Its been over 24 hours and no takers on this question just peekers.
Answers posted as/per Robert O,Connor
At least 2 feet of main supply plenum without a supply take off is recommended by most MFG. About 3 to 4 feet before a take off is considered a better practice.
It is considered as poor practice to have a take off cut into the head of a main supply plenum. I consider this as a defect if installed without a damper and a accessible damper handle to balance the system.
Personally when installing duct if there was a drop within the first 10 feet of the plenum I would not come off of the trunk line connect straight to the plenum and reserve the trunk line for the extended drops.
Robert will be back on board Monday he can discuss ACCA with you.
My Manual D is buried right now, but I believe you are correct that they also recommend no take-offs within 12" of the supply plenum end (in addition to minimum distances to fittings and take-offs from the equipment).
However plenum head take-offs are found on systems. I consider it at least poor practice, and a defect if there are no accessible dampers with handles to balance the system. I am also very mindful of the flow at registers (feel the air flow with my hand) when I see that, as the registers from the head take-offs are often over supplied (and thus under supplying other registers).
I just recently observed a head take off last week as a matter of fact appeared the owner had installed an additional run for a garage convert (no damper) I personally have never installed one. (bad news)
I have another question for you in particular. I make it a rule to always observe the amount of drops that are attached to a specific trunk line in the attic I do not measure CFM at the registers as an HI. The amount of 8 inch drops attached to a 12 inch trunk line gives me a ball park idea as to the CFM distribution that I will be having when I operate the systems. I prefer 3 drops as excellent CFM, 4 drops as exceptable CFM, 5 drops un-exceptable.
I do realize that CFM is calculated by the size and the length of the various ducts but this is not within the scope of a HI. What would your take on this be?
It really does depend on actual flow required, but in general I specify and look for a reduction in the duct size after 2 to 4 max take-offs (3 max preferable … especially for 8" take-offs) on residential systems. This ensures there is enough back pressure in the larger duct closer to the unit (caused by the transition restriction) so that the take-offs on the larger upstream end get enough supply air. This is called a “Reduced Plenum System”. If layed out correctly, the system operates very well, with also little balancing (if any) required.
Another type of system is called an “Extended Plenum System”, where there is a constant size main supply plenum up to about 16 feet long on either side of the air handler (some say 24 feet max), and the system is then balanced with damper handles at the take-off’s. In my opinion the Extended Plenum System is an inferior system, but is used by some who don’t really know how to calculate out the ductwork. When I see that I am very mindful of balanced register flow, which can easily be roughly checked by just holding you hand up to the registers to get a comparison. It can work fairly well if balanced right (rare), as registers near the end tend to be over-supplied or noisy.
Also, some installers make the mistake of locating equipment on one side of a house with a long extended plenum … almost always problems with these systems in my experience … and in fact some registers near the equipment can actually be drawing air into the ducts … :shock:
And last, and least of all, is the “Radial System” with a very short supply plenum and flex everywhere. Some installers call it a “Spider” system. It’s a sign of a real cheap system (lower end builder special) that doesn’t work very well in my experience.