What's the best heat duct material?

I have a two story home with 2000 square feet of living space. 1100 down stairs, and 900 up stairs. The house has forced air/natural gas heat. I'm going to replace some of the old heating duct. Should I replace them with the galvinized tin ones, or the plastic coiled/insulated ones? Does it help to insulate the tin type? I have a basement, but the house has two additions with crawl spaces.
Also has anyone used a secondary inline fan to help push heat though the house? Home Depot has them, ans says it helps save money on heating bills.
Thanks,
Steve
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I just wanted to add, that it appears you may have some problems with the design of what you have now. If that is the case, you should be well served to have a qualified professional come in and determine what you really need, not just guess. A good distribution is critical to a good system. I would rate it as the most important part.
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I'm not sure what you mean by the "plastic coiled/insulated ones". Do you mean the round dryer vent type? Regardless, what you need to look at is a material that allows the air to move with minimum friction. I learned this from the installation of a dryer vent. Steel venting allows the air to travel more freely allowing for longer runs. Given such, it would seem that anything "coiled" would work against proper air flow. Also, the coiled ones would present you with a real problem when you went to clean them. And yes it does pay to insulate the ducts, especially if they are in an unheated area. You also get the added advantage of cutting down some of the noise from your heating system when you do this.
I don't have any personal experience with the inline fans but I have heard that they are not as good practically as the theory behind them. It is better to make sure your system venting is balanced properly to ensure proper air flow to where it is needed.

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wrote:

The "theory" is flawed. The system fan only puts out so much air. An inline fan is just a way to send more flow to one room vs. another.

Yes... dampers can probably do the same job with no electricity.
To the OP: You need to do a lot of reading on duct installation if you plan to DIY. Use metal wherever possible. Flex duct will reduce flow. You will need some tools like a duct end crimper (or whatever they call those tools). You will need lots of supplies.
It would help to find a local HVAC supply that sells retail with a nice friendly counter man who is willing to spend a few minutes with you to make sure you get all the stuff you need.
If your system needs balancing and adjustment, call a pro instead. Only replace ducts with the same size and only if you like the way it all works now.
Bob
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snipped-for-privacy@hotmail.com (X) writes:

Metal ducts are the most efficient. There are various grades and sizes of the fiberglass ducts. Some are lined with plastic liner, and are moderately efficient, some are unlined and absolutely horrible. If you use fiberglass ductwork, increase the size 2" over metal ducting, and use suspension saddles if you hang them.

You need to balance your system. That can include installing a new return air duct as well as adjusting the air flow to individual rooms.
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