Ducts at end of Supply Plenum?

[George Sambataro]

I am having some work done on my existing HVAC system have met with 2 contractors.

One contractor has pointed out that I have several ducts at the end of my supply plenum and that is a NO-NO. He suggest that this should be corrected by making the supply plenum larger and having all ducts come off of the sides of the plenum. Another contractor is not concerned about this and suggest that the use of manual dampers is sufficient.

How much of a NO-NO is it to put supply ducts at the end of the supply plenum? We could use better air flow at the ends of the house and I was wondering if the larger plenum with all ducts to the side would help.

Thanks for any advice you can provide.

George

[Joe Bobst]

Simple laws of physics always work. You want more air flow, use larger ducts. Too many HVAC installers make tidy looking installations that disregard simple rules. Maximum airflow depends on plenum and blower design and should follow manufacturer's recommendations. Your post will likely trigger many responses from HVAC experts. Some may have practical advice for your situation. You may even need to talk to one or more other contractors to arrive at a logical conclusion. Good luck. Joe

[Joseph Meehan]

To answer your last question, someone there will need to do the work of measuring what you have and doing the numbers to determine what you need using, I believe, that Manual J calculations or is it "D" I don't do this work so I don't really remember.

[Zypher]

George;

Yes, it is considered 'not a good idea' to take branch runs from the end of the plenum unless it is an 'extended plenum of considerable length.' Residential duct systems run at a very low static pressure and if the branch takeoffs are directly facing the blower, there's a tendency for the pressure to take the path of the least resistance. It's better if the branches take from the sides so all the ducts are pressured the same. (Although sometimes that's just not possible.)

Dampers are fine for 'small balancing' but not recommended for large corrections. Dampers will not 'increase the pressure' in adjacent ducts, but will reduce the air flow. It's dependent on the design of the ducts (if it was designed at all.) ACCA Manual J is for load calculations on a Residential / Light Commercial Building and ACCA Manual D is for designing low pressure duct systems. You should seek a licensed HVACR contractor who uses these guides for design work on your HVAC.

The best way to find a reputable contractor for you would be to seek recommendations from your friends, relatives and neighbors. If they had any 'bad experiences' they'd let ya know. And please don't get more than 3 - 5 quotes. It does cost for a contractor to give a bid.

What exactly is it you want to do? Or what problem are you solving?

[John Hines]

Yes and no. You need a larger duct feeding the it to flow more air, and it needs to be reduced in size as it goes along, to keep the pressure (?) up.

I remember seeing a laboratory in a national lab, built after WWII to hold a room full of vacuum tube counting equipment. Talk about a cooling load!

The ducts were like 2'x4' at the beginning and ran to the end of the room, and had ducts coming out every foot on both sides, and shrunk by like one inch after each pair of ducts, until it was just a single 8" for the last pair. I was there in the 70's and what had been a 6' rack cabinet had shrunk down to about an inch wide in a rack chassis.

Even in modern houses, I see that, where the main air duct reduces down in size, after feeding ductwork.

[Joe Fabeitz]

...and this reply helps the OP, how?