[wengang1]

Hi all.
I posted a while back about moving the ductwork to the ceiling, and like a lot of projects, that was put on hold.

Now I'm settled on going with metal ductwork (not flexible) in the attic, running a single trunk line with branches to the various rooms.

The house is ranch style, about 68 x 27'. The ceilings are all 8.5 feet. The air handler is in the crawlspace, and I'll be running a duct up through what used to be closet space to get it to the main trunk line that will roughly run the 68' length.

I have a pretty good idea about what to do, but one thing that confused me was reading a very detailed chart on cubit feet per minute and duct diameters. At the end, I just need somebody to tell me the right answer.

I have a four-ton system for my house, and I need to know the correct diameters to use for:

1. the duct that comes up from the air handler into the attic to the trunk line (which will have to form a T junction since I have rooms going in either direction)
2. the trunk line (if different from the duct coming up)
3. The branch lines running to each room (say 8 lines averaging 10' length each)

The reason the chart confused me (with no HVAC experience) is that it lists different ratings for main lines and branch lines, which made me wonder about the whole idea of CFM. I was thinking the air handler must push the same amount of air no matter what, but the ducts themselves have a limit to how much air could be pushed through them. So I was concerned that having the wrong measurements would cause either too low or too high air pressure through the ducts and maybe cause problems, if not just uncomfortable heating and air. At that point I decided to seek clarity.

I may not have provided enough information here for the right answers, since I haven't done duct work before, but if that's the case, please ask and I'll do my best to get the answers.

Thanks.
Gary

PS I'm well aware by now of why a lot of people don't want the ducts in the attic.

 

[beenthere]

A 10x12 bedroom on a southwest wall, takes more air flow to cool it then a 10x12 bedroom on a northeast wall.

Perhaps you can make and post a drawing of your house and its rooms, and then trunk layout you'll be using along with lengths.

 

[wengang1]

THis is a rough layout of the house with North and South marked. The large peach-colored area is one big opening with no doors to close. The bedrooms and bathrooms all have doors to shut them off from the other rooms. The big red block is where the air handler is located in the crawl space, and that is where I intend to run the duct up through the floor and ceiling into the attic. I didn't overlay the ductwork because I thought it would disrupt the rest of the picture too much. The idea is that I'd run a trunk line from one end of the house to the other, roughly in the middle.
As for vent placements and lengths, I haven't completely decided. One HVAC guy came to the house to price the job and told me that all the vents should be placed over doors and windows. If that's the case, it would mean all branch lines would have to be about 10 feet long to reach the outer walls of the house. If that isn't the case, then maybe I could shorten the ductwork significantly by having it on the inside walls of each room. I also didn't know if it was necessary to place supply vents in the closets or if they would just heated or cooled from the bedroom vents (the doors are pretty much always closed). I assume each bathroom will need a vent, and I further assume (maybe incorrectly) that there should be a vent in the kitchen, one in the dining area, and one in living room #2, then one more in living room #1, and maybe one in the utility room, though it has no door, just a doorway. So, there is still a lot that is undecided at this point, as I'm still soliciting opinions from experienced HVAC people. Last thing, the grid marks are all 1 foot.

 

[beenthere]

The main trunk line should start out at 10x28. Bedroom 2 and 3 should both get 7" supplies, the bathroom a 5".

Since your running your duct work in the attic, and using ceiling supplies. You can put your supplies near the inside wall. And use 3 way registers to throw the air toward the outside walls.

The utility room should get a 4 or 5" supply, even though it has o door between it and the living room.

 

[wengang1]

Thanks.
That's helpful information.
What about the big open areas, the two living rooms and the kitchen? And bedroom 1? And just to follow up, no need to put vents in the closet?

Also, I was thinking of using round ducts, so I'm wondering if I could convert 10x28 to ~19" ducts.
Are there any other components to consider? Is it as simple as connecting all the vents to the trunk line and connecting that to the air handler?

Lastly, I have only one return in the whole house, located roughly where the red square is, in the hallway. Is that still adequate?
Thanks again.

 

[beenthere]

Not having done a load calc on your house, I don't know the CFMs each room needs. So can't tell you what size for each room without it being a guess. That could be off.

Generally only need to heat a closet if your going to use it as a dressing room, or have a water heater or water pipes in it.

An 18" metal pipe would be okay for the main trunk line, and reduced as velocity drops by more then 300FPM.

You can use saddle taps, or wyes to branch your supply runs off the pipe trunk line.

 

[wengang1]

Thanks.
That's plenty of information.
I guess I'll have to have an HVAC guy out to the house before I do the work to get the final calculations.