Code where I live requires makeup air for vent hoods over 400 cfm. We have makeup air because our vent hood is 1200 cfm.

Most locations follow the same code which requires it over 400cfm.

I can't speak to the code requirements in your area, but in ours there is no code requirement to ventilate a cooktop. Nada. So the easiest way to make the problem go away is simply to ditch the whole hood thing. You can comply with code and save money, too.

Now, back to the real world...

Kitchen vent fans serve the important function of venting combustion gases produced by gas-fired appliances, cooking odors, and moisture. As Cook's Kitchen notes, ventilating a cooktop over an island is more challenging than a cooktop with a wall hood. Get someone who knows what they're doing to design that for you and install a make-up air system if you need one.

A bigger issue is, Not everyone turns them on :) None of my clients bar one, in all the kitchens we have installed understood the code for stove top ventilation. Some even block them off to stop the cold air...

Brolly Renos - then you haven't been installing them correctly. There should not be cold air coming in (!). I also bet the duct going out of the house is not sized properly. If the duct is too small, the hood won't work efficiently and will be very noisy, which are likely the reasons "none of your clients bar one" turn them on.

I turn mine on when I am even scrambling eggs. Works great as well for smoky things like bacon and steak.

I didn't need makeup air for a 300 CFM hood. Mine was not on an island - very short run on an outside wall. I found 300 CFM adequate in that situation - 36" Wolf gas cooktop. I turned mine on only for steamy, smokey cooking as the fan noise drives me crazy. I have since moved to a place with an OTR microwave with no reasonable way to get an external duct. Even though the old ventahood was only 300 CFM and noisy I would love to have it now.

300 CFM is the minimum for a 'normal' range. If the most you do is cook something like bacon - it will probably do. However, if you have a 'pro' range with high BTU burners, and you're going to use a wok, etc - then you need higher CFM.

With an island - you have four open sides - thus - not all the air will be exhausted directly from the range - which reduces the efficiency of the fan to exhaust cooking steam / grease / odors.

The concern is if you remove a lot of CFM from your home - it will create low pressure, and physics dictates it will be 'made up' some how. There is a possibility that you will suck air down the chimney, water heater vent, etc - which is not good.

Perhaps opening a window ( an 1" or 2" )would be enough.

That is a huge assumption to say it's a bad install Shannon_WI (not to mention rude). I personally have never lived anywhere that a cold draft didn't find it's way through one of these vents during the winter. It's just something you learn to live with. Obviously the codes for installation will vary by the area as well. And noise levels in my personal experience has more to do with the cost of the unit. I actually have a down draft in my gas range that works well - and is a good solution for an island. It does tend to suck some of the heat away from the nearby burners when it's on though (and yes we have a bit of a cold draft on the very cold days of winter).

Wow! Some basics are needed here.

First, no air goes up the hood, through the duct, and to the outside that didn't get into the house. Seal the house and the hood flows no air.

Second, the hood will try to move air and if it can't the house pressure will fall to the zero CFM value at the left edge of the hood blower's fan curve, perhaps a few tenths of an inch of water column. This pressure, and likely the pressures corresponding to a good portion of the fan curve (less the losses from hood filter to outside), can cause back-drafting of combustion appliances. Back-drafting is a carbon monoxide hazard. Some combustion appliances can back-draft at a mere 0.03 inches of water column. Fireplaces are just a bit higher than that number, depending on draft achieved.

Third, while low CFM levels may be supplied by the house leakage as make-up air, this is not usually good for the interiors of house walls, and can pull dust into the interior.

Fourth, there are many ways to supply make-up air (MUA) and generally some heating of the air is needed (possibly required by code) in northern climes.

Fifth, the comments about wall vs. island hoods are valid, but the result is somewhat confusing. Without a back wall, the island hood needs to be larger front-to-back (deeper) to accommodate the rising expanding cooking plumes it is to capture from the rear burners. This larger area still requires the needed flow velocity (90 ft/min suggested), so island hoods will in general require more CFM than the same performance hood mounted to a wall. (Feet per minute equals CFM per square foot of hood entry aperture area.) If drafts are accounted for, including those caused by moving people, the island hood may need to be wider as well as deeper.

In some cases, cold MUA can be brought into a room having an oversized heater (think Modine type) and preheated that way. In others, an electric or hydronic heat exchanger may be needed somewhere in the MUA ducting. Low CFM requirements might be addressed by an existing hot air furnace, but usually these are sized for normal house heat loss. In all cases, even in houses with separate MUA for combustion appliances, the MUA configuration should be imagined as having ducting commensurate with that needed for the hood system.

Last, all of the above is related to having a hood that removes most of the cooking effluent, leaving the walls clean and the air relatively odor free. As far as I know, there is no Code requirement to keep one's house clean and thus the options of no hood, or of a barely filtered recirculating hood, or a barely useful low CFM exhausting hood are available choices.

Could someone clarify if I’d need make up air for range hood with 300CFM in Wisconsin?

Are you asking from a legal/code point of view, or will it work?

i prefer you start your own post, and register.

Legal/code. Thank you . Also started a new post