farthest rooms,so corrcting this may increase air flow to the bedrooms.

If we were on site we could test and tell which it is or both.Can't tell from here.

Code in Florida today,new homes,would require a return in each BR,so you really should address that.Supply air can't get into a room,if it has no path back to the return.Feel the air flow from the grille,thenclose the BR door,if the flow drops,the existing return path ,under the door,isn't enough.