Alumaflex vs. rigid ducts for furnace?
I’m getting bids to replace the duct work in my 1930 bungalow…existing ducts are plastic flex ducts that are 20+ years old and sagging and have holes from animals etc. All duct work is under the house (one story house) with 18-24″ crawlspace, less in places where 6″x8″ beams run the length of house under floor joists. First bid was for rigid ducts and second bid they suggested using alumaflex ducts instead. He basically said it would be a pain to install and properly insulate with rigid ducts due to crawlspace height and suggested the alumaflex instead – he said the alumaflex is more expensive but overall it will be less as labor will be less to install as it’s easier to work with in confined spaces and is pre-insulated (r6). I asked about comparability in performance, durability/longevity, and resistance to animal damage and he said alumaflex is equal on all fronts to rigid ducts. He said animal issues would be less as they can’t chew through either duct type but can strip insulation off rigid ducts, whereas they would have a tough time doing this to the alumaflex.
The bid came it at half that of the rigid duct bid, so certainly that is appealing but I don’t want to be suckered into an inferior product – any thoughts out there on the comparable performance and durability of alumaflex?
Replies
I would be more comfortable with ridgid duct, but that's just my opinion. Here's an old post that may interest you. http://forums.finehomebuilding.com/breaktime/energy-heating-insulation/thoughts-flex-duct
I gather that "alumaflex"
I gather that "alumaflex" isn't a brand but rather a generic type. (At least I couldn't find a web site for it.) So there are no real standards, and impossible to say how good a product you'd be getting. However, since the stuff (generically) is reasonably rigid it would perform significantly better than plastic flex if intalled reasonably well.
Offhand I don't see how the insulation on the flex would be harder to chew off than insulation on rigid -- they would presumably be similar materials.
If you go with the flex it needs to be well-supported. It's more rigid than the old saggy stuff, but still needs support at least every 5 feet or so.
If dgale goes flex, that is an easy DIY job, and cost would be about 1/8 of pro installed rigid.
I'd support flex every 16" myself. On a DIY job, would also box it in with surplus plywood with additional insulation around it in the crawl space, with lots of mouse traps or bait if animals have been a problem.
You have no knowledge of
the product under discussion, but have to add your "advice"? Worse than useless.
I don't understand why the flex duct would be more expensive. There is zero fabrication time, it requires less space to transport as it's all folded up in a box, there are less connectors and fasteners, and you just unpack the stuff and install it.
I also don' t understand the point about flex being easier to insulate. Doesn't seem right to me....the stuff is all floppy and loose.
If the old flex duct got damaged by animals, so will the new stuff. I would go with rigid myself.
Flex is more expensive to purchase I'm guessing. A lot more goes into the fabrication and there may be twice the materials in it as rigid. As far as handling and install, it's way cheaper (I do mean CHEAP).
The alumaflex itself is more expensive but the labor to install is less and the total cost would be less than with the rigid due to cramped space making it difficult to seal and insulate the rigid pipe. My understanding is that the Alumaflex ducts are pre-insulated and the insulation is inside of an outer wrap that is supposedly pretty tough, whereas the rigid pipe gets wrapped with insulation on the outside. He said the traditional insulation is to wrap in fiberglass, which would be challenging to do right due to cramped space and more prone to animal damage. He did say there was bubble wrap and other insulation options which I think might have been more animal resistant.
As far as being floppy and loose, my understanding is that this stuff is much more rigid than the plastic flex duct. Again, I'm not familiar with this particular product and my default reaction is to be a bit leery of flex vs rigid, just wondering if anyone has any experience or knowledge of the stuff.
Well, there's little doubt that it would be better than what you have. It's a question of how much better you're willling to pay for.
At this point I'm leery enough to ask them to give me a second bid for rigid ducts instead. I guess I'm just trying to understand how much better (and why) the rigid is so I can weigh that against the difference in bids. I'm always in favor of paying more to do it right but I just want to make sure I understand how and why rigid is better and to what degree so I can make sure it's a sensible decision.
What you should do is ask the guy to show you a sample of the stuff and see how it seems. It should be reasonably stiff -- only bend when forced -- and, while it won't be smooth on the inside, it should have a consistent diameter and little tendency to crush or skew in a way that would reduce airflow.
