Plenum sizes and return air questions [Merged threads]

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Old 10-23-11, 01:03 PM
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Plenum sizes and return air questions [Merged threads]

I really would appreciate some input before my contractor returns as I requested them do on some issues that I am letting the world of experts suggest in their own opinions.

My most pressing issue is the plenum sizes as one part to this and secondary is my questions about return grilles sizing affects to the system.

I recently had this contractor replace my old 4 ton unit. We can get into details later, but my concern at this moment is I have learned after the fact, my flex duct system was inadequate as I have read most people's are and depending on how large they can be and how much I can get them to move it and the space allowed will determine a lot of my questions and concerns below as they did not want to do nothing other than put this unit in the same location as the 4 ton unit and at this moment that is the situation.

The problem at this moment is that I learned reading around online is that a 5 ton unit really requires return on each side of the unit and they were not going to do that, but I made them. They were only going to put a standard 1" filter on the one side like I had with the 4 ton unit. I made them go to an APRILAIRE 2210 4" filter on each side. The problem is space in the attic is working on their side and against me to make them do it more correctly and unfortunately, I am obviously not the expert, but I am not complelely naive, except I probably shouldn't have used this contractor. Now, I have ran lots of numbers and I am currently using that HVACALC program and hope to figure out all the correct CFM for duct sizing and I also found, shockingly a local company that does air balancing, but they said that for really accurate measuring it needs to get hot outside for getting accurate readings, so that will have to be next spring. Probably, a discussion for a later time there. For now, before these guys measure my unit hopefully, I will get them to move the unit over as much as possible, but I have to know these questions below if possible.

I understand you need @ 1000 sq in of return free area of grille space and ideally @ 300 fpm passing through both filters. I know that is 1,250 sq in physical size of the grille itself or like one @ 40" x 31".

I have three 12" return and one 10" return. Facing the front of the unit I have the one 12" return on the left side of the unit connecting directly to the furnace rack, which I know is bad, but remember they were going to have nothing. So, the other two 12" and one 10" are on the right side. After they leave, I will change one of the ducts, probably the 10" to 12" and one of the others to 14", but for sure minimum four 12" ducts return. According to the charts that is 1,920 CFM. I will have four 12" ducts supply also to equal out.

Can a supply plenum be larger in cross-sectional area sitting on top of my furnace?

Can a return plenum be larger in cross-sectional area than the filter's physical opening (21" x 20")?

I think I have room if they move the unit over of about 16" to 20" between the filter rack and my hot water tank, so can I have them make the plenum go out that distance and up say 40" so I can have them connect two ducts on the one side, thus having two ducts on each side of the unit? I just thought for better balance to have two ducts on each side, but maybe that is not necessary. Good or bad idea?

But, how would you get the FPM down because if you only had those four ducts connected each to a 12" return grille that is @ 750 CFM per duct and close to that in FPM, when you need 300 CFM or lower passing through those filters. which is another change I am going to do after they leave is to move them from in the wall pans to the ceilings and put 12" square boots, as I made them do that one, but it is noisy at the moment. Anyway, if you only had the four ducts and four grilles only you will only have 624 sq. in. that is way short. You would need at least two more grilles and you would be short still and in the mean time you are adding ESP, but seems like you would have no choice there. But, if you start randomly adding additional branches, which I have in my home and assume most homeowners have if contractors put returns in individual rooms as you could not physically have a separate duct for each room returning to your system. So, you have to either branch them or use jump ducts (if that is the right terminology). So, the ones that exist now they reduced down one size, but isn't that conterproductive? I assume they did that to keep velocity up? Don't we want velocity down? So, when I remove these branches I want to have the same size branch as the trunk, is that bad or good? If I do this, take for example if you have one 12" trunk and you split it into two what happens to CFM and FPM if you measure that at the grilles? Now, what happens if you split it again two more times and now you have 4 return grilles, what does the CFM and FPM do? I have an anenometer and have been taking measurements and could do that afterwards, but I am pressed for time and do not know what my limitations willl be with what size plenums they will install.

I have more questions, but I thought this would get me enough so I can ask my contractor to make these plenums large enough to accomodate these ducts.

I would attach some pictures, but I don't know how do it.

Thanks everyone for their time with this!
 

Last edited by GregH; 10-25-11 at 06:58 PM. Reason: Please keep all questions on the same project in one thread.
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Old 10-25-11, 04:52 PM
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Return air estimate

If you have a 12" round flex duct return that is 20 feet long and there is
750 CFM and lets say 750 FPM also is measured, but now you split it
using a wye and you connected a short 12" duct lets say 5 feet each on
this wye, what would be the @ CFM and FPM reading now on each return?
 
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Old 10-25-11, 07:00 PM
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Don't know what your looking for but 12 inch flex duct is to small for 750 cfm of return air. 500 cfm would be more like it 400 would be ideal
 
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Old 10-25-11, 07:07 PM
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It must be late because there is too much info and too many questions here to absorb.

The part about only being able to do accurate air measurements when it is hot does not make one bit of sense.
If you want to be technical there is an increase in static pressure drop across an a/c coil when laden with moisture but this would have little impact on your solution.
My suggestion would be to put the brakes on the job and let someone who works in this field take over both the design and installation.

To post a pic here you need to upload it to a free host like Photo Bucket and either provide a link or use the button at the top of the reply box to have the pic appear in the post.
 
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Old 10-25-11, 08:21 PM
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Greg thought I was loosing it tell I saw the merged threads post
 
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Old 10-26-11, 04:29 PM
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I am looking probably for a lot more information than I should and I understand I should get another company. But, the guy did not show up until last night and he resisted wanting to put a plenum where he connected the return directly to the filter rack. His logic, not as good as a pull, but I understand the farther away the better. So, I asked that question for two reasons. One to understand what does that CFM and FPM do as you keep adding the same size wyes as I want to do, but too many will be bad too. Anyway, I think CFM stays the same and the FPM reduces some amount and I wanted to know even if I didn't give a figure what does it do? Does it get halved every time you divide it that way? Also, I have an anenometer and this one return that I am talking about reads those values going from my ceiling straight to that filter (@27 feet away). I understand ideally you want @ 300 to 400 FPM across that filter. This grille in my ceiling is noisy as you can understand, but I have two other 12" and one 10" returns, but they are all on the other side of the furnace. Can I have some suggestions in how to accomplish that and maybe some reasons why it is so high?
 
 

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