Flexible versus rigid heating duct


About 10 years ago, I had three 10" diameter runs of insulated flexible duct installed by an HVAC contractor to go from my basement forced air furnace plenum to a 2nd floor attic room, hoping to provide adequate cooling and heating to the 2nd floor attic area. Each of the 3 runs is about 25 feet long.
I have never been able to get really adequate cooling or air flow on the 2nd floor, and can only assume that the static pressure drop on these flexible ducts must be much higher than rigid ducts of the same approximate diameter and length. The installer was very careful to avoid any sharp bends or turns in the ducts when installing them, but they really seem to have a lot of pressure drop.
Is there any easy way to predict how much of an improvement I might expect by tearing out these flexible ducts and replacing them with the same size rigid 10 inch circular ducting?
The HVAC contractor who designed and installed this arrangement apparently over-stated / over-estimated the flow / volume capacity, and the CFM delivery rates were highly optimistic. I get very little cooling, and better but not great heating.
Thanks for any opinions in advance.
Smarty
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Have you tried a booster fan or increased overall blower speed in furnace?
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Thanks for your reply.
I have 3 separate runs, so I have been reluctant to add booster fans to each one. Not only is there is the costs for installing 3 more blowers, but I really don't want the extra noise, operating cost, and additional parts to fail if I can avoid them with a lower impedance / resistance duct system. Increasing the furnace blower speed in the summer to move air conditioned air definitely helps increase the CFM, but the 2nd floor room still gets a small fraction of the CFM it needs.
Smarty
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The real problem is in trying to push that much cold air up into the attic. Bigger/better ducts might help a little but you should really have an air handler up there if you are serious about cooling it. I understand that this may be a foreign concept in places where the heat is on more than the A/C but from a physics aspect they are two entirely different things.
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Thanks for your reply. I'm a little perplexed. Putting an air handler on the second floor is a possibility, but I then have a configuration where the evaporator coil is sitting in the basement in the furnace plenum, and a secondary blower is running on the second floor being used as a "booster fan". I take it your suggestion doesn't also include adding a second coil, outdoor compressor, etc. for cooling the second floor. This would be a highly unusual and extreme solution for the climate I live in, where a single central (basement) furnace with evaporator coil provides adequate 2nd floor cooling for both older and new build homes.
I am in a Northeast U.S. climate where summer temps (July-September) seldom go above 90, are most often in the mid 80's, and the temperature drop / difference I need to achieve in the 2nd floor is typically 15 degrees. An 8000 BTU/hr window air conditioner used to cool the space very adequately, and I was (improperly) advised by the HVAC "expert" that 3 ducts each with 10 inch diameter could deliver way more CFM and BTU/hr cooling capacity than required. This has not proven to be the case, and my first (and perhaps erroneous) conclusion is that the ducts cannot deliver enough CFM because they are restricted. My intuition is that these flexible ducts impede the air flow a great deal more than rigid ducts would. And thus my question........
Smarty
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You are making a compromise when you have one system for heating and cooling. Down here we compromise the other way. The air always comes down from above so the A./C works better and the heat sucks. If you had your primary cooling in an air handler in the top floors your cooling would be a lot more efficient. They just don't do it that way. Do you have a return up there? If you are not sucking the hot air out the cold air will never do much. The heat will always pool at the top
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If you dont have a return in the attic it will never cool even with more supply, but since you dont heat you dont as you know get enough air. One thing right is they are insulated as your replacement should be. Can you up blower speed for more heat.
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wrote:

furnace
flexible
expect
size
apparently
I also had the same problem with 2 upstairs bedrooms (new house-lousy builder). I consulted a HVAC contractor. They added an additional return from the basement through a 1st floor closet, into the attic (insulated duct)and down to the ceiling between the 2 rooms. I was skeptical, but the problem was solved. There was enough supply but not enough return.
Clint
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Thanks to all for your help. I have a large return duct already installed using rigid rectangular duct (I think it is roughly 12 to 15 inches wide and about 5 inches tall) which brings the hot air from the top of the attic room back to the air handler / furnace. It passes thru a first floor closet and is maybe 20 feet long including a couple 90 degree turns it makes coming up to the attic. No doubt it is less optimal than using a 2nd floor separate cooling system, but I am totally opposed to the idea of adding all the extra equipment and complexity and cost.
The original question really is trying to understand how well or how poorly flexible duct compares to rigid duct. I was able to find a good engineering article on the web comparing the two, and have now learned that flexible duct can be fairly similar to rigid duct ***IF***** it is not compressed, and is properly supported without sag or severe bends. The tests conducted by this PhD ASHRAE engineer showed a difference of as much as ******TEN TIMES THE PRESSURE DROP***** for improperly / compressed flex duct. This is most likely my problem, since the CFM I can create in the attic seem far lower at the outlets than the CFM available at the other end of the 3 runs. See:
www.mmmfg.com/pdfs/060601_CC-KW_DuctTechPaper.pdf
I think I have perhaps answered my own question in part, and now need to determine if replacement ducts, booster fans, or some approach makes more sense.
Again, thanks for suggestions and assistance.
Smarty
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Thanks for that very interesting info.
I have a lot of rather crappy flexible ducting in my crawlspace. This confirms all of my own impressions.
In practical domestic HVAC applications, the situation gets even worse. In my case, the ducting does not appear to be terribly well insulated and in some spots the insulation is clearly damaged. I strongly suspect there is some significant leakage as well. In fact, when I purchased this property (it's ~15 years old) the home inspector noted that the ducting had become completely detached from two of the registers. So a significant amount of air was just heating/cooling the crawl space.
When you consider those problems AND the likely pressure loss, it tells me flexible ducting is pretty much a disaster area to be avoided!
I've made a few repairs here and there but I don't think there's much else I can do in short term other than ripping the whole lot out. Then I might was well replace the 15 year old furnance and AC too with high-efficiency units and some independently controllable zoning. And that gets fairly expensive :-(
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Flex duct definitely has more resistance than round sheet metal of the same diameter. However, the difference is not so much that simply replacing your flex with metal will likely solve your problems. There are a lot of issues -- exactly where these ducts are tied into the main ductwok, and also how many CFM your system is putting out. Check with other a/c contractors, but I seriously doubt that merely using flex duct is your problem. Larry
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