Extended Plenum vs Reducing Extended Plenum

[mofotech]

Wussup Folks

I was reading up on ductwork in my school books and I came across section on extended plenum's and reducing extended plenum's. I understood what the book was explaining , where extended plenum are mainly used when the air handler located center line of the building and building may not be very large. The reducing extended plenum are used mainly in larger buildings and also when the air handler is not installed center and will be located at one end of the main trunk line.

But what I wanted to know what are the pros and cons between these two ductwork in the real world? Where are they best used in a given situation when on the field? (Obviously all hvac design is accounted for including Manual J and Manual D)

[hvacker]

I thought extended plenums were uned when the tinner wasn't sure how to design a duct system. I think they are a waste of metal.
What's the point of having the same size duct on the first takeoff as well as the last.
I've also seen on RT units that often have a small supply and return at the unit being continued as the same size for the duct run. There are some guys out there that should look for opportunities in the fast food industry.
Maybe some one else here can have a reason for extended plenum.

[Saturatedpsi]

Here's a link to an RSES topic offering some thoughts. :yes:

http://www.rses.org/assets/serviceapplicationmanual/630-148.pdf

[ACFIXR]

I use a reducing extended plenum design all the time in light commercial. I like to use them because they offer the best strategy for performance and economics. With a reducing extended plenum that is in the middle of your service area you can immediately divide your volume in half. After splitting your volume you can then use 1/1 aspect ratio duct and help further your performance and economics. I further reduce the split branches after a 50% reduction in volume. It is the single extended reducing plenum runs that PE's like to use that I find have consistent flow issues. For example having an extended plenum return that is as long as your supply. :gah: or trying to put a supply tap close to the air handler.

http://i1201.photobucket.com/albums/bb359/Chuck_Phelan/HVAC/Ductwork/Reducing_extended_plenum.jpg

[genduct]

Life and this business is series of choices. Pretty deep huh?

Thing is the decision to reduce the duct size has to do with whats works and what costs the least. So fittings cost more than straight duct SO if your run is short, why reduce the duct size? If you are going to continue for another 20 or more feet of run then you'll want to save the material and money and change sizes.

The problem comes when someone who may think that you NEED to reduce to KEEP UP THE VELOCITY to deliver the air. THis thought is not correct, in fact at the lower velocity the friction rate is reduced. There is a point where you don't want the air to move too slowly, there is more of an opportunity for heat duct transfer.

That's a lot to throw at you in writing, so if you have any questions with your school work, check my profile and call me. my cell is there. If you got me at a bad time , we will make a date to talk.

Good Luck with your studies, my social security is based on you understanding this stuff and making lots of money to pay for us old guys.

Happy New Year, Mike

[mofotech]

Life and this business is series of choices. Pretty deep huh?

Thing is the decision to reduce the duct size has to do with whats works and what costs the least. So fittings cost more than straight duct SO if your run is short, why reduce the duct size? If you are going to continue for another 20 or more feet of run then you'll want to save the material and money and change sizes.

The problem comes when someone who may think that you NEED to reduce to KEEP UP THE VELOCITY to deliver the air. THis thought is not correct, in fact at the lower velocity the friction rate is reduced. There is a point where you don't want the air to move too slowly, there is more of an opportunity for heat duct transfer.

That's a lot to throw at you in writing, so if you have any questions with your school work, check my profile and call me. my cell is there. If you got me at a bad time , we will make a date to talk.

Good Luck with your studies, my social security is based on you understanding this stuff and making lots of money to pay for us old guys.

Happy New Year, Mike

Funny you say that. My instructor has that same view. He will hold up a Trane ductulator and tell me this is all you need. He tells me to maintain air velocity. When I ask him about what about going the route in where you find the friction rate for your equipment? He will roll his eyes and just tell me "Why make it more complicating?"

Ya we'll have to keep paying for SS and also pay back all the money we have borrowed from the Chinese.

[mofotech]

I use a reducing extended plenum design all the time in light commercial. I like to use them because they offer the best strategy for performance and economics. With a reducing extended plenum that is in the middle of your service area you can immediately divide your volume in half. After splitting your volume you can then use 1/1 aspect ratio duct and help further your performance and economics. I further reduce the split branches after a 50% reduction in volume. It is the single extended reducing plenum runs that PE's like to use that I find have consistent flow issues. For example having an extended plenum return that is as long as your supply. :gah: or trying to put a supply tap close to the air handler.

http://i1201.photobucket.com/albums/bb359/Chuck_Phelan/HVAC/Ductwork/Reducing_extended_plenum.jpg

PE's? Who?

[genduct]

Funny you say that. My instructor has that same view. He will hold up a Trane ductulator and tell me this is all you need. He tells me to maintain air velocity. When I ask him about what about going the route in where you find the friction rate for your equipment? He will roll his eyes and just tell me "Why make it more complicating?"

Ya we'll have to keep paying for SS and also pay back all the money we have borrowed from the Chinese.

The whole truth is a little more complicated. The residential MAN D is a simplification of the more complicated commercial approach. You can keep it simple and be OK, although if you understand that the friction drop for a take off depends on how fast the trunk duct air is moving past the Take-off hole then you'll begin to understand the subject a little more thoroughly. Thing is if he does understand this he may not have enough time to do the whole explanation and still get to the other lessons. If that is the case you'll need to do your own research if you would like to understand this depper

[ARTICHILL]

So static pressure means nothing???? Try blowing through a straw then reduce the size you will get more pressure, im mean im only thinking of physics. Are units design with certin static pressure depending on the applaction????

[genduct]

So static pressure means nothing???? Try blowing through a straw then reduce the size you will get more pressure, im mean im only thinking of physics. Are units design with certin static pressure depending on the applaction????

I didn't say static pressure is not important. Only that it is a little more complicated and MAN D simplified some of the complicated stuff because with residential the systems are simple and the max velocity is usuall only 900 for supply.
Blowing through a reducing straw is a good example of why it is complicated. think of the sum velocity and static like they were Watts. So with the straw the total pressure would not change if the reduction were perfect but the staic pressure would drop. The velocity and static pressure are convertable. Again the complete explanation is a little too long for a post

Bottom line is you need to size the duct so that the amount of static you have with the unit is not exceeded by duct design resistance to flow. And most of that resistance is not from the straight duct but is from the fittings where you have asked the air to change direction or size