I went by and checked a section of the proposed "Alumaflex" duct out and was actually quite impressed. It was very rigid and didn't bend or flop at all when I held a 10' piece up by one end. Nothing like the floppy flex duct I'm used to seeing. The inside was far more laminar than typical cheepo flex duct and was reasonably stiff - <1/8" deep grooves on the inside and he said he installs a larger diameter of this stuff than he would rigid for the same job, which more than makes up being less smooth inside than rigid duct. While the stuff would definitely crush if someone tried to crawl over it and put their weight on it, short of this is seemed pretty tough and I can't see how it would get crushed or skewed. Also, it comes pre-insulated with a tough vapor barrier on the outside, so it would be difficult for critters to get at the insulation and impossible for them to actually chew into the duct itself.
Anyway, I was impressed with the quality and am going to go for it. I'll try to find out the actual brand name for the stuff if anyone is interested, as "Alumaflex" is not a brand name.
I'm leery
... and you should be. A good contractor would have given you the option. A good contractor knows what the difference is. Assuming rigid is an option and you are in the frame of mind to 'do it right', you should ... do it right. Flex reduces air flow substantially over smooth walled rigid. Any good HVAC design professional will tell you that. That's the simple benefit. It translates into energy savings and comfort by getting the air to go where you want in the quantities you want it.
Going rigid ... make sure they seal all joints w/ duct mastic and install balancing dampers to allow you to adjust air flow as necessary. Both inexpensive additions that will benefit you.
It may be difficult to find a contractor who says it's best to go rigid, but those who do/would, would be on my list of good options. Those who tout flex are simply lazy and don't care about performance and quality install .... that's my opinion.
My general rule is to always say NO! to flex duct. It affects air flow performance. This can be substantial particularly if not installed 'right'. If you have difficulty accessing the crawl w/ rigid, MAYBE use flex, but I'd guess you should be able to get e.g. 4 ft lengths in.
Flex is used by contractors ... it's easier for THEM, but it doesn't give you the same performance. At minimum, if you use flex, use rigid at the registers (i.e. elbows). But only use flex if you absolutely need to ... and then take great care in doing it.
If you want, look at the static pressure loss per length compared to rigid ... particularly if you have bends. Irregularities on the inside of ducts substantially changes the air flow characteristics.
>>>it's easier for THEM, but
>>>it's easier for THEM, but it doesn't give you the same performance.
Right; totally forgot about that part.
Can be noisy too...all that swirling air trying to get through.
The most significant
cost in any ductwork installation job is the labor. Flexible duct, installed properly, will perform better than rigid (galvanized snap-lock pipe).
Aside from the cost difference, the trade-offs are that the rigid duct will last forever, leak more (with any competitive installation) and requires external insulation, that is a weak part of any (again, reasonably competitive) installation. It also requires a level of skill and experience mostly absent from the "competitive"residential trade.
Flex duct (alumaflex is just regular flex duct with a silvery refective cover in lieu of the standard black poly - a wasteful, useless scam IMPO) is very easy to install, and is, unfortunately, the material of choice by most of the cheap, lazy and unqualified installers, and most DIYs, too. That, like lower cost equipment, is the reason for the bad rep, not the actual product. It is pre-insulated and continuous in 25 foot lengths, with proper connections, it is vitrually leak free. It must be installed correctly, protected from physical damage, supported more frequently and care taken to avoid collapse at turns, connections and obstructions.
Air flow through fully extended flex is comparable to that or rigid duct of the same size, if sized correctly. At higher velocities (>1000 fpm), the rigid duct will perform better, but there should not be higher velocities in a low pressure, residential system.
Personally, I prefer rigid ducts, but not exclusively. Each have their place. In basements, I use rigid ducts always. In attics, if necessary, and other concealed areas flex duct can be the best choice.
Get and keep the rodents out. Don't waste money on alumaflex vs "standard" flex (IF you were to have cooling ducts in a very hot attic, a bad idea to be avoided, the alumaflex would be beneficial for a year or two, until it was covered in dust).
My $0.02