Hot rooms and blocked supply plenum. Can I DIY this?


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Old 05-12-17, 12:53 PM
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Hot rooms and blocked supply plenum. Can I DIY this?

I live in AZ. Hot. I have two rooms on the west of the house that are uncomfortably hot. I was hoping that I could find some help on how to solve this myself.

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My system has a large rectangular metal duct which supplies all the rooms. This duct seems original. Added on to that are two round ducts running to two rooms. Those two rooms don't see the brunt of the sun and are cooler than they need to be.

When I look up at the main supply plenum that carries cold air from the AC, there is a large branch that supplies the two flexible ducts. This branch is before the rectangular duct and blocks almost 75% of the descending .

I feel removing this branch will improve air flow to the hot rooms. I've no idea why the previous owners did this. But, my hunch is they ran out of money before they finished what they wanted to do with the duct work.

Any help will be appreciated. Thank you for your time. Image 1 shows my layout and Image 2 is a picture of the supply plenum with the large branch obstructing the air.
 
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Old 05-12-17, 01:04 PM
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Could you provide a few more pictures of the duct and layout?
 
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Old 05-12-17, 01:21 PM
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Happy to provide more pictures. The main rectangular duct is hidden under insulation and hard to see. Thanks Airman, let me know if there is any specific picture that would make my situation easier to understand.

This picture shows the plenum with the cold air coming out of it. At the bottom you can see the two round ducts.

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This, below, is a close up of the two round ducts emerging out of the plenum.

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This below, is a picture of one of the round ducts going to the south room.

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Old 05-12-17, 02:59 PM
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The take off on the end of the main trunk needs to be relocated to the side of the duct. Dumb mistake.
 
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Old 05-12-17, 03:02 PM
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Do the two hot rooms have return air ducts ?
 
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Old 05-12-17, 03:10 PM
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>>> The take off on the end of the main trunk needs to be relocated to the side of the duct. Dumb mistake.

It would certainly seem so skaggsje. If I understand you correctly, you are saying that instead of the branches coming out of the main trunk (where it is blocking most of the trunk as seen in the picture). It should come out of the side of the rectangular duct, where the registers are for example.

There is only one return air duct, in the hallway. In the pictures it is approximately in the center of the main supply rectangular duct.

Can I just remove the take off and seal up the main trunk? will that somehow unbalance the whole thing. The AC unit is from 1996.

Thanks!
 
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Old 05-12-17, 03:21 PM
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You will have to measure that take off, probably 6", and buy a 6" cap. This will help the airflow to all the rooms. With that takeoff on the end of the trunk your static pressure is low.
I'm a little confused about the duct inside the duct, very strange.
 
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Old 05-12-17, 03:32 PM
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hmmm, I am not explaining something correctly. I think its because I do not know for sure what things are called. Please bear with me. Let me draw an elevation schematic.

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So in this picture. The pink round ducts coming off the main green cold air duct go to rooms that do not need it. They also seem to block the air going into the main rectangular duct that supplies all the other rooms.

I would like to remove the pink ducts entirely and seal off the main green cold air duct.

1. Is this advisable or will it unbalance the system?
2. Would it just be pulling out the pink ducts from where they meet the green duct and sealing up with a cap and some mastic. The pink duct is quite large, I'd say about 10 inches.

Thank you all for your time. If I can just get this done before the real heat starts. I can be a hero. Until, our 1996 AC dies and we can get this sorted out my a professional.
 

Last edited by MrSmithNV; 05-12-17 at 04:01 PM.
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Old 05-13-17, 06:10 AM
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Ok that drawing helps a lot. Question, do the blue boxes on your drawing have flex connected to them ?
 
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Old 05-13-17, 06:18 AM
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Glad that helped. I felt I was not being clear.

No flex. The blue boxes are the registers. They open into the rooms. There are short (2ft) smaller rectangular ducts that lead off the main rectangular duct and lead to the blue registers.

At this point, I think I am going to disconnect the pink takeoffs and seal off the green main supply. Can you tell me how to go about doing this? or even what such a procedure is called because I don;t know how to search for it

Can I just screw a cap on there and use mast
 
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Old 05-13-17, 06:38 AM
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Neal, I agree, get rid of the pink duct and cap it off. Is the green duct a round or rectangular duct ? There really is no procedure here, its all custom work.
 
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Old 05-13-17, 07:01 AM
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Thanks skaggsje for your time and advice!

The green duct is round. I meant the broad outline of how to do this. Can I just screw a cap on the hole after I remove the pink duct and use mastic to seal.
 
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Old 05-13-17, 07:24 AM
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I would pick up some sheet metal at Home Depot. I think they come in 2' x 4' sheets. Screw a piece over the hole and then use Mastic. I would remove the register on the west end of main trunk and relocate it to the south side of the duct. If you don't want to hassle with removing the register you could just screw a piece of sheet metal over it. On the east end I would cover that register and then run the unit for a few hours and see if that work better.
 
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Old 05-13-17, 07:29 AM
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Thanks! okay, I have a plan. I will do it one step at a time and see how things improve.

Will report back, thanks for taking the time skaggsje, appreciate it!!!
 
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Old 05-13-17, 07:37 AM
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Your welcome, Yes let us know.
 
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Old 05-19-17, 03:44 PM
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So this worked out perfectly!! I was able to disconnect the branches and put on a cap. Air flow has improved tremendously. And I'm an instant hero! Thanks skaggsje for your advice!

I did have a follow up question regarding insulating the sheet metal ducts that are in a dropped ceiling. The ducts are open to the attic on the top side with only blown cellulose over them.

Can the dropped ceiling be sealed off bringing ducts into conditioned space.
If rigid foam is used for this, can it be left exposed in attic or does it need to be covered with drywall?

There is a discussion of this very thing here: but it does not go into great detail as to the best solution.
Capping/air sealing, insulating dropped ceilings - Home Energy Pros
 
 

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