HVAC Question: Rigid metal vs. flex duct

[nhkabotab7800]

pretty hard, no impossible to clean flex duct in the future...just sayin'

 

[rtimgray]

Looks like consensus is rigid ductwork, which is how I was leaning before. I appreciate the comments.

As for duct placement, ceiling/floor, I don't really have much choice because the house is built on a slab and the second story is "under roof", so there is neither an attic nor a crawl space. All of the branch ducts have to be run in between the 2nd floor joists, while the trunk line will run in a closet/mechanical chase.

I agree with floor registers being a pain sometimes, but I don't notice it too bad. And yes, I've had to fish things out of them that my kids thought would be funny to push down into them.

All of the rooms are getting return ducts as well (actually panned joist spaces) and there will be filters in each of them.

Thanks again for the comments. Metal it is.

 

[rtimgray]

But what kind of metal?

It looks like the choice is largely between 30 gauge and 26 gauge. I will be fabricating by trunk line from 26 gauge sheetmetal, but what about the (mostly) 6" branch lines. There is a significant cost savings by going 30 gauge, and the ductwork will not be subject to damage (without extensive damage to the house as well), so is there any reason that the round ductwork would need to be 26 gauge?

Thanks for the comments.

 

[rtimgray]

Just for the sake of closure, looks like 30 gauge will be fine for my application. I went to a supply house over the weekend and handled each type - I'm almost certain that what I used in the original installation was 30 gauge, and there were no issues with it.

Thanks again.

 

[schmism]

Just added 3 ton AC coil to the new furnace in the farmhouse we bought last year (PO put in a new furnace but no AC) the AC guys poo-poo'd the small 8" ridgid duct work that supplied the registers. most said they were to small. but there is no way you can convince me that the flow rates through 8" flex and through 8" rigid are anywere CLOSE to the same. (rigid has higher flowrate) I believe all there thinking is based on flex duct because rigid isnt used anymore today unless they are made to.

 

[gorj]

Be sure to use good quality tape between joints and insulate the duct well. I use 1" fiberglass board for returns. The fiberglass board tends to isolate air handler noise better than metal duct.

 

[clemsonfor]

When were individual room returns made popular? I did not even know that it should be done untill a few years ago watching "Holmes on Homes". I obviously dont have it in my house just a 30"x20" filter that i change monthly (with the most expensive or second most expensive filter) as i have cats and we "make" a lot of hair and have lots of dust.

I have 2 HVAC units in my home and have flex ducts, i extended one and it must be 25to 30 feet and it still seems to flow pretty good volume, but i can easily see how they cut flow drastically.

Anyway i have been in newer homes in my state and not seen room returns? Is it a northern thing? I think even in my brotherin laws home they may have a few return airs but not each room, i will try and pay attention next time (he lives in a $400k house). His lake house im not sure about the returns either, i will look as its only 1 year old.

 

[Bird]

When were individual room returns made popular?

Maybe when they started putting the indoor HVAC equipment (blower, evaporator, etc.) in the attic? I don't really know myself, but I bought a new 4 bedroom, 2 bath home just north of Dallas in 1972 (very small; only about 1531 sq. ft.) and another new 4 bedroom, 2 bath home in Dallas in 1977 (about 2171 sq. ft.). They were built by the same builder, had Carrier HVAC (electric cooling; natural gas heating), but the indoor equipment was in a "closet?" along with the water heater in the hall. That closet had a raised floor and louvers below the door, so that was the only return air and it was sucked up from below. Of course, bedroom doors were designed to leave a gap at the bottom so the air could be sucked out of the bedrooms into the hall. So I had a single rectangular filter in the bottom of the HVAC unit in that closet. But I had a metal, custom filter made that I washed with water, let dry, and sprayed it with a sticky filter coating spray periodically instead of replacing it once every month or two. And the HVAC ducts were rigid ducts in a dropped ceiling.

And now we live in a little 3 bedroom, 2 bath home (1295 sq. ft.) that was built in 1991. It has the HVAC stuff in the attic and a return air vent near the bottom of a wall in each bedroom and the living room. And there's an Aprilaire 2200 filter up there. I'd never seen one before; accordian like material in a big frame (huge filter), and I change it once a year (usually in September, but did it earlier this week). And all the ducts are the round flex stuff; part of the ductwork is covered with 13" of blown in insulation.

We have some friends about halfway between Dallas and Ft. Worth who live in a house with a return air vent in each room, and a filter in each of those vents. The vents are different sizes, so he keeps either 3 or 4 different sized filters on hand. I don't know when that house was built, but that's the only one I've seen like that.

And while I'm sure I'm in the minority, I still say hanging would be too good for the guy who came up with the idea of putting the HVAC equipment in the attic.

 

[MFWD]

Hey XTN,
You said that you designed systems and only put flexible at the last 3 or 4 feet before the register........is that to keep things more quiet??????



MFWD

 

[Tig]

Interesting thread. I'm just putting the finishing touches on the installation of my duct work in the new house. I bought 8' of flex to complete an awkward run across six joists. From what I have read you go up one inch to allow for the losses in flex. After reading some of the comments on flex I'm reconsidering using it at all. That means doing it the hard way, as usual....
As for flow rates. I have a few branches in the 30' range using 5" and 6" duct. Staying with velocity targets, the flow rates were spec'd out as follows;
30', 6", 85 cfm
36', 5", 68 cfm