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.

[dash]

The stand may have n opening much smaller the the unit,if so this kills return air flow.Another practice is two elbows of flex in the stand.

Both common in your area and both cripple the airflow to the farthest rooms.

Is the existing return ducted from the closet,if so what size duct to the return grille??

[Isailer]

The return isn't ducted. The air handler sits in a closet on top of plywood. Directly underneath it is the return, they cut a whole in the wall and installed a filter grill. They didn't seal anything either and is why it was pulling air from the attic.

[udarrell]

The return isn't ducted. The air handler sits in a closet on top of plywood. Directly underneath it is the return, they cut a whole in the wall and installed a filter grill. They didn't seal anything either and is why it was pulling air from the attic.

It may have been one of my posts that you saw the warning concerning hot air from the attic with the return under the air handler.

I have seen that too many times, therefore checks for Return air leaks ought to be done every time & logged on the systems record for the next tech. IMO, service logs should be kept by both the tech & the customer; saves time on future service calls.

Another way to check that situation is to check the condenser air discharge temperature verses the indoor SA/RA split & wet bulb. If the condenser split is real high & the indoor split is low, check the wet bulb or humidity latent load, that will tell you if there is a discrepancy, indicating hot air is entering the return some point.

That, & low airflow situations explains a lot of the high utility bills. - Darrell

[dash]

The return isn't ducted. The air handler sits in a closet on top of plywood. Directly underneath it is the return, they cut a whole in the wall and installed a filter grill. They didn't seal anything either and is why it was pulling air from the attic.

Is the plywood cut out to match the size of the air handler inlet??

With that setup,your return isn't all that undersized,just the grille.It is usually diffficult to add a return to your type setup,other than a hole in another wall of the closet,doubt that will make much difference .

Are there any closed doors between the return grille and the BR area??


Check the BR's for airflow ,with the doors closed and then open,if there's a large difference,you need jumper returns from the BR's.If not you likely need more supply air to the BR's.

[Isailer]

Dash,
The air handler inlet size is good. You're correct that it's just the grill that is undersized and I can't install a larger one because there isn't enough space.

It wont be too difficult to expand the return since I'm going to fur out the wall beside the closet to enclose the additional duct.

There isn't any closed doors, it's merely a entryway approx. 3' x 7' leading into the hallway. The hallway is approx 30 feet long and connects all the bedrooms which are approx. 165sq ft each. The house is approx 3000sq ft to give you an idea of the overall size.

I tried shutting the doors to the bedrooms and 3 of them get about 1-2 degrees cooler with the door left shut (after several hours). I didn't get a chance to check the master bedroom which is a bigger room.

ETA - There is still about a 2-3 degree difference between the rest of the house and the hallway/bedrooms.

Darrel,
Since I've sealed the leaks I can already tell that the system runs for a much shorter period of time. In fact, at night it short cycles more than anything.

[dash]

"I tried shutting the doors to the bedrooms and 3 of them get about 1-2 degrees cooler with the door left shut (after several hours)."

That is strange.Sure it wasn't just getting cooler outside?

[Isailer]

I just rechecked since I had left one of the doors shut and the bedroom is 2 degrees cooler than the hallway. There are no supply vents in the hallway only the bedrooms. I know this is true during the heat of the day as well.

I'm pretty sure the master bedroom may get warmer since it is already warmer than the other rooms (reads 79 when the living room is 76) but I'll check it tomorrow with the door shut.