Hello Kristen, no answers huh? Without dorking out completely, the CFM's don't really matter unless you're directly vented to the outside. I'm not a mechanical engineer, I just play one on tv....

But that's not helping you because you still need one. Just saying, don't get caught up in CFMs. It's hard to give you a suggestion because I have no idea what the rest of your kitchen looks like. If you want to share an Idea Book, I'm dying to incorporate those here on Houzz. Shoot me a message if you'd like to give it a shot.

Have fun,

-f

Assuming you are venting to the outside since all XtremeAir USA hoods need to be ducted to the outside: If this is a remodel you may be able to get away with anything, else you do need to get caught up in CFM since anything over 400CFM requires a makeup air unit which can get expensive especially if you live in a colder climate where you will need a heated makeup air unit. Guess how I know. Exceeding 400CFM will result in negative pressure in your home and cause problems with your fireplaces (if any) and other vents which may act as air inlets to compensate for a higher CFM being vented out of your home.

Regardless of your heating cooling external draw, you will need a makeup air unit if your hood exceeds 400CFM. With a 30" range, assuming it does not have a grill or a griddle, I recommend you stick to a hood with 400CFM to avoid additional expense. County inspectors are extremely fussy across the country and trying to follow code; there are exceptions of course.

I bought an Imperial hood between 500-570 cffm; furnace and water heater bring in their own air.. I crack open a window when I run it in high, which is not often, just to be safe.

Kristen, A 6" duct can at most support 600CFM, check Thor hoods; a 900CFM hood needs a 7" duct. Sure you could install a 600CFM hood and likely get away with it especially if insulation inspection has already occurred and no one may object however it's something you should ask your builder. Conclusion: don't exceed 600CFM and hope it works out fine.

In our case we started with an 8" duct for 1200CFM unit however by the time we got around to it we liked an updated model with 1800CFM and had to call my HVAC guys to replace 8" with 10" duct. We have a 48" gas range with griddle and all. Our 1800CFM four speed controls the quantity of air replaced by our heated makeup air unit. I had no choice since the inspector saw the earlier 8" duct and demanded we install a makeup air unit; a very expensive proposition when installed but does an amazing job, zero cooking smells no matter what; I put it all down to my HVAC system cost which is scary and rediculous anyway.

By the way I'm an engineer and completely designed (not using the word architected) our home down to the stud, nut and bolt level.

build.com has lots of range hoods on its site, many with consumers' reviews. Home Depot too.

Consumer Reports has a free Buying Guide but does not test them.

How long is this 6-inch duct?

D E Electro Industries EM-MG20-240-1-14

Below you can see I had it installed high on the outside rear wall

And the 54” (corrected) with 3" gap on each side between wall cabinets, hood responsible for this debacle, visible on the east wall of the house which is some 100’ away from the mechanical room.

+1 Pinebaron. Some areas allow you to get away with just motorized damper

https://www.homedepot.com/p/3-1-4-in-x-10-in-Make-Up-Air-Kit-W10446914/204358178

Others [most cold climates] require make up air to be heated. [I think really stupid imho, because wholehouse HVAC could do a better job cooling or heating. But because in general tstat may not even notice the temperature change ... make up air heaters are required.]

@BT: Areas that may allow you to get away with a motorized damper are likely ones where temperatures rarely fall below 50's.

A lot depends on what you cook, how often, and how important the air in your home is to you.

More: https://www.gardenweb.com/discussions/5161173/hood-faq

If you don't cook often and never sauté, pan fry, stir fry or use other methods that produce a lot of effluent then you may be OK with 400 CFM (and as mentioned above, there's a lot more to what you do than just CFM's). Otherwise you may find 400 CFM's to be very inadequate.

Keep in mind that if your home is very tight that a 400 CFM hood will likely only perform at about 100-200 CFM without powered makeup air.

Your builder is either extremely ignorant or very dishonest to say that you can use any hood with a 6" duct. As noted above you're realistically limited to about 600 CFM.

