Duct work

Randy Clayton · 05-09-2008, 07:30 PM

I'm running across this more often in new construction;duct installed directly into the back of the the plenum box they say this is ok,but I still have a problem with this any ideas???

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**Jim Luttrall** · 05-09-2008, 08:14 PM

What is your problem specifically. Are you used to sheet metal?
Your photo looks like all the production builders in this area.
No problem with the principal other than there is more resistance with distribution box and flex ducts as opposed to sheet metal with smooth transitions and wye's.

**Markus Keller** · 05-10-2008, 07:43 AM

Not sure of the problem either. I don't like lots of flex, especially straight off the unit. I've looked into this here. It's Ok if:
- combined snakes have sufficient CFM as to not choke the airflow
- if sleeve is metal not vinyl
- our Code has length limitations for snakes, vinyl and metal, especially in Res. Commercial code is different also

**Markus Keller** · 05-10-2008, 07:45 AM

Sorry, FOIL, not exactly metal

Randy Clayton · 05-11-2008, 07:12 PM

Ok this is what I see is that there is no back pressure being present; that with that duct being installed directly into the back of the plenuim box the majority of air like water will take the path of least resistance; and will flow straight through the box and not out the side ducts with out this back pressure?

I may just be old school or am I missing something that I'm overlooking?? Still does not make scince to me.

**Jerry Peck** · 05-11-2008, 07:18 PM

Originally Posted by Randy Clayton

Ok this is what I see is that there is no back pressure being present; that with that duct being installed directly into the back of the plenuim box the majority of air like water will take the path of least resistance; and will flow straight through the box and not out the side ducts with out this back pressure?

I may just be old school or am I missing something that I'm overlooking?? Still does not make scince to me.

Randy,

Depends.

As in "depends on where/what that duct goes to.

It may go to another distribution box, in which case the entire duct would reach some static pressure level, depending upon the design of the system.

You don't want that going straight to a supply register, no.

**Billy Stephens** · 05-11-2008, 07:23 PM

Originally Posted by Randy Clayton

---- will take the path of least resistance; and will flow straight through the box and not out the side ducts .

Randy,

The End Duct is Smaller than the side ducts.

Randy Clayton · 05-11-2008, 08:06 PM

Well that is true about the size of the duct;still in my mind would never put it there;old dogs can maybe learn new tricks?!?!

Ron Bibler · 05-11-2008, 09:07 PM

I think randy is correct. I have 2. 6" duct at the end of my plenum box and i get more pressure then from the side 4. 10" duct. If it hits the back and then finds its direction the pressure to each will be the same.

But then they may have a redirect in front of that end duct.

I think

Best

Ron

**Billy Stephens** · 05-11-2008, 09:13 PM

Originally Posted by Randy Clayton

Well that is true about the size of the duct;still in my mind would never put it there;old dogs can maybe learn new tricks?!?!

The longer the run of the duct the more static pressure .

If the end vent is smaller and longer more force is needed than a larger and shorter run.

**Markus Keller** · 05-12-2008, 06:14 AM

I used to carry a regular electric tester. Then a couple years ago I upgraded to a "Fieldpiece" brand. Not only is it an electrical tester but it is built to accept many different attachment heads for different purposes. I have the AMP clamp, the carbon monoxide detector head and for this thread the CFM meter head.
When I'm especially suspicious of a system, I'll pull out the head and measure the CFM at various registers and grilles. It really comes in handy to prove a point. Also cuts the BS from the hvac guy way down.
Fieldpiece Instruments - Hand Held Instruments For HVAC/R Installation And Maintenence
I've had mine for a couple years and they work great, even after many falls.
Fluke and some of the other companies are also making this type.

**Allen Lott** · 05-13-2008, 05:15 AM

Markus,

That is one very nice piece of equipment you have there. I spent some time looking through the online catalog. Which model do you carry? How much should one expect to pay for it? (I didn't see any prices on their website)

**Markus Keller** · 05-13-2008, 06:17 AM

Allen, I have the HS36 multimeter as the primary unit, the AC current clamp head ACH4, the carbon monoxide ACM3 head and in relation to this thread the air velocity and temperature head AAV3.
The multimeter was maybe 120-140 it came with the AC clamp and a bag. The AAV3 was maybe +/- $100. The carbon monoxide head was more though. I think a couple hundred. It's been awhile. There are more heads I'd like to get, just waiting for the right job to come along to need them.
I bought the units through my HVAC supplier, I think that's where they are primarily sold.