View Full Version : Insulated metal vs. flex in attic
09-11-2009, 10:03 AM
Right now my furnace is in the corner of my ~40'x40' house and all of my return ductwork in the attic is 6" R6 flex. I'm having a new system installed soon and I'm debating on having a large metal trunk run about 15' down the side of the house and then turn 90 degrees and head towards the center. The trunk will be insulated (not sure of the R value just yet). The room on the other corner of the house will still need about 18' of flex coming off of this trunk.
So, my question is which option would be better?
A) ~39' of new 8" R6-8 flex duct (relatively straight shot)
B) 15' of large trunk
90 degree turn (which is about my only option when doing the trunk)
Additional 15' of trunk
18' of 8" R6-8 flex duct
Obviously option B will be a longer run and it'll have the 90 degree turn, but will it still be better than the long run of flex?
Would a large insulated metal trunk be better than several runs of small flex duct? I would assume that you'd have more total surface area when adding all of the flex duct, so I would think it'd be worse, but I'd like a professional's opinion.
I live in southern Indiana, which gets it's fair share of humidity, but I don't think I've ever seen any condensation on any of the uninsulated return boots up in the attic.
09-11-2009, 10:40 AM
What kind of air flow are you needing to get out of the 8" flex?? Did you say that all of your returns are 6" round?? You mentioned your home was @1600 sq. ft.? Something that always is a problem when installing ridgid duct is the location of the indoor unit sometimes is on the outside wall, and the roof rafters will not allow any room for a duct of any size. Attempting to "hide" the home comfort system sometimes causes problems with performance. Too bad that most home designers never take courses in HVAC! See way too many "beautiful" homes that nobody is comfortable living in!
09-11-2009, 10:48 AM
Total area is ~1680.
All of my returns are currently 6", but I'll be swapping most of them out for 8" as well as adding a 12" in a hallway near the thermostat. I haven't completed a room by room heat load calc. (I know, I know:)), so I can't say exactly what I need for the room, but I did figure the total area needed for the 3 ton / 1200cfm system. With the new runs, the flow should be no more than 175-200 cfm in the 8" return. I'll be getting a variable speed furnace, so hopefully the ducts won't be seeing that high of flow most of the time.
09-11-2009, 11:02 AM
With respect I have to really encourage you to get a professional to at least do a proper heat load calc and detail the equipment and design of your new system if you plan on installing it yourself. It is just a fact that it takes a lot of knowledge and training to do just that and then remember about 80% of a systems efficiency is whether or not it is properly installed after the design and equipment needs are determined.
09-11-2009, 11:26 AM
Don't worry...I'll definitely be having a pro do the installation. I like to tinker on a lot of things, but HVAC isn't really one of them. I actually brought the idea of installing the 30' trunk line to my dealer/installer. I think he would be fine doing it either way, so I was just trying to figure out if it would be worth the extra money to do it or not (or if it would even be worse than using all flex). On a side note, he did say that he seals all of the seams with mastic, which I liked.)
I've been reading about issues with long flex duct runs, which is how I came up with the trunk idea in the first place.
My installer did do a heat load calc., but he essentially treated the entire house as one room. I could ask him if he could do it room by room as well as doing a manual D.
09-11-2009, 01:09 PM
I think the problem with companies is that they are constrained on how thourogh they do jobs due to having to outbid others. Also they get into a routine of just going on experience of having done similar houses etc... A room by room heat load calc is the proper way to do it. A room you are going to use as a den or reading room will not need as much cooling as a "party room" with big tv's and large amounts of people expected to be in them. Plus how much insulation you have in the walls and ceiling is huge.The difference it can make from best insulation to worst can be several tons! Or an attic fan.
09-11-2009, 01:37 PM
I'll see what the dealer says about doing a more complete heat load calc. He seems very knowledgeable and willing to listen to my questions and concerns. I did receive lower quotes from other dealers, but I felt the most comfortable with this particular gentleman.
