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Old 08-10-2012, 05:24 PM   #1
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Plenum maximum output


I want to make sure my unit is strong enough for the amount of air being distributed throught my house. Is there a maximum amount of ducts you can add to a 4 ton split level plenum? If its capable of 48k BTU's then i would think there is a certain peak amount of ducts you can splice into your plenum before you reach diminishing returns.

In a nutshell, how do i find out how many ducts can be used for my 4ton and size of each before i hit that diminishing returns point? Any calculators online?

I measured my ducts.
Plenum is 3ft 5in x 2ft 7in (41inx31in)
the plenum extends to the downstairs from the bottom of the plenum at a size of 1ft 3in x 1ft 1in (15inx13in)
There are 5 8inch ducts upstairs in attic to each room / bathroom
once the plenum hits the bottom floor it branches out with 2 8inch ducts, 2 4inch ducts, and 1 open cut in the plenum of 6.5inch x 11inch (not a duct, the plenum is right next to the wall on the lower floor at the ceiling and there is just a 6.5 inch x 11 inch hole cut out at the Sheetrock)

For a 4 ton is that right? Im no pro so im trying to understand the logistics behind air distribution.


Last edited by ang1dust; 08-10-2012 at 06:02 PM.
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Old 08-10-2012, 06:31 PM   #2
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Plenum maximum output


adding supplies won't diminish returns. it just does not work that way. adding supplies past the design of the system will diminish the air flow out of original designed ones. in your case if you have a 4 ton it is designed to push 1600 cfm. I will give you the cfm's of the round sizes you list so you can do the math. 8'=200 , 7"=150 , 6= 100, 5"= 75 4"= 50. these are actually number rounded off(eg. 8" really is something like 225 but most use the numbers I gave). No matter how many supplies you add you will have 1600 cfm being delivered from the blower and pulled into the return. you will loose velocity and your further supplies will get no flow.

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Old 08-10-2012, 06:48 PM   #3
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Plenum maximum output


With that said, if i have 5 8inch at 200x5 = 1100
2 8inch downstairs at 200x2 = 400
2 4 inch at 50x2 = 100
1 11x6.5 = 400?

im getting a total of 2000 am i right?

Not to mention a couple of these 8 inch are split off for 2 bathrooms and such.
In addition, i was looking at my handler up in the attic and its a 75k BTU furnace. Correct me if im wrong cause im a newb at all this but throughout all my research isnt that a 3 ton handler? I cant find anything on the handler online.

Ruud Zephyr series MFD 1/05. Oh hell let me just upload the picture.
Model 80PJ07EBR01
Serial GE5D302F020501314

Hourly input rating 75,000 BTU/Hr (21.97 KW)


Looks like the pic is going to be ultra small heh cant get full sized for some reason like everyone else i guess its permissions.

So confused by this, if i have a 3 ton with a 4 ton compressor and evap thats why i cant get enough air to my rooms?
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Old 08-10-2012, 07:07 PM   #4
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Plenum maximum output


So my newbness is coming out heh. Found this http://www.allreds.net/WS2/docs/rhe/80pj_specs.pdf on my handler. If it is rated for 1600 at .5 ESP then its a 4 ton unit? But even then the calculations for the ducts that i have puts me at 2000. So if its rated for 1600 and i am set up for 2000...thats bad right?
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Old 08-10-2012, 07:20 PM   #5
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Plenum maximum output


Now tell us about your return.
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Old 08-10-2012, 07:27 PM   #6
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Plenum maximum output


11X6.5 does not = 400 cfm. if it is connected to a register depending on the type of reister may be more like 150 -200 cfm. some of your questions confuse me, sorry.

"So confused by this, if i have a 3 ton with a 4 ton compressor and evap thats why i cant get enough air to my rooms? "

That question does not make sense to me 3 ton what? 4 ton compressor (do you mean outside unit)? your OP said you have a 4 ton unit and asked air flow questions makes most think you know you have a 4 ton indoor unit. I am confused.....
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Old 08-10-2012, 07:44 PM   #7
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Plenum maximum output


My return is 16 inches, the box it connects to is 25x25 (least thats the size of the filter) and there is a 6 inch hose that goes down to the lower floor back to that box. Ultimately the return is 16 inches.

Dont dwell on my dumb questions. They are merely me speaking aloud. If it annoys you i can delete them :P.
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Old 08-10-2012, 08:00 PM   #8
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Plenum maximum output


Your 25X25 air filter is ok for 1000CFM at less then 400 FPM velocity through the filter. So you need more return in order to get more air flow. So most likely none of your supplies is moving nearly as much air as you posted.
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Old 08-10-2012, 08:06 PM   #9
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Plenum maximum output


So if i increase the return from my 25x25 from 16 to 20 inch you dont believe it will increase my air return?
and the 11x6.5 as i stated before is not connected it simply is a hole cut in the plenum for the lower level of 6.5x11 inches long, i found a conversion site online that says its close to 400cfm for that kind of output think it says 350. Ill link http://www.electromn.com/pdf/EC001.pdf. Yes it says 350 for a 11x7. In my calculatoins i forgot to add the 1 6inch duct i added near the thermostat to help calibrate the air which puts me still over 2kcfm for my current setup thats only made to handle 1600 if its even pushing that with my 16 inch return.

Last edited by ang1dust; 08-10-2012 at 08:26 PM.
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Old 08-11-2012, 04:20 AM   #10
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Plenum maximum output


Filter will still be too small.

Just because a duct is 8" round, it doesn't mean you have 200 CFM going through that duct.

Thats like thinking that because vehicle has a 600 HP engine, it can go 300 MPH.

Ducts have a resistance to air flow, your blower needs to be able to over come this resistance. An 8" duct is often used to move 160 CFM. Using a generic form to size or calculate how much air a duct is moving is somewhat futile. The air volume has to be measured to know how much is being moved.

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