Adding another supply outlet


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Old 03-06-21, 11:17 AM
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Adding another supply outlet

I want to add another supply outlet to my master bedroom. It currently only has 1- 12 x 8 register fed by a 8” flex line. The room is approximately 16’ x 12’ x 12’ high ceilings. The house is approximately 1850 sq.ft. The ac system is a 3.5 ton Trane. I wanted to add a 12” x 7” supply register fed by a 6” flex line. Can someone please assist me with some professional advice on what I want to do? Reason I’m adding the additional outlet is bc the room gets warm, especially during summer months. I’m located in central Florida. I will try to attach some pictures of the duct system in the attic and what I’m trying to accomplish. The 1st picture attached shows the 1st supply box off the air handler, where I want to add the 6” supply outlet to the bedroom. I will also add a damper to the collar. The 2nd picture is of the wedge-shaped box that feeds the only register in the bedroom and other rooms in that part of the house, however, doesn’t seem like there is any room to add anything to this box. With what I have described, will this be ok? Looking for some professional advice please. I understand the current duct design in the attic is horrible but until I can get a professional to re-design it, I thought this would help.

This is the supply box I want to add the 6” supply to the master bedroom.

Wedge shape supply box
Thank you in advance!
 
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Old 03-06-21, 01:30 PM
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Wow, that’s a hideous duct system there. Not too much done right.
That duct needs to be supported and stretched tight. Shouldn’t be run to branch boxes like that.
 
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Old 03-06-21, 02:12 PM
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Thanks for your help roughneck77. I didn’t ask for any criticism, just some professional advice. If you would have read my post, I clearly stated that it was a horrible duct job. I didn’t install the system, it was installed in the house prior to us buying it. If you don’t have any advice, move on.
 
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Old 03-06-21, 03:13 PM
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Professional advice is to repair the duct work.
Non professional advice is to go ahead and connect that additional line to the distribution box.
 
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Old 03-06-21, 03:44 PM
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Adding a duct wouldn’t change the performance, it would just add to your current issues.
By your description, my professional advice would be you likely have an oversized unit on woefully undersized duct. For 1800 square feet 3.5 tons is enormous.
Id focus on redoing the duct rather then adding to it. It’s not going to improve the situation.
 
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Old 03-07-21, 05:34 AM
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roughneck77: How can it not improve my situation? By adding another outlet to the room, won’t I be forcing more air to the room? If the 3.5 ton is oversized like you suggest, won’t adding another outlet help the situation? Just trying to understand your response
 
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Old 03-07-21, 07:09 AM
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By adding a duct, does not mean the extra air is sitting there waiting to be tapped into and utilized. Your duct system, with tremendous overuse of flex laying bunched up on the attic floor, is incapable of moving the proper amount of air in the first place. And if you do add a duct, it will likely further starve the ducts that are there now.
At best, the 6” duct will flow 75 CFM if installed perfectly. If it looks anything like those ducts there, cut that in half.
I strongly suspect the air handler is incapable of moving enough air at all. You paid for a 3.5 ton unit but you are not getting 3.5 tons of capacity out of it.
To add to this, you never put a duct takeoff on an end cap. Your projected placement is directly across from the duct going to the air handler. Doing that will starve those other ducts coming off the side, and be very noisy.
Flex is designed for short runs and takeoffs. Not to construct the entire duct system. It’s often done because of low cost, and because the installer has lack of skill to make metal ducts.
At 3.5 tons, you need to be flowing around 1,400 CFM at a reasonable static pressure.
What does the return side look like? What is the main supply coming off the air handler to feed those boxes?
 
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Old 03-07-21, 10:10 AM
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roughneck77: I would also install a damper on the new line at the take off to control the air flow. The supply plenum off the air handler istarts out 24” x 21” that transitions to 18” x 16” going into the attic. Im assuming due to the trusses in the attic. As far as returns, I have (1) 24x24 return, near the floor in the dining room, which is centrally located. I also have a 12x12 return in the ceiling in the master bedroom. Again, until I can afford to redo the entire ductwork in the attic and also find a reputable company to do a proper install, I was hoping this would help my situation, temporarily.
 
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Old 03-07-21, 08:48 PM
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What size ducts are coming off those return grills?
You don’t even have enough supply for 3 tons of cooling. You can’t tap in to what isn’t there to begin with.
A damper will not help with noise issues that will result from putting a takeoff on an end cap.
 
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Old 03-08-21, 09:28 AM
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The 24x24 is on the opposite wall the handler is on and there is an additional 8”:flex line that runs from the return box in the garage to the 12x12 box in the master bedroom ceiling. I wouldn’t be taking the supply off the end cap. I would be taking off the side of the box. I understand, it’s not ideal especially being directly across the main line coming from air handler. That’s why I would install the damper.
 
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Old 03-08-21, 10:05 AM
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So you have an 8” flex line and a 12” X 12” duct for return air?
 
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Old 03-08-21, 02:58 PM
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Sorry, I eyeballed the flex line and thought it was 8”. I got closer and was able to read the size on the flex line. It’s actually 10” size flex line that’s cut into the return box that travels into the attic and into the 12 x12 in the ceiling
 
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Old 03-08-21, 04:21 PM
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That gives you around 980 CFM, or less then 2.5 tons of airflow. How are you going to add a supply to an air handler, that’s already being choked down by insufficient return duct?
You need around 1,400 CFM of airflow for the system to operate properly. Return should be a bit oversized. Helps cut down on noise and velocity.
 
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Old 03-09-21, 02:26 AM
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So by adding an additional return would solve my problem in the warm bedroom during the summer months? What size do you recommend?
 
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Old 03-09-21, 05:58 AM
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No, adding return will not fix the issues in your bedroom.
No one thing will repair your issues. But to get additional supply air, you need a source you currently don’t have.
At a minimum you need to stretch and support that flex duct. Add returns. Probably upsize the supply to the bedroom.
You want to add supply air to the bedroom but you don’t have enough air to feed that new duct right now.
 
 

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