This is why I would never recommend Zline for a hood. The sell against science. Plus the quality control issues.

Thanks M.

Live wire, not helpful. I've had a Zline hood on my 36" BlueStar for four years and it's worked like a charm. I used an 8" duct on that larger hood but was unsure of what to do on the 30".

Without knowing all of the pressure losses, and suspecting that the blower is rated at 900 CFM in free air (not in the hood), I would only assume, at best, 2/3 of the rated volumetric flow rate and start with an assumption of 600 CFM.

Next, using the sectional area of different common sized ducting, I would calculate the air velocity in the duct if 600 CFM were actually being moved. The duct diameter should be large enough to not exceed 2000 ft/min and small enough to not fall under 1000 ft/min. The lower number is better if much of the duct will be cold.

Great. Do me a favor and do the math.

For 600 CFM, the 1000 ft/min speed occurs at a diameter of 10.4 inches, while the 2000 ft/min speed occurs at 7.4 inches diameter. So for modest length, minimal bend ducting, 8 inch duct will do. Long ducts might be better in 10-inch size. Six-inch is too small, unless ....

Checking this part at Z-Line and its installation directions it is clear that it uses a squirrel cage blower. These can be more sensitive to pressure loss than radial or axial blowers, so 600 CFM will perhaps be optimistic, even with decent MUA. I would go with 8-inch ducting.

You hood entry area is about 4 sq. ft. so the actual flow needed is 90 x 4 = 360 CFM. This may be what you will end up with due to pressure losses. Unfortunately, if your AHJ imposes code requirements, he will demand MUA for 900 CFM, so that part of your plan needs to be fleshed out, so to speak. Passive MUA with a filter and damper should be sufficient.

Thank you.