Plenum top take-off vs. side takeoff

[The Tommy]

We're getting some mixed messages from HVAC contrators and would appreciate any insight anyone has regaring the issue at hand.

This is for the installation of a air handler in a new construction - specifically the configuration of the plenum.

The air exchanger is situated in a closet and has a 16x16 plenum. This plenum need sto feed 2 8x16 registers. The first HVAC contractor suggested we take 90 degree knees and feed each of the channels from the top of the plenum (Option 1). This seemd like a good solution to me as it seems very clean and simple. The second HVAC contractor said this is a big no-no and that take-off shoudl be on the side only (Option 2). The problem is that there is not enough room for that so the walls (rough framed only) will need to be reconfigured to allow this. I'm not too crazy about this option but I do want to get the job done right. The third HAVC guy didn't care at all and said he'll do it whatever way we want - which also concerns me as I'm not really the expert.

This is a big investment for us so we would appreciate any input to guide us in proper HVAC contractor selection. Many thanks

http://www.schroecker.com/downloads/Top-takeoff.jpg

Some additional information that may be helpful: The unit is a single floor, approximately 900 square feet, heating is fed through 12 vents in the ceiling. Air handler is situated approximatey in the center of the unit and feeds to 8x16 channels, each of which channel feeds 6 vents. There is a space of approximately 10" on ABOVE the ceiling in which the 8x16 channels run

[Mr Bill]

How "tall" is the Plenum? is it 16" tall also or 36" tall?

[The Tommy]

How "tall" is the Plenum? is it 16" tall also or 36" tall?

The plenum hasn't been built yet but from the top of the air handler to the very top of the ceiling which will be the top of the 8x16 channels is around 4 feet. The air air handler hasn't been installed yet - one fo the HVAC guys said it has to be lifted up about a foot because it has the return air in the bottom so that # may go down to 3 feeet.

[Mr Bill]

The air air handler hasn't been installed yet - one fo the HVAC guys said it has to be lifted up about a foot because it has the return air in the bottom so that # may go down to 3 feeet.

So are you saying that the "return" will be only 1' high and only as wide as the Air Handler?

[The Tommy]

So are you saying that the "return" will be only 1' high and only as wide as the Air Handler?

I think return air is around 12x24

[Mr Bill]

I think return air is around 12x24


For 3-tons or 38,000 BTU of air? :eek: You might need to address the return first before the Plenum.

[The Tommy]

For 3-tons or 38,000 BTU of air? :eek: You might need to address the return first before the Plenum.

This is a heating sytem - 38,000 BTU of heat. The Air exchange is fed hot water from a centralized boiler in the building. The AC coil inside the air exchanger is only around 1 ton.

I belive the return air is appropriately sized as everyone is telling me the same thing on it. The thing that concerns me is the configuration of the Plenum - top take off vs. side take off.

[Mr Bill]

Ok, so what you have is a 38,000 btu 1-ton Air Handler 400 cfm's , man was I confused.:D

[jaygee10]

Round the inside corner (throat) on the top takeoff and the differences are negligible from the side takeoff. Both fittings will be equal to 10 equivalent feet of straight duct. Assuming duct sizing, static pressure, and other design issues are correct, install balance dampers and forget about it.

[kls-ccc]

The biggest problem I see with option 1 si the throats are square which causes turbulance. Why not make a "T" plenum with curved thoats. With the room you have it should not be a problem but if you need extra space for the curves you can bevel the inside of the wall which gives you more space yet looks the same from the outside. I don't see a problem with the return as long as the space under the AH is at least as big as the return.

[Mr Bill]

I don't see a problem with the return as long as the space under the AH is at least as big as the return.

You lost me here, space under the AH is at least as big as the return????

[Mr Bill]

Never mind my last post, was on first cup of coffee.

[JJDH]

I dont like option 1. Option 2 is much more common. However a tee plenum with the inside radius curved is ideal.

[beshvac]

I don't think you have found the right contractor yet or maybe you did and don't know it.......

If I was bidding the job you would have a manual J and Manual D results with actual duct sizes going one way or the other or both. I would also design proper register sizes for every room.

But, I am not a high volume new construction contractor and only work for select builders or homeowners who are building (their own house or will leave the builder out of HVAC decisions.)

[heaterman]

Either will work fine. The radiused throat would help but with a system this small, don't sweat it.

[deejoe]

I agree with "heaterman". Either way will be ok. if it were me I would go with the simple "T" method and the square throats and heel would still be ok.
And if Mr Bill posts you again just tell him to turn off the coffee pot and go back to bed.

[REP]

On this small a system either will work.
With the loads you are claiming your ductwork on both sides is too big.
You should use 8x8 on both sides of the supply and with that an 8x14 or 8x16 would work on return .
I don't have my register book but I'd bet that the supplies could be smaller to get a decent throw from them.