48 Inch Range and 'Make UP Air'

lniaAugust 5, 2012


Are there certain rules regarding make up air? We will be installing a 48 in range with 54 inch hood. We want to do 600 cfm with 8 inch duct to hopefully eliminate the possibility of needing make up air. It will be rare that we are using all 6 burners simultaneously. Not even sure this is possible as all I read is recommendaiton of 10 inch duct and ~1000 cfm.

The size of the kitchen is ~300 square feet and the wall of the range is up against the garage wall. The duct will go directly out to the garage (horizontally) and then bend to go out of the side of the garage (~15 feet or so).

We are using r19 insulation so our home isn't as airtight that foam would indicate. Not sure if this is used in calculation for need of make up air.

Any guidance/thoughts would be appreciated!

THanks so much!!!

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Sophie Wheeler

First of all, you cannot vent through a garage unless you build a drywall chase around the duct. Second, your fan doesn't sound adequate for your range. Third, makeup air will probably be needed no matter what the CFM of your hood is, but whether or not it is required is up to your particular town's rules and regulations. You should plan to have it though if you have any other combustion appliances in your home such as a fireplace or gas fired hot water heater.

    Bookmark   August 5, 2012 at 11:58AM
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It is possible to get a 54" hood with a 600 cfm blower set with an 8" duct. Having said that 600 cfm is not enough draw for a 54" hood, let alone a 48" range.

MUA is no where near as daunting or as complicated as it seems from reading this website, nor is it necessarily crazy expensive.

Speak to your contractor and inspector to find a way to address the issue of MUA, if it is in actual fact a problem in the first place.

You may find out that in your situation you don't need MUA.

    Bookmark   August 5, 2012 at 5:27PM
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In many places makeup air is not required for 600 CFMs although I'm not sure you can find a hood as large as you need that only has 600 CFMS. Some places require makeup air for anything over 400 CFMs. Check your local codes. Also, you mention a 48" range with only 6 burners - is the rest of the cooktop a grill or griddle? If a grill, 600 CFMs make be inadequate.

    Bookmark   August 5, 2012 at 5:28PM
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We will have 6 burners and a griddle. Honestly, we like the look of the 48 inch range and that is why we are going with it. It will be a very rare occasion when we are using all the elements of the 48 inch range. That is why I don't think we will need the CFM hood/blower that is often noted with a 48 inch range and the 10 inch duct

    Bookmark   August 5, 2012 at 8:26PM
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As Weissman advised, check you local codes.

As Trevor said, 600 CFM spread out over a 54" hood does
not provide that great of an exhaust rate. Sorta okay,
but not good-great. Depends on what you cook (would be
fine for me, but I don't sear anything, fry, etc.)

Do you have anything inside the house that can be back-drafted by the hood fan? Examples would be a fireplace, a gas water heater (that gets air from the house, not from direct venting), a gas furnace, a gas clothes dryer. If so, you may need MUA to prevent pulling exhaust gasses back down their vents.

From a practical viewpoint, you can just open a nearby window when turning the hood vent to high. Code will not allow that as a choice.

Here in San Jose, the make-up air rules are apparently there to address back-drafting. For this rule, I can have about 400cfm for my hood (5-something minus my 100cfm bathroom fan) for my 3500 sq ft home. If I isolate my water heater closet from the house envelope I don't have to comply.

"Buildings with gravity appliances such as water heaters, furnaces, gas fireplace inserts, etc, within the conditioned envelope must provide make up air if the combined exhaust rate of the two largest exhaust fans (including clothes dryers and kitchen hoods) exceeds 15 cfm per 100 sq ft. "

    Bookmark   August 5, 2012 at 10:37PM
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There are also less obvious reasons for adequate air flow at the hood than just pulling in air.

One is that for baffles to work correctly in removing grease particles from the larger part of the particle spectrum, the air must be accelerated around the baffles, thereby achieving centrifugal impingement. 600 cfm for such a large area hood seems to me to be marginal for grease collection. Fan sizes vs. baffle hood sizes listed in the back of the Wolf Design Guide may provide guidance.

A second is that the upwelling effluent has momentum, and when it hits parts of the hood it will tend to reflect away. It is desirable that the air velocity at the hood aperture be comparable with the effluent velocity of approximately 3 ft/s. (Comparable means not worse than one half, as then the reflected momentum will tend to become transverse flow that will be captured at the next baffle.)

A 600 cfm at zero static pressure fan pulls 10 cubic feet per second, but in a duct/hood/baffle configuration, even with perfect MUA, will probably only pull around 7, depending on the particular fan curve and all the losses. With no MUA but a closed leaky house, maybe the actual amount is 5 cubic feet per second. Now a 4 ft x 2 ft aperture is 8 square feet, so the velocity averaged over the aperture is 5/8 feet per second. (Actual aperture vs. exterior hood size will vary, and there are various internal shape factors that can influence the effective aperture.)

5/8 fps is on the low side for cooking that generates significant grease and smoke. Other considerations such as cost and aesthetics compete with effectiveness, so compromises must be made.


    Bookmark   August 6, 2012 at 9:07AM
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