I agree. No overkill. HVAC guys always seem to be right-sizing things when kitchen-obsessed people want it big.

I agree also that this is just about the first post ever to deal with duct size alone.

Fantech and Broan have fans that run on 6" ducts. b.t.w.

If this is to be a simple run, almost straight, you could be very happy with 6". Upsizing to 7" makes a big difference. Upsizing to 8" makes another big difference.

Going way bigger can be a problem because airflow has to be fast enough to carry greasy smoke out of the duct or else it is just one big decantation chamber, not a duct. Of course I'm exaggerating but I'd like to make it clear that 8" is big.

Annemarie29 you haven't said what size CFM you will be blowing out the kitchen regularly.

Is make-up air a known concept to you, or a new concept?
A lot depends on your climate and your house construction.
Whether you are happy blowing a lot of air outdoors, or not.

I'm sure I've only touched the surface of all the things to take into consideration.

Without seeing your place and without knowing what you like to cook and whether you want to be at the high end of the range of possibilities or not, I couldn't say whether you would be happiest with an 8" duct or not.

Just eyeballing it from here, I'd guess an 8" duct would work best given what you want to do.

But can you talk the HVAC guys into installing one that size?

No disrespect, but I disagree with Davidro1. And there have been many threads on duct size on this forum, but they may have fallen off by now. IMHO, it is crucial to have a duct that is 8" or 10", and not 6". I speak from experience. I have a 6" duct, and it is the ONE disappointment in my kitchen reno. The ONE thing I wish I could change. Unlike you though, I did not have the luxury to install ductwork for my exhaust; I was locked into what was already there, which is 6" size. If you are lucky enough to be able to choose, get the 8" (many on this forum have 10", but 8" will be fine). Don't let your contractor push you around. Really, this is important. Ask yourself--what difference does it make to him if I get an 8" duct? Well, it'll probably be a bit more work for him to squeeze in an 8" duct rather than an easy-to-slide-in 6", so that may be why he's telling you 6" will be fine. It's YOUR kitchen that you'll have to cook in for years, long after he's gone and forgotten. Tell him you want the 8" duct installed, no debate.

You have already hit on one reason not to get a 6"--it will severely limit your choices in hoods. Meanwhile, if you have an 8" duct, your choices will be wide and varied at all different price points, designs, brand names.

A second reason for a larger duct is efficiency. A 6" duct simply won't be large enough to allow for cfms above 300-450. 300-450 may be adequate, but on those days when you are frying hamburger or fish, you'll wish for more. Believe me, I know. As to overkill--I would rather have high cfms that I may only use once in a while, with the flexibility of lower settings for the rest of the time, than to be permanently hampered by low cfms due to a 6" duct size.

Per the Appliances Forum FAQ on Vent Hoods: "The rule-of-thumb is 10CFM per 1000BTU of burner capacity. Thus, for a 60K-BTU cooktop (4 x 15K), you should have a 600CFM hood. Since you rarely, if ever, have all burners on full at once, you have excess capacity that can be used when you really need it. Most hoods have variable-speed controls, which allow you to choose the airflow---and noise level---appropriate to the task."

Another reason for a larger duct is noise level. When you are pushing air through a narrower space, it will be more noisy than pushing the same air through a wider space. Of course, the length of your duct run and the number of bends will also affect noise level, but 6" will guarantee a higher noise level than 8".

Although duct size has been discussed a lot on this forum, I have never once seen a thread saying "AW SHOOT, I WISH I'd installed a smaller duct!" On the contrary, we see the opposite sentiment, "If only I had a larger duct..."

The easy solution is to tell the HVAC guy that you need a 10" duct and have them run it. If you choose a vent hood that works better with a smaller duct, it's a simple matter to run that duct inside the larger duct and up through the roof. It's a LOT more work to try to go larger if you have smaller, but relatively easy to use smaller if you have larger.

All three replies have been very helpful. It is good to know I could run a smaller duct inside if I needed it. What I find confusing is that futuro has 48" 940 cfm hoods with only 6 inch duct outlets. I would have thought there was a direct correlation with cfm and duct size? I'm leaning toward 8", as I may go with a BlueStar 48" cooktop. Regarding makeup air, as part of my HVAC they are installing a air cycler, which will supply makeup air, but I am curious if I'm not running my HVAC and running my vent hood, if it comes on, guess I need to ask the HVAC guys, this seems to be a new science.