I have a 36" hood that is 600CFM. It was purchased from and installed by the best appliance dealer in town - very knowledgeable people. At no time was "make-up air" ever mentioned. Is this always necessary? Or does it depend on the age of the house and how the hood is vented?

A 3" overhang on each side is actually considered best practices. It works noticeably better than a hood with no overhang. Just picture what the cooking fumes do. They don't rise straight up, but instead they rise in an expanding cone shape.

The need for a make-up-air system depends on a bunch of factors. Most immediately, it depends on building codes in your local jurisdiction. If your building inspector tells you that you need a MUA system, then that's simply not something you can argue with.

From a practical point of view, there are two or three main reasons for MUA.

1. the air that is being vented needs to come from somewhere. If you have other gas burning appliances (e.g. water heater or furnaces), there is a good chance that you could create (partial) backdraft conditions. This will suck exhaust into your house. It also deprives the other appliance of oxygen, making it more likely to produce some amount of CO (carbon monoxide). This is an odorless gas that is highly toxic even in small quantities. Carbon-monoxide-poisoning is insidious as it affects your judgement and makes it less likely for you to actually go outside where you'd be safe.
2. the harder it is to find outside make-up-air, the more your vent hood has to strain. And the fan is simply not built to do this efficiently. The upshot is that you might have paid for a fancy 1000 CFM hood, but you really only get 400 CFM (and lots of noise), because it has no way to draw more air.
3. if you live in a part of the country where it gets cold, your energy bill will go up, when the vent hood sucks lots of cold air into the house. Also, you will feel cold and miserable.

If your building code doesn't say anything about requiring a MUA system, you don't live in a cold part of the country, your house is old enough to have plenty of natural openings anyway, and your other gas appliances are outside of the normal envelope of the house, then you could skip a MUA system and see how well things work. It is always something you could add after the fact. Or you could get into the habit of always leaving some windows open. That's the poor man's MUA.

Unfortunately, the harmful parts of not having a MUA system won't necessarily be apparent (e.g. CO poisoning), but the other parts (less efficient venting) should be quite obvious immediately. Only you know whether your house has other gas appliances. But let that be your foremost deciding factor.

My in-laws have a wider cabinet over their range with a 30" hood and it looks ridiculous. My understanding is a hood slightly wider than the range will be more efficient at capturing steam, etc.

Think about it. If your hood can truly vent 600 cfm, that much air has to come in from somewhere to replace it. Either you have doors and/or window open, and/or your dwelling leaks that much air. Newer houses in colder climes are more air tight than older houses. I do know that when I used the 600 cfm (mfg claimed equivalent of 900) in my former house which was around a century old at that time, I needed to open a window or door to let in air as leaky as the house may have been.

I just installed a newer model of the same hood in my new home (which is comparatively more air tight) but it is summer and I have doors and windows open. In winter, it will be different. My climate is relatively mild so heated incoming air isn't that important. There is a fresh air intake in the basement that I have put a sprung damper in. I'll see if, and how long, it will take to open when I have the hood running in winter.

If you are keeping the cabinet above, then buy a 36 inch under cabinet hood.

Or can you remove the cabinet and buy a chimney style hood to fit the space? Not sure what your kitchen looks like but maybe post a pic.

The cabinet should be centered on the range. With a 36” hood.

MUA is required in all situations above 400CFM. If the home is old and leaky, the onus is on the homeowner to do a blower door test to prove that. And if it’s leaky enough for a range hood to not backdraft, the house has air infiltration issues that should be solved.

Thank you. So this is a picture of my existing kitchen (yes its ugly) without the new stove. I currently have electric but my new stove will be natural gas which is common in my area. I currently have a downdraft. I am aware of the make up air issue so I will consider this in my purchase. And I’m aware I will need to decrease the width of my upper cabinet for proper clearance. I’m more concerned about aesthetics (so yes Im getting it a mini-refresh as well). Plus I’m on a budget hence the reason for the new stove being 30”.

I would still use a 36" hood.

https://www.ajmadison.com/cgi-bin/ajmadison/AK7136AS.html?utm_source=google&utm_medium=cse&utm_term=AK7136AS&gclid=CjwKCAjwyMfZBRAXEiwA-R3gMw5_FRu0cYVlFCn8UGNCr9YISzeAVpvq11sl_S9s-JLj_O4B7o5H1hoC7pgQAvD_BwE

https://www.houzz.com/products/stainless-steel-under-cabinet-range-hood-36-prvw-vr~80703161?m_refid=PLA_HZ_80703161

Opps, I meant I will decrease the length of the upper cabinet not the width.

Its going to take extensive carpentry, electrical, hvac, and a whole host of other skills to do anything the way it needs to be done here. That isn’t a simple project. At all. You might as well go whole hog as to spend 10K to just replace the range and vent.

As in, find a better layout that doesn’t leave it hanging on at the end of a run. You’re gonna have to tear up enough stuff that you should look at total replacement. Of everything.

Thanks everyone! I’ve ordered the 36” wide one.

As Sophie alluded to, the cabinet above the stove looks to be too low to provide adequate clearance between the hood and the cooking surface. The installation manual of your new hood and stove will have dimensions for the minimum allowable clearance. You don't want to be bumping your noggin on the new hood when you're cooking nor do you want to have to duck to see the back burners.

@zsmith Yup, aware. Im making sure the clearance is the appropriate distance 28-30” from the new stove. Thanks for your thoughfulness. I feel so educated about all the details (calculating Cfms needed by btus, exhaust size affecting noise, filter types, sones, steel grade, lights, weight of units etc)! Its been a process. :)

Now, researching roof caps to avoid squirrels and bugs, leaks, proper venting, etc!

Looks like a brand, new kitchen, CV! Enjoy. :)

Looks fantastic! Can u show more pics? It gives people hope that don’t want or cannot afford a full redo!