So, as a general question I guess....would it be better to run a long insulated metal trunk with short flex branching off of it or several shorter flex ducts for returns in the attic? The metal trunk will probably end up costing about 2-3x more (quick guesstimate on my part), but if it'll be better I'd be willing to pay it. It would also clean up my attic and make it less likely for damage to occur in the future. The supplies in the crawlspace are in a similar state, so I am most likely going to have him do something similar down there. I'll be encapsulating the crawl in the near future and either conditioning it or getting a small dehumidifier.
09-11-2009, 01:57 PM
It's a balance between cost & efficiency. Flex-duct is harder to move air through than metal. As long as when it's all said & done you have the proper cooling & heating delivered to each room and good efficency-no high utility bills- you are in good shape. Take insulation for example. No insulation compared to the worst insulation is huge in savings, but then as you go to higher and higher r-values your savings get less & less. If you have a reputable contractor that you feel comfortable with then sit back with your warranty in comfort when he's done or call him back via the warranty if there are prblems. But let him do his job after letting him know your concerns.
09-11-2009, 02:16 PM
I understand about calling the installer if I have a problem, but the issue is which method is better?
It'll be hard for me to judge by differences in energy usage due to the large change in my system. I'm going from an 80% w/10 seer a/c to a 96% w/18 seer heat pump.
I guess I'm just looking for someone to tell me that this method is generally better than that one...and how much better it is :)
09-11-2009, 02:36 PM
Hands down metal insulated duct is better than flex-duct. Get dampers for supply take-offs from the trunk if you want to perfectly fine tune your system. Not vents with dampers. This way after installation if your various rooms needs change you can adjust the amount of air each supply is delivering to overcome some rooms being hotter or colder. Example:maybe later down the line you cut a tree down that was providing shade to a rooms wall and now it seems too hot all the time. You just adjust your dampers to a little less air to the cooler rooms and open the damper more to the hot room. Money & knoweldge are the only things standing between anyone & perfect comfort.
09-11-2009, 02:49 PM
duct leakage is a big factor also.
hard pipe will leak more than flex unless all connections
(90's, seams of hard pipe and all conncetions) are mastic
flex leaks at either end of flex (unless flex is damaged or cut)
hard pipe leaks a lot. most companies do not seal all seams and
joints of mastic, but rely on duct wrap to minimize duct leakage.
duct wrap does not do this as it is to insulate only. it covers leaks
but does not seal them
Personally I would invest in the mastic seal of the flex.
Properly sized flex can provide proper air flow.
09-11-2009, 03:10 PM
Thanks for the tip on the dampers. If I go this route should I step up the size of the supply ducts to ensure sufficient flow without getting too crazy of a speed?
My installer said he seals all joints with mastic, but I understand what you mean about leakage. I guess my main concern with the flex was when used for long runs. I thought I read somewhere that flex isn't recommended over about ~20-25 feet. I guess as long as the size is large enough to overcome the additional resistance, it shouldn't really matter, correct?
09-11-2009, 03:23 PM
You've got too much time on your hands like I do, but I love talkin about this stuff when I do have the time. There is too much involved to explain to you about sizing ductwork. In a nutshell it's all about volume, pressure & velocity. The size of your ductwork will be determined by your contractor who has all of this knowledge. The last guys post about metal ductwork leaking is only correct if it is not, as he said, sealed with mastic. Flex duct certainly works correctly when sized and installed correctly. Remember, 80% of how efficient & well your system works is whether it is installed correctly-whether it be with metal duct or flex duct.
09-11-2009, 03:40 PM
I do but I don't have too much time on my hands :) I have my first kid on the way (end of October) and I'm trying to get everything figured out and installed before then. This means I need to hurry up and get the ball rolling, but the engineer in me wants to figure all of this stuff out and get the best system I can get (within reason). I'm pretty sure I'd like to have the metal duct work, but I'm just trying to decide on whether to spring for it or not. I could really use the extra money if I skipped it, but I'm afraid I would kick myself in the future if I didn't do it. I guess if I'm already spending as much as I am on the system, what's a few more dollars on metal duct work. But then again, that money could buy me a lot of diapers...see my dilemma!! :)
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