In the southern states, flex is common. Using rigid round pipe is a step up. Designing with a trunk system is seldom seen. Probably because there is no basement. When laying pipe in the attic, it is almost impossible to use trunk design, without sacrificing the majority of possible attic storage.

Flex is expected in small retail stores, and migrated to residential during the last 20 years. The difference in spider and trunk systems allows anyone to throw duct into a house and have a 60% chance it will kinda work.

When rigid duct is run in the basement, it can be easily installed. Flex in an attic is easily installed.

I would ask for bids with a minimum specification of rigid round pipe with insulation wrap, unless the insulative foam is already in place. It may not be possible to use rigid at that time.

The difference between engineered square duct and flex is probably about 5x the cost to install, if done at the proper time. Otherwise a simple flex job that costs $1800, can jump to $30,000 if a trunk system is installed at the wrong time.

As far as quality, both will perform similarly if enough design was done, otherwise both systems can fail.

rachelh,flex duct is fine when used properly.That duct design is a joke.They need a trunk line.I would demand a better designed duct system.You can use ductboard trunk and flex,if designed correctly you will still have plenty of storage room.I always try to keep flex as short as possible,12' or less.Balancing that duct system you have designed will be a nightmare if not impossible.That design would not pass code here.good luck.

I hate to say this but the design will never work. None of the runs could be balanced, static will be to high, and sizes are off.

I am the husband in this senario and have had some experience in HVAC (about 5 years worth) but I have been out of it for some time now. We spent $225 to get a manual J done and to have this abortion of a duct design done. Just one look at that and I about hit the floor laughing. So I am wondering, if the duct design looks like an eight year old did it should I question the Manual J or just assume they used a reputable heat loss/ heat gain software and plugged in all the correct info?

I cant imagine that was done with any software.It is some what commical.lol.Search for some good contractors in your area.good luck

Looking at the design, I see a basic design bastardized. The various runs will move air, but probably not enough air to all areas. IMO it can be balanced, but will need rebalancing every season, in the present design, until you give up and just suffer. For instance the air into the den looks to be less what is needed?? When designing a spider system, care in sizing branches is important. If they are the wrong size, not enough air will move thru them to the extremities. The system needs way to many dampers, that need adjustment during rough in. Depending on the heights of the various rooms, this might be the best that can be done without spending a LOT of money. On the otherhand, it sure looks dumb on paper. Returns are important in making a duct system work, and there are not enough returns in this design. Forget the inlet area for now, the locaztions of the returns will make balancing very difficult. Although flex is considered a good duct material, I do not want anything plastic in my attic. Either I will crush it doing a repair, or it will melt in the heat. The one positive about plastic is when it is snaked thru the attic, the very design of it allows maximum air flow without excessive turbulance, which results in lower system. static pressure.

As to the design, I don't like to use duct less than 6 inch diameter for a run longer than 4 feet. The smaller diameters restrict air too much. Although they are right for small rooms, the rooms never seem to get enough air. If it is at the end of a run, the air is again not enough.

If the person did the heat loss/gain with software it should be easy for him to do a detailed printout. This should show the square footage for walls, windows, & roof. Then you can do a quick check on him.
I'm assuming this diagram is supposed to show a vertical unit in the garage with the ductwork running in the attic.
It should not be difficult to run two reasonable lengths of fiberglass ductboard trunks in the attic and then flex to the diffusers. I also think one of the returns or a third one should be more in the center of the house.

RachelH:

The whole thing looks like it may have been designed and printed with one of those house design software programs.

If someone actually submitted that kind of drawing as a scale representation of their intended work, I wouldn't trust their Manual J calculations.

If that was an off the cuff quickie layout to give you a feel for their intent, make them do an exact scale layout showing all details including balance damper placements.

The poster known as "Rabadger" does duct design for a fee, but can not offer his services under the rules of this forum.

I can tell you about him because I have no connection to him and I have read only positive posts about his work from those on here who have used him. That sort of contact will have to be directly with him at his private or business email as soliciting on this forum could get you thrown off.

Good luck!

Fairyprincess

Proper ductwork design is a science, as it needs to be done right for efficient operation of heat & air conditioning.
The link might provide you with some helpful info.
udarrell _ 2007

Here is a link that might be useful: udarrell.com