This system has a 12" round and a 8"x20" split trunk. Duct system is all metal.
The tech is up against some retro-fit type issues because of structure limitations. I was adamant that I would not cheap-out and settle for a basic design. The tech is going through all the motions I see advised here and is consulting up his chain. I do not feel I could ask for any more attention to the design.
He does have a full 1600cfm (normal design w.c.) of return from 5 locations. With the second story being slightly under a positive pressure.
I know much of this useless from a critiquing standpoint over the internet.
Then he using .1 FR, and saying its 4 ton duct, he's going by rule of thumb.
Using .1 FR, that gives you roughly 900FPM velocity.
A VS blower can most likely move 1600 CFM through it.
If you go 4 ton, make it a 2 stage!
Thanks to everyone for your patience.
They just had a 4ton VS air handler and 3.5 ton HP delivered to the site.
Does the fact that one third the system supply (12" round) is going vertical 20' and downfeeding 6 registers from the second level ceiling play into this decision at all?
I trust this board above all else, as you guy's practice what you preach. I have no problem paying for an ex-change.
Yes ,the length and turns make a difference,really everything about the duct effects what size it should be.Manual D is the standard.
Originally Posted by mlo1
"He does have a full 1600cfm (normal design w.c.) of return from 5 locations." How do you or they know it's designed for 1600 cfms,what's is normal design w.c.,what "number"??
I enquired about a manual D, which the tech said he would gladly bring a partner of his in that does them frequently on the commercial side to do. But, he felt the unfortunate nature of the construction and design hoops that had to be done to accomadate the sytem, would be difficult for a manual D to be copied exactly. He said he really was happy with the final numbers and design of the system, only achieved through my willingness to pay and persistance in demanding something other than the normal install. I would like to think this was a rationale based comment about the manual D?
The regional manager was somehow swayed from a matching 3 ton system to a 4 ton AH and 3.5 ton HP as well. I would like to believe that was also based on the facts at hand. The tech commented they do not usually reinforce such a design.
I have never heard of this line about matching. When we register a Nordyne product, it just asks model and serial #.
Personally I'd want the 3 ton myself if that's what the house needs. But if you want bigger, the 3.5 ton on a 4 ton AH is just fine, make sure the dealer is smart enough to check the duct static pressure and set airflow to 1400 CFM. At least in the modular blowers, Nordyne only has the 1200 and 1600 anyway.
I had miss-understood the Tech about the zone manager authorizing un-matched sizing. The zone manager owned several local companies for years and the tech considers him the sharpest HVAC guy he knows. They do reccomend matched sizing and when he is enclined to deviate, likes to consult with the zone manager. As long as the units are listed to be compatable there is no warranty issues.