HVAC Question: Rigid metal vs. flex duct

[rtimgray]

Once I get my house addition dried-in, one of the first things I'm going to work on inside is the ductwork. I've got my plan for my main trunk line taken care of and was pretty sure how I was going to do my branch lines - pretty much 6" round ducting. All of my ductwork is going to be run in between the floor joists - registers for the downstairs will be in the ceiling and registers for the 2nd floor will be in the floor (I'm building on a slab). This is the same arrangement as I used when I built my first house.

In my first house, I used all 6" sheetmetal round ductwork and insulated it. This was my plan for the addition. I was talking with an HVAC guy yesterday and he said if he was doing it, he would put all the branch lines in flexible duct since there is no danger of damage. He proposed that is would either be cheaper or the same cost for material and the installation would go much faster. He also said that I could be assured that the flex duct wouldn't "leak" like the joints on metal ductwork might.

I thought that air flowed better through metal, and that the flex duct tended to reduce the velocity. I've also heard folks say that flex duct can sometimes be noisier because it might flap a little bit while the air is flowing.

Obviously, cost is a concern, but not overwhelming. I will be doing all of the installation myself, so labor cost is not a factor. I know that I'm happy with the metal duct I installed earlier, and have all the tools neccessary to install it. Any thoughts and experiences one way or the other on this?

Thanks.

Good luck and take care.

 

[s219]

With exception of the main manifolds, our entire HVAC system is flex duct (some insulated, some uninsulated). No complaints at all.

I extended two lines in the attic to a finished room over the garage with insulated flex duct, and it was an easy install and worked well. Metal ducting would have been a pain to install there.

 

[rtimgray]

For what it's worth, all of my runs will be straight with a register boot on the end, either pointing up or point down. No bends, no turns, no obstructions to work around. Thanks.

 

[Ed of all trades]

Was a HVAC foreman for 3 years doing commerical work. We never used flex unless we had to because it cut down on flow so bad. We had to have the flow because it was often checked. My sister has a mess in her house. one run is 30' of 6" flex and the flow is almost none. Go with metal and get the thin "crome" tape that is used on duct, don't use duct tape, it will turn loose quicker.

 

[mmorisoli]

We have had nothing but problems with critters chewing our flex pipe over and over again. I guess they enjoy all that nice cold air pumping into the attic just for them.

We will never use flex again...

 

[s219]

My sister has a mess in her house. one run is 30' of 6" flex and the flow is almost none.

That is way too small of a diameter for a 30' run, regardless of the type of duct. It causes the velocity to be higher, and the losses go way up.

 

[BeezFun]

We have had nothing but problems with critters chewing our flex pipe over and over again. I guess they enjoy all that nice cold air pumping into the attic just for them.

Yes we had same problem at work. Maintenance guys were constantly rewrapping the stuff after it got torn apart.

 

[timinsanborn]

i`ve been in the hvac field for almost 30 years, and if it were my house....i`d go rigid metal duct, tape the seams and insulate. don`t forget return air ducts also for each room

 

[Bird]

I've had rigid ductwork in the past and would prefer that, but not because I have any real knowledge; just a gut feeling.:laughing: However, we currently have the flex stuff in this house and been here more than 6 years with no complaints or problems with it. So, I actually have doubts that one is really much better than the other.

 

[Ed of all trades]

That is way too small of a diameter for a 30' run, regardless of the type of duct. It causes the velocity to be higher, and the losses go way up.

Yes I agree the people who did that job did not care or know what they were doing, they put up duct with no rime or reason that I could see, unless it was $$$