I want to know the pros and cons about flex duct vs. steel duct for hvac units?

Good question, there are important differences.

The pros-

(1) Cost is lower for flex duct, for both material & labor.

It's easier to install, taking less time and less skill- cheaper labor.

It uses less fittings, such as elbows.

(2) Installation can be done in spaces that make metal duct impossible because of the flexibility.

The cons-

(1) Despite claims, flow in flex duct is less than that of same-size metal duct. It's also easy to pull around an obstruction or have it stepped on in an attic, and have it collapse to 1/2 or less of the normal size.

(2) Flex duct is not a really durable or permanent product. It's a very thin plastic liner with a spiral wire, then insulation and a plastic bag shell over that. I've seen much of it which has become brittle with age that will instantly disintigrate when touched, usually in 10 year or older installations. Plastics are affected by heat, losing their elastomers, and the application assures they will be thermally cycled. Because it's often used in places where it is hard to put metal, but this means the replacement is an even tougher job.

(3) It's easy to damage in the short term as well, such as tearing the cover, having heat from plumbers work melt the skin and so on. I've even seen dogs chew it up where it was accessible, and mice chew through the plastic walls.

I'm a custom design/builder and GC with 30 years in the business. While cheap houses always go flex, I don't allow it on the job without advance permission, and only allow it's use under a few conditions, like a return duct where metal can't be used.

Flex Duct Vs Metal Duct

the benefit to flex duct is it is quick to install and can go in a lot of areas not feasible in metal.(money wise).. its quieter (some say)

the down fall is resistance of the air flow if not stretched out all the way

metal duct can be made into any shape to fit any space it just costs allot more to do the same thing.

then there are all the connection joints that have to be sealed and some are inaccessible to do so making for an inefficient system.

over all i would say that the main trunks should be done in metal with branch run-outs done with both metal and flex (say the text book) but you can do the main in metal and the branch in flex

STAY AWAY FROM DUCT BOARD

.the problem with rigid duct work is that every 45 degree angle equals 5 more feet of duct .meaning more Resistance.thus less volisity..it also can be forced into the floor joists.duct work on the venturi prince able .meaning that u start large and end up small .because of the correct sizing your heat will appear at the end of the run before e the first takeoff .look at the burner tube on your stove u will see what i mean about venturie effect

Pros- It's really fun. Makes for funny memories. Awesome photos. Fun to do if your drunk. A good way to prank some one if they pass out at a party. A good way to get rid of duct tape. Cons- You are stuck to a wall. You are at the mercy of your friends when they are there with you. When the duct tape gives out you fall (I fell on my face and broke my nose), You are going to rip your body hair off and some skin. You can rip paint/wall paper off your wall. These are Pros/Cons I have experienced in doing this.

Flex duct is cheaper, faster and easier to install.

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Pros: Super fun, super funny, makes for a great memory. Cons: You are stuck there, your friends can beat on you, they could leave you there, incredibly painful to get the tape taken off, may ruin your wall, hope you don't have to go to the bathroom. Do it.