Well, I'm one of those people who spouted off the top of his head about rigid ducting. I'm a health economist by training and self-taught in HVAC, electrical, plumbing, etc. So you need to take everything I say with a grain or two of salt :)

There are different types of flexible duct. Heating often uses wireflex, which is insulated and the inner coating is plastic. This is good for forced air heat, but not for venting a stove.

There is metal flexible duct-- like the kind you probably have behind your dryer. Then there is rigid metal duct. I did the venting for my hood and used rigid duct because my GC who was doing the framing, said that is what I needed. Perhaps it would have been fine to use flexible metal, I don't know. I have no data on which is sufficient for code.

Flexible and rigid metal differ with regard ease of installation. Rigid is a pain to work with, so your contractors are likely to want to use flexible.

I would take the statements that your hood is too big with a grain of salt. When I worked with Prestige to identify our blower volume, they recommended a min of 1000 cfm based on my needs. Unless your contractor knows your needs well, it is hard to make blanket statements about what is too little or too much.

Sorry if I couldn't be of more help with regard to code specifics.

I don't have any specific knowledge about flexible vs. rigid duct other than what I just found by searching, but if I were in this situation, I would tell them this...

Zephyr states a minimum duct diameter for rigid duct of 7". If flexible duct is installed perfectly straight (no sagging) it would need to be 8" (7.7" according to hart & cooley's calculator) to carry the same amount of air. Zephyr's specified minimum is the only authority that I believe when it comes to how much air flow is "plenty" for this hood.

Also, I wouldn't be surprised if not following the installation documentation would void the warranty on the hood.

BTW - I just installed duct for this same hood, but installed 3 1/4" x 10" duct down the wall and into the cabinet. Tell them to stop whining and to watch out for sharp edges. It's like juggling knives.

You may be able to look up the recommendations in the National Uniform Mechanical Code standards, and they will back what you are saying.

I posted this link the other day http://www.keidel.com/design/select/vent-hoods.htm, which is the most comprehensive piece of info on the web I've found regarding hood venting.

It says, "Range hoods may be connected either to round or rectangular ducts, but the ductwork must be metal. Smooth ducts are preferable to flexible style, as that is not recommended by the National Uniform Mechanical Code standards.

Plastic ductwork, especially 4" dryer duct should never be used on a range hood installation"

If you don't use the proper duct you will lose efficiency of the hood (lower CFMs). The hood manufacturer's instructions should specify what type of duct work is recommended. Also make sure that if you do not use what they recommend (I would not advise doing this) it could void the warranty. I also want to reiterate that you should check with your local jurisdiction regarding the code if you want to do something different. The last thing you want to do is do something that will prevent you from passing inspection and having to tear it all out (then your contractor has a reason to be annoyed).

Personally I would tell the contractor to do what the manufacturer recommends.

And another thing... if everyone jumped off a bridge would your contractor jump too (can you tell I have kids)?