use of flexible duct

[jasjam]

I am having conventional A/C installed in a 2 story older brick twin (heat hot water)air handler will be placed in the attic of the home.

The installer said he will use 6" & 8" flexible ducting in lieu of metal suct work. He assures me that their will be no problems and that the system will function properly. The air handler will be placed in a catch pan utilizing a drainage pump.

My question are will the system cool the house properly using this type of duct work? Can it be used vertically dropped one floor to 1st floor ceiling? Can it be compressed if needed?
Can the air handler lay on attic floor or should it be suspended?

NOte; Attic is insulated and floored with 2" and 4" brick vents utilized for ventilation. height is 27"

[contactor]

Too many variables & lack of info without seeing the job.
Maybe you need to get more estimates, or ask these questions to the install Co.

[Irascible]

My question are will the system cool the house properly using this type of duct work?
The type of duct is not important. Flex duct works very well if it's designed and installed correctly. My first reaction is that if it were really just 6 and 8 inch duct then some of the duct runs are going to be too small. But I don't have enough information to know that for sure.

Can it be used vertically dropped one floor to 1st floor ceiling?
Yes.

Can it be compressed if needed?
It comes from the manufacturer compressed in a box. But you want to avoid compressing it during installation. A little compression in a cramped space when you have no choice is not a big deal.

Can the air handler lay on attic floor or should it be suspended?
Functionally speaking, yes. Some jurisdictions say no.

You'll get responses from knee jerk types who say flex is always bad and that you must always spend 2, 3 or 4 times as much to get sheet metal ducts. But in fact a good quality flex duct (not the builder grade stuff) is superior to sheet metal in a number of ways. They both have their advantages and disadvantages. But ultimately flex can work very well if designed and installed correctly.

[trapeze]

I have absolutely no problem with flex duct provided:

1) it is not installed in a crawl space due to likelihood of vermin infiltration,
2) it is accessible (not buried in walls or ceilings) so that it can be replaced if necessary,
3) it either lays flat on attic floor or is properly suspended to avoid kinks/crimps,
4) duct design takes flex friction losses into account

Since you are installing it in an attic then conditions 1 and 2 do not apply. In addition to watching out for item 3 you should make sure that the flex has a fairly good thickness of insulation (flex is available in different insulation values) because an attic is usually ventilated to the great (cold/hot) outdoors. Low end is R-4.2

As noted above, do not accept builder grade flex, the better stuff usually has a silver mylar outer jacket.

I personally like to suspend air handlers/furnaces when installed in attics so that I can get a little better access to and drop from the condensate drain. I also prefer the suspended-variety of vibration isolators that would attach to rafters via all-thread rods. However, with only 26" to work with that may not be an option for you.

[lbjheat]

Local code should dictate how much flex duct you can actually use. For residential use in Chicago Il. your not allowed to use any flex duct. Each surrounding suburb has a different code anywhere from 6 feet per run to unlimited. Follow the prevailing code and you should be fine. Personally prefer metal duct, doesn't get crushed easily easier to clean. Just my choice. Have seen many all flex installs that have worked fine.

[steve3871m]

Now take the info the others offered and hire a pro who will perform a Manual J and D.

If they so no or don't know what it means, move on to the next who does.

[hvac hero]

Originally posted by lbjheat
For residential use in Chicago Il. your not allowed to use any flex duct.

Man thats hard to believe, I've seen million dollar houses here in TN that have nothing but flex. It is a ugly site too.

[hvac wiz 79]

flex is good for its uses .. i like the fact its insulated already .. who wants to sleeve hard pipe ? or wrap it otherwise ? all duct should be insulated ..conditioned space or not ... yes hard pipe may look nicer, & have less friction loss .. but like metntioned by someone else before if sized correct can work well .. i am against people coming right off plenum with like 12 or so flex runs ... thats crap .. here in nj last i heard was in commercial apps its no more than 10' .. but ive seen new commercial buildings with like 25' .. who knows .. they say resi is 25' but again its up to the inspector i guess .. R6 is code now i believe

[lbjheat]

That's Chicago for ya. Can't use and inch of flex for HVAC, dryers, bath exhaust fans , range hoods. You might get a break if your using it for a fresh air intake and the inspector is having a good day. Beware for those that use it in Chicago. If the city catches you they will make you take it out. Funny thing about Chicago, they will give a license to anyone that has proof of insurance and has no outstanding debts with the city, yet they have some of the strictest(antiquated-dumbest) codes.

[jenet85]

That's Chicago for ya. Can't use and inch of flexible duct for HVAC, dryers, bath exhaust fans , range hoods. You might get a break if your using it for a fresh air intake and the inspector is having a good day. Beware for those that use it in Chicago. If the city catches you they will make you take it out. Funny thing about Chicago, they will give a license to anyone that has proof of insurance and has no outstanding debts with the city, yet they have some of the strictest(antiquated-dumbest) codes.

go for the rigid duct

[genduct]

think about all the extra surface area of all flex vs the simple trunk duct with flex branches.
You would be better served with a trunk duct with branches, flex only in areas that are accessible so if they disconnect, you can see and fix them

[stonewallred]

There is a guy in my area that does million dollar plus homes, and he has no problem running flex lines for 60 or 70 feet. And yes, that is a lot more than the manufacturer says.
But he has a engineer in his pocket who stamps his designs, and I will give credit, his stuff works very well.
He is just a pain to do work for. All metal duct work has to be assembled in the shop, and all joints must be sealed using silicone. Including pittsburghs. And then each piece of finished duct must have 6mil plastic taped over the open ends to prevent any dust or dirt getting into it before installation.

[b26440510]

If you can afford metal duct and the return on investment is good for you I'd say do it. Seems that too many companies take shortcuts when using flex and the customer ends up with a poor install. Finding a company that will install metal instead of flex may be a bit difficult, but it may also be an indicator of a company that works better.

If you can, take time with getting this done. Finding a company that cares about their workmanship and you is of the utmost importance.