@ Pinebaron: Areas that may allow you to get away with a motorized damper are likely ones where temperatures rarely fall below 50's.

To the best of my knowledge not required in Indiana, however code require you to account makeup air into hvac design.[good luck really]

900 CFM through a 6" duct, particularly with any length or bends, would require a very powerful (and expensive) blower and would produce a lot of noise. Realistically 900 CFM would be an 8-10" duct.

@BT: Sorry you are mistaken; All codes are generally universal. Per Indiana Artical 18. Mechanical code:

(f) Insert Section 505.2 to read as follows: 505.2Makeup air required. Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.

(f) Insert Section 505.2 to read as follows: 505.2Makeup air required. Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.

Not what I meant, HEATED MAKEUP AIR is not required. Damper is ok.

Then between November to March, when temperatures often drop below freezing, raw freezing cold air would be working against the HVAC system. Fine if the intention is to just 'get by' but not fine if someone is building a new home and considers impact of raw cold air entering the home while cooking/using range exhaust on high. Some people may just open a window in the kitchen and that's fine too if it works for them.

If you don't want to add a makeup air system Zephyr makes a line that you can set. If you find you need more power, you can turn it up.

http://www.marketwired.com/press-release/zephyr-launches-new-airflow-control-technology-act-keeps-focus-on-power-energy-efficiency-1764372.htm

Normal residential HVAC heating is not designed to adequately heat make-up air for high hood flow rates unless it was designed for a very drafty house. For better sealed housing, the HVAC is designed to compensate for radiation and conduction losses through the walls to the outside.

One might think about MUA heating as falling into two categories: One uses an in-line heater in a ducted MUA system, the other uses a space heater in the room into which the MUA is introduced (not the kitchen, but connected to the kitchen). By space heater I mean a gas, electric, or hydronic heater such as Modine makes. For in-line heating, check out Fantech's MUA systems as examples.

Heat requirements by flow rate and differential temperature (to the outside) may be seen in the graph below, previously published here. (Click to expand.)

ksc36: I think you are missing the point; any hood capable of exhausting in excess of 400CFM generates a need for makeup air to compensate. Almost all hoods are multi-speed and Zephyr's ACT is only a sales pitch and even they sell makeup air dampers. I doubt a county inspector will accept a statement from an owner promising to run a hood only at 400CFM setting even though it can go up to say 1200 or more CFM; it won't fly. Then again new homes are pretty tight, would one want to live in a negative air pressure environment even if you could get away with it? A home with negative air pressure can result in dire consequences. Air will get in by any means necessary, and with this unfiltered air comes some serious dangers.

Good design no doubt costs $$$$

The way I designed our home is the heated makeup air enters the home at the opposite end of the home, which not only helps compensate the hood exhaust cfm difference but also complements the hvac system hence heat is not wasted as the warmer air travels through the corridor, family/living areas then the kitchen; due to the size of the home there is no perceivable draft. Right next to the massive makeup air inlet grill in the corridor ceiling, is a large hvac return air grill. The makeup air unit is programmed to 'just about' exceed hood exhaust cfm and the excess air is fed back into hvac return hence heat is not lost while maintaining zero negative pressure. Considerable design engineering went into it; I was primarily responsible for its final design with the help of a commericial hvac company that did the entire hvac systems including makeup air unit and tankless heaters.

I know more than one inspector who has passed a Zephyr ACT unit, including my own. I set mine on 600, but rarely turn it up more than halfway. It's just an option, especially for someone who lives in an older house.

Good luck with that!

There's no luck needed, just common sense.

Here's a useful engineering question for you. If I had a chimney fire, how high would I have to turn it up to snuff out the blaze?

http://docs.zephyronline.com/docs/brand_essentials/products/specsheets/anzioc_spec.pdf

"an exclusive feature built into specific Zephyr hoods which can limit the maximum blower CFM to meet local code requirements. AIRFLOW CONTROL TECHNOLOGY™ (ACT™) TM "