Greeting All,

I am looking into a new 6 ton Waterfurnace split Envision install and visited several of the HVAC contractors jobs.  They are a very reputable contractor in the area and the work looked wonderful.  My question for the group is the use of 100% flex duct for the air handler in an attic??   seems cheap or spec for such an expensive system??  am I wrong or just being picky?

thank you in advance
brian

Depends on local practices. Flex is definitely cheaper and more vulnerable to damage and sloppy installation. It can work fine if done right and may knock 1000s off a system's cost

A reasonable compromise might be to allow flex for runouts only

Engineer is right, however as an old school installer flex to many of us older guys is like fingernails on a chalk board, I have to admit if sized properly and installed as straight as possible with as few dips and turns it will work fine. Be sure to follow the new international building codes when doing this, I believe R8 is the minimum requirment for flex in an attic situation. your local inspector may have more to say about this too.

For flex to be sized and installed properly, most likely wouldn't save much money at all. Unfortunately, you're most likely not going to get that type of flex installation. The reason being that the flex installers chose it because they can cut "costs", when actually, they're cutting corners...

Most everything works if it is designed for. There is additional friction loss to the flex which means if they are using the standard 6" you may need more openings to get the CFM called for by geo.
When working in an area where insulation is required using flex takes a step out of the labor as it is already insulated. It also is practical in existing home retrofits like one we did recently where the shallow roof pitch made it difficult for our guys to get ducts closer to the rooms' perimeter. If that family had insisted on hard pipe it definately would have cost much more. Instead our guys were able to fish ducts already assembled to areas we wanted to condition.
I would consider this to be the key question- "how many openings are you planning?" then "how many openings would you have with hard pipe?".
If the numbers are the same I'd want to know why.
As a municipal inspector I have actually seen flex used in basements of the "big box builder's homes". In that case where there is no insulation requirement, it actually costs more (in material) to use the flex. So why do they do it?....... Untrained labor is much cheaper (a kid that was flipping burgers yesterday often can't use tin snips).
Good luck,
Joe

thanks all for the replies... I guess it all boils down to the contractor and how reputable they are. I did ask my installer and his response was pretty much carbon copy of the above but he did offer to use rigid duct work if i preferred. That offer with the explanation of sizing the duct right was what i wanted to hear

thanks again
pb

I have a water furnace split system with the 2 ton air handler in the attic in an energy star home.The air handler runs all the time.They say now never put the handler in the attic,to cold of a space.They are in the process of shortening all the ducts they can and then their going to blow insulation on top.I hope this solves my problem,i can only get 78 degree air at the register.

The only way to install flex properly is in lengths of less then 15 feet , so there is no good way to install a system with 100% flex
the standard install includes a reducing plenum trunk with 2-3 flex run outs between reduction of the hard pipe ( I prefer round in attics )
thats not to say that is an international standard just ours . as joe said if length and resistance is considered and size is used to compensate to achieve the target static we could use sheet rock, plexy, custom fabed square , prefabed round , or flex , and it would work great .
2 quick points if you want to spend extra on good ducts go spiral round and move them inside the living space , the energy savings on ducts inside the envolope are worth the expense , while paying extra to keep the system 100% flex free in the attic isnt .
on the return side I prefere flex because bends and turns are essential on reducing system noise

Who says never put an air handler in the attic?
I'm not a fan of it but mainly because of condensation leak risks.
You might want to start your own thread. I have some thoughts on your situation but have some questions.
J

It seems to me that flex vs hard duct is the same cast iron vs ABS/PVC for drain lines and copper vs PEX piping for water distribution piping. If it is installed properly it should work just fine.