Checking heating duct system

I have two furnaces in my house, using two separate duct systems in the attic. This is a one-story house. I checked the registers and found that the registers in one of the furnace systems have twice the air-flow than the registers in the others system.
I suspect that one or more of the ducts in the low-flow system may have opened up or may have collapsed. The ducts are the round, insulated flex-ducts type and are about 25 years old. The furnaces are new.
Beyond a visual inspection, which is difficult due to access problems, how can I determine if there are leaks in the duct system in the attic?
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Walter
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Walter,
A visual inspection using a fiber optic probe may be in order if you can't enter the attic and this low flow is creating problems. I'd recommend calling a service tech who has a fiber optic video camera set up to look at your heating system.
Dave M.
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Thanks for the suggestion. I also wonder if I can use so-called smoke candles to see if there is a leak somewhere in the ducts. Ever heard of it?
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Walter
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wrote:

Unfortunately, you didnt use a professional HVAC company or you opted the cheap route and didnt have your flex ducts replaced. Plastic flex ducts dont last 25 years. They deteoriate. Get them all replaced. You'll be glad you did and so will your pocket. Bubba
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Well, I thought this was a self-help group. Of course, I can go out and spend $ 5000 to replace the ducts. That's what it costs and that's what I am trying to avoid paying by fixing the ducts myself. Ducts don't look like high-tech to me.
Walter www.rationality.net -

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Are the ducts in the under performing area longer? Flex ducts have a significantly high resistance to air flow than do smooth metal ducts. Bends (elbows) exacerbate this. Your problem may be nothing more than this.
Using flex ducts should be a last resort. Builders like them because they're easier to install, but they are inferior to smooth metal ducts.
Do a web search and see the pressure drop per foot for the same cross section and air volume flowing in flex and metal dusts and you'll see quite a difference.
Boden
Walter R. wrote:

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