An 8" round duct is required from beginning to end (including the termination on the roof or outside wall) for a 600CFM Vent-A-Hood. Alternatively, you can transition from round to rectangular ducting, as long as the rectangular ducting is equivalent in size. An 8" round duct encloses approximately 50 square inches. You don't mention the type of cooktop or range that you will be venting. If it isn't a pro-style gas range, you very likely could go with a 300CFM model which requires a 6" round duct (approx. 28 sq. in.). I have a 600CFM Vent-A-Hood over a 36" Gaggenau gas cooktop. About 95% of the time it runs on the lowest SensaSource setting with only a single blower going. It vents very effectively (and quietly) at that speed. Unless I'm cooking something over very high heat that generates a lot of smoke, I almost never turn on the second blower. Your contractor should be able to find a way to install the appropriate sized ducting. If not, call an HVAC/sheetmetal contractor. Installing a new, properly-sized duct through the attic, through joist spaces, or through a side wall shouldn't pose an insurmountable problem.

You can do it but the tools and time aren't worth it. Call around to locate a metal fab shop,contractors have to use a shop on more than half of their handler swaps. If you can draw it,they can build it.

The Benefits of a Duct System Analysis. https://www.air-conditioning-ductless.com/ A duct system analysis provides you with a professional assessment of the condition of your ductwork. This invaluable service has a number of important benefits, including: Cost savings. Reduce the cost of energy in your home as all your conditioned air reaches your living space, and heating and cooling equipment runs at optimal performance. Increased comfort. Experience even temperatures throughout your home, and increased airflow. Improved health. Those who suffer from allergies or asthma will benefit from the enhanced air quality. Prolonged lifespan of HVAC equipment. Clean, well maintained ducts circulate clean air through your home, preventing undue wear and tear on system components due to dust or other particulates.

Insulating HVAC Ductwork. https://www.heating-air-conditioning.net/ When insulating HVAC ductwork, use a foil, faced fiberglass insulation with an R-6 or higher R-value. Use the type of metallic foil duct tape recommended by the insulation manufacturer to seal and hold the insulation in place. Make sure the foil surface of the insulation is free of dust before applying the tape. Peel the paper backing off the tape as you apply it to prevent the tape from sticking to itself. If your existing duct insulation is in fairly good condition, an additional layer of duct insulation can be applied over it. If the old insulation is in poor condition, remove it and replace it with new insulation.