My thought is that she’s clearly a designer, not an engineer.

It’s time to change to a 6” duct.

Clearly not.

Nu-uh.

An 8" duct would be more appropriate and is called for on some of the choices. As well, a makeup air supply would be advised for safety. Good design having the hood 6" wider than the cooktop.

Even a 6" duct will not be adequate for a 42" hood with 1200 cfms. It's going to sound like there is a jet airplane in your kitchen every time you turn on the hood, and don't go blaming the hood. If you are doing a kitchen renovation, now is the time to open the wall and put in an 8" duct to the outside. It may not be as hard as you think. But your KD does not appear up to this level of expertise, so I would not go to her for help on this - unless that is, that she suggested a larger duct and you said no and so the solution in the OP was all she could do given the constraints.

Other than the XO hood with the mesh filters that I advised against in your other post, all the hoods you list specify either an 8" or 10" duct. You ignore the specs, and you will have a straining noisy hood, and likely void the warranty to boot.

I think reducing the size will also increase the noise of your hood.

People on this forum can get a little excited about ventilation.

This vent hood is going over a 36” induction range. The Home Ventilating Institute (this is a real thing) recommendation for cooktops against a wall is 40 to 100 cfm of airflow per linear foot of range width. According to HVI you need a vent hood that moves 300 cfm.

Fifty people are about to chime in and tell me you need more, but unless they have references from a non-commercial source it is possible that both they and their source have an intrinsic bias.

So how do you get a vent that moves 300 cfm in a 42” space? You move to a different type of system. Instead of buying a huge metal vent hood, replace your duct to a 6” duct and buy a vent liner. Build the surround to accommodate a 42” capture area.

We are staying with a SS range hood due to cost, time and not wanting to use anything other than the paints/stains we already haves used.

In any case, you all agree That my kitchen designer 1) doesn’t know what he’s doing or 2) is trying to avoid correcting a mistake now that the tile has been put in. Or both.

Looks like another battle will be held in this war to get this kitchen done that should have been completed way before Covid 19 struck. We tore our kitchen out between Christmas and New Year’s.

I forgot to mention that there is a bunch of plumbing behind the wall where the range hood goes that he seemingly “forgot“ to move prior to closing up the walls. A 4 inch pipe will fit Between the pipes. A 6 inch or 8 inch duct will not.

Then it’s probably time to open the wall and fix the forgotten piping issue so this can all be done properly.

“Fifty people are about to chime in and tell me you need more, but unless they have references from a non-commercial source it is possible that both they and their source have an intrinsic bias.”

I only have the non-commercial reference and bias of actually having had a 300cfm hood with a 6” duct. So I lived with that kitchen with that hood for about 6 years. I can assure you it was inadequate for anything smokier than frying eggs. It was also really loud. I seared steak in a cast iron pan all the time anyway. I just lived with it, shrug; people make compromises all the time in their homes. Hey, I wanted a bigger master bath too, yet lived with the small one just fine. However, given the opportunity, when walls were open, I chose an 8” duct and more powerful hood than the measly 300 cfms I’d been living with. What a difference! Worth every penny and more.

Having said that, I think 1200 cfms the OP mentioned is overkill and definitely requires makeup air. 600-800 should be good. But that may also require makeup air depending on your home. Mine is older and leaks air like nobody’s business.

Your Zephyr Venezia calls for an 8" duct, the Zephyr Siena Pro calls for 10". And remember that the make-up air diameter must match the hood vent one...put your designer to work and figure out where you can fit that.

I admit to having an intrinsic bias against smoke and odor in the air and grease on the walls.

Unfortunately, HVI doesn't specify on their website the basis for their kitchen ventilation value of 100 CFM/ft. of stove. Although Greenheck is listed as a member, they don't follow Greenheck's recommendations for commercial hood flow rates. Or the IMC's minimum of 200 CFM per foot of hood. Why would residential hoods need less air flow? Do the commercial kitchens like paying more for air movement when it is their highest continuing cost? If I had to guess, I would guess that HVI is still stuck on a 1955 electric cooking surface as a baseline, with bacon simmering the source of their highest velocity grease plume. Surely Capitol, Bluestar, Wolf, and other pseudo-Pro cooktop manufacturers call for higher flow rates than 100 CFM/ft of stove width. HVI doesn't even seem to recognize the need for addressing the fan curve factor in blower selection.

if installing new duct, go with a 10" duct. There're more range hoods to choose from with a 10" duct than an 8" duct.