I have several questions related to my Goodman Furnace (Model GMS90703BXA - 69K BTU / 93% efficiency), which is installed HORIZONTALLY in the attic and SUSPENDED about 14" from the attic floor in one long line as follows: [right to left] return plenum (3' wide x 14" tall x 24" deep) / furnace / spacer duct (12" long ) / A Coil / supply plenum (2 pieces: one 4' wide and one 2' wide; both x 21&1/2" tall x 16&1/2" deep)

Duct Placement ?
I have at least 8 flexible ducts to attach to the supply plenum (two 4"; five 6" and one 12" which will supply the basement ducts) and at least 2 large ducts to attach to the return plenum (one 12"; one 10 " which runs to the basement). What should I keep in mind when mounting my flexible ducts into the supply plenum? For example: where should they be placed relative to a bypass humidifier (see question two)? Does it matter whether they are mounted on the top / bottom / end of the plenum? Should they be equally distributed (by air volume) on opposite sides? Should they be mounted down the center line of the plenum, or slightly above/below, or staggered on either side of the center line? Have I missed anything?

Humidifier Placement ?
I have a Honeywell Bypass Humidifier (model HE220A) and the installation instructions are very conflicting. Which plenum should I install the humidifier in AND what is the ideal location on that plenum (considering the relative position of the supply ducts). Obviously, the 6" bypass duct will install on the opposite plenum, but I'd also like to know if there's an ideal installation location for it as well (again, considering the relative position of the return ducts). Have I missed anything?

Insulation Effectiveness ?
Lastly, what is the most cost effective way to insulate my attic mounted HVAC system? Should/can I wrap the furnace and/or plenum(s) If so, with what? Or is it more effective to insulate in between the rafters? Or should I use a stiff insulated/reflective sheet mounted across the rafters? Is there an ideal combination of treatments? Also, I'd like to suspend some of the insulated flexible duct (to maximize floor space for storage). Is that a bad idea? Have I missed anything?

Thank you in advance for your time...

Yuk. you are going to hang a furnace in the attic and use poorly insulated plastic, combustible ducts instead of metal. Read:
http://heatpro.bluedomino.com/duct/ductloss.html
and
http://heatpro.bluedomino.com/service/solidair.htm

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When citizens are entitled to food and medical care there is no poverty and no need for retirement security.

No, I'm not GOING to hang a furnace in the attic. It's ALREADY BEEN HUNG, including the flex duct and ceiling vents!

What I'm trying to do is figure out the best/cost-effective way to improve on a poor design.

With that in mind, do you have any constructive feedback?

Thanks...

The constructive part of the references after you read them is to do something that will make those weaknesses go away, like eliminating the flex duct, using metal ducts, sealing them with mastic and wrapping them with full insulation of the value you would use for your walls.

www.heatpro.info heat/cool right-sizing

When citizens are entitled to food and medical care there is no poverty and no need for retirement security.

Make the attic in conditioned spaced.

The insulation goes on the underside of the roof and none on the attic floor.

There are lots of details that need to be worked out. And if might not be practical for a retro fit.

But that is really the best solution is to expand the conditioned envelope to include the HVAC system.

You can see several houses that have conditioned attics here.

http://www.buildingscience.com/housesthatwork/default.htm

and you will find more information under technical resourses.

Did you ever get an answer? I have a similar problem, and was curious what you came up with.
Don't you just love it when you ask for advice, and get insulted....

Exactly what kind of an insult is it to be told that plastic duct of the thickness of a shopping bag is not a good duct and that it should be replaced with metal duct that is well-insulated instead of just 1-1/2 inches? Is it always an insult to be told how to do things the proper way and not the cheapest, most useless, shortest-lived way? Did your parents insult you by telling how to put your pants on after you put them on backwards? Do we have to be politically-correct and pat someone on the back for doing a really bad DIY job? I guess that's the way schools want learning to be these days. Don't do it right, just get a smiley.

THE ANSWER IS RIGHT IN FRONT OF YOUR NOSE. All you have to do is read. If you don't want to be told how to do it the right way; don't ask.

www.heatpro.info heat/cool right-sizing

When citizens are entitled to food and medical care there is no poverty and no need for retirement security.

To Escher and Wheelman -
Sorry to break into your "conversation" with HKestenholz, but his whole attitude compelled me to say something.
While it's evident that HKestenholz has knowledge and experience in HVAC design, it's also evident that he's a complete know-it-all self-aggrandizing jerk. If HKastenholz had his way, he would have you rip your whole house apart to improve the efficiency of your heat by 3.5 percent. The scary thing is he actually thinks he's helping.
What HKastenholz fails to understand is that sometimes you have to make do with what skills you have. Not everyone knows how to run ridgid ductwork, or where to buy the tools and supplies in order to do so. And not everyone has the money to invest in replacing their whole heat delivery system. Flex duct work is used all over the world in new and old construction. It's economical, it can be installed easily in many scenarios where ridgid ductwork would be impractical, and it's modular, where most ridgid ductwork must be custom-fabricated. To compare flex ducts to plastic grocery bags indicates a special kind of bullish attitude that is worth ignoring.
So, Escher and Wheelman, go ahead and keep researching how to optimize your flex duct system. Just think, while you're taking active steps to improve your heating system yourself, and learn some stuff along the way, HKestenholz will still be here insulting people and wandering around his house trying to find elusive pockets of neutral air pressure.

eyestrain has a right to his opinion; but it is smart to examine what he gets out of it.

Inadequate insulation has become a sore point in the industry over tha past decade. Codes have called for an improvement of duct wrap to R6 from R4 in flex duct. While this is an improvement it is ridiculous in a year when fuel costs have almost doubled to have ducts throwing heat away at 1960's insulation standards. Walls are now a minimum of R14, and so should the ducts that carry heat and cooling. If eyestrain is successful in lulling homeowners into thinking that inadequate insulation is OK, then the industry ripoff salesmen can continue to sell cheap installation, while running homeowners into fuel poverty.

He has another gain in his mind as a flamer. He becomes one-up by putting down. Here is a link to explain the flame game:

http://64.226.150.132/hydronic/service/girly.html

Here is one of the ploys he is using:
"Another flame approach to watch for is another newbie entering the site to comment about the way you talked to someone else. As they have no real business of answering the original question, their only reason for commenting is to give a political harangue on your character. If you explain clearly the reason behind challenging the original poster, they will respond with a flame that you were showing bad character. Another tag team member will then come on to comment upon your response. Neither of these tag team members will give any useful answer about the original post unless you challenge them to respond to the first post to show that they can answer the question better than you can."
http://64.226.150.132/hydronic/service/girly.html#TagTroll

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When citizens are entitled to food and medical care there is no poverty and no need for retirement security.

HKestenholz,

With all due respect and genuine appreciation for the time and expertise you have given on this and other forums, I believe that you've missed the essential message (and gift) of the feedback given to your comments in this thread.

The ideal - and the reasons that make it so - which you are advocating are wonderful and certainly merit thorough consideration.

However, the primary theme of the feedback to your posts is not addressing the validity of the ideal you are advocating. It has more to do with the WAY in which you are referencing the ideal.

In other words, it's not the facts you are sharing that are rubbing people the wrong way. It's the distinctly arrogant, dismissive and defensive way that you make your point that is at issue.

Again, please do not misunderstand me. I am VERY grateful (as are others, no doubt) for your time and expertise, but the beauty and merit of sharing your time and expertise has been overshadowed by the ugliness of your relatively insulting comments.

I hope the time I have taken to offer you a new perspective on your communication style is received in a way that benefits you and those you love.

Again, thank you and happy new year.

Escher Van Korlaar