MUA - Make up air for range hood

scottvdJanuary 26, 2011

Boring details - 48" range, 23k btu burners + grill, custom 48" hood, Fantech FKD 12XL.

I'm considering building a passive MUA system-

(3x) 3.25" x 14" rectangular riser ducting tied to 14" round duct leading to outside fresh air. 14" duct will have passive louvers, screen, and electric damper.

The rectangular risers will be installed in the wall behind the range, venting into the void directly underneath the range. The damper will be opened when the hood fan is turned on.

I understand the fresh air won't be heated/cooled by a heat exchanger, but it seems pointless to me to treat the MUA since it will immediately be exhausted through your hood. Unless your MUA input source is substantially far away from your hood (undesirable) - what does it matter?

The passive system is very economical, quiet, and the air flow dynamically adjusts according to the demand - so there's no complication with syncing the variable speed exhaust and MUA fan speeds as with an "active" MUA system. Thoughts? TIA..


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My thought is you should post this on the appliances forum and the HVAC forum They have lots of knowledge (not that this forum doesn't) and you will get lots of responses. For what it's worth, my house isn't buttoned up tight, so we just crack the kitchen window a couple of inches when we have the vent on, the cool breeze is usually welcome because cooking over that range is HOT!

    Bookmark   January 26, 2011 at 2:24PM
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I just found out today that I need MUA with a vent over 400cfm. 400cfm??? That's so low it's crazy to me. I'm planning a monster Capital Culinarian with 6 burners, 23,000btu per burner so I darn well better have more than 400cfms. Now I need to figure out what, exactly, a MUA system is. Your post as helped me. Sorry I can't help you.

I'd do what Bee suggests. Post this on the other two forums as well to see what they say. I'll be watching to see what the answers are. This whole vent issue has been giving me a headache for months.

Good luck!

    Bookmark   January 26, 2011 at 2:29PM
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The specifics of a MUA system are often dictated by local building codes. You need to check and find out what is required in your community.

I don't see why your passive wouldn't work, but it may be a significant source of heat loss when you're not using your range, depending on your climate.

    Bookmark   January 26, 2011 at 3:18PM
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I have a passive MUA system of sorts - a dog door near the kitchen. When I wok and am using all 1200 cfms available from my hood, I will also crack open the kitchen window. The air flow goes directly from the window to the hood so no concerns about altering overall indoor house temperature. I should add I live in So Cal, so weather is usually not an issue.

Your plan sounds good to me, unless dust bunnies located below your range find themselves in your food.

    Bookmark   January 26, 2011 at 3:22PM
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Jon--that sort of MUA system was acceptable for code in your area?

    Bookmark   January 26, 2011 at 3:26PM
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Will you have a remote switch to close the vent in case of a stovetop fire? You don't want fresh air/oxygen feeding any fire.

    Bookmark   January 26, 2011 at 8:08PM
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rob from nj

There must be a reason the instructions for the CC say "any openings in the wall behind the range and in the floor under the range must be sealed".

    Bookmark   January 26, 2011 at 11:11PM
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I don't understand why you think it is undesirable for the MUA source to be far away from your range. You don't want the MUA to be sucked up your vent, you want it to RELACE the air that is being sucked up your vent. I can't imagine any modern home that has interior seals that would prevent the free flow of air within.

I'm one of those who has a code compliant MUA system. The MUA enters the utility room in my basement, and straight into the HVAC ductwork. It certainly prevents any drafts. And it cost a bundle....

    Bookmark   January 26, 2011 at 11:36PM
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Thanks everyone for the replies and feedback!

@beekeeperswife- Where I come from, cross-posting is a four letter word - but I'll give it a shot per your recommendation! Thanks.

@warmfridge- Thanks for the thoughts. From what I've read building codes and inspectors for new construction are often ignorant on MUA systems - perhaps this was more common in the past? Not sure. But in regards to heat loss, the ducting is sealed off when not in use by a normally closed electric damper. The damper is tied to the hood fan, when the fan is on the damper is open, otherwise closed. So in the event of a fire turning off the hood (beneficial of course) will also close the passive vent, good observation though.

@itsnotrocketscience- I'm in central cal, so weather isn't too big of a concern here either - but I was trying to avoid the window cracking thing. But same principle applies as you noted, for all practical purposes this is a "cracked window" below the range that automatically opens with the hood. (:

@robj- I assume by CC you mean Capital Culinarian? I searched their specifications PDF found here:
and other data on their website and couldn't find a reference to the instructions you cite - could you please provide a link / page number? Thanks.

@cooksnsews- I'm not specifically concerned with if the MUA is "sucked" into the hood again or if I'm "replacing" house air with MUA air - in short I want my hood exhaust to run without creating negative pressure inside the home relative to the outside air pressure. That being said, if air is leaving via the hood exhaust, new air must be introduced at the same rate to prevent negative pressure.
The further that new replacement air (MUA) is from the exhaust point, the more that air will impact the house climate. For this reason I cite the MUA input source should be (within reason) in close proximity to the exhaust point (if that MUA air is not temperature treated.)
Let's assume we brought the MUA directly under the hood - what would happen? The exhaust fan would run sucking air from the MUA and not creating any negative pressure in the house (desirable). But of course this scenario isn't beneficial for removing range heat / odor / steam / etc. So if the MUA source is underneath the range, we can capture those additional elements and still not be too concerned with treating the air.
Why go though all the hassle, expense, and energy of heating/cooling your MUA with an exchanger if it's going to be exhausted immediately, it doesn't make sense? Why suck out already climate controlled air in the home through your hood exhaust and replace it with MUA that needs to be re-heated/cooled again by your HVAC? Neither makes sense to me? But I could totally be missing something?

    Bookmark   January 27, 2011 at 12:55AM
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Intuitively it seem to make sense for makeup air to come in under the range, and be drawn up past the front of the range and up into the hood, hopefully containing the smoke and effluent. I've thought of doing the same thing, in my case the makeup air duct could run under the floor to a register under the range.

However, airflow isn't always intuitive, and it might not work that way. Perhaps the cold air coming from under the range will stay low, because it is cold, while the hood will draw warm room air that is closer to the hood's height. Perhaps the volume of air coming from under the range will create turbulence that makes the hood less effective. Perhaps the amount of room under the range is too small to smoothly flow as much air as the hood will be pulling out.

I don't know the answer, but you might want to read this article. I recall it recommends make air be introduced into the room at a distance from the hood, though I am not sure if it explains why.

Davidro may see this thread, I believe he has looked into similar issues.

Please, if you would, report back on what you learn, whether by asking or reading, getting a professional design, or simply doing it and seeing what happens. I would be very interested.

    Bookmark   January 27, 2011 at 1:27AM
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rob from nj

I see Capital doesn't have the installation guides for the Culinarian up yet. The instructions for the Precision Range (page 34, step 3, item #4) state that the area immediately around the range must be sealed.

I'm guessing it has to do with drafts affecting the flame but could be a fire safety issue as well.

    Bookmark   January 27, 2011 at 8:54AM
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Perhaps I'm misunderstanding cooksnews point but I agree with what I believe they are saying. If the MAU orgin is very close to the Exhaust ven then rather than sucking out the smoke/heat it will only pull new clean air into the house and then exhaust that new clean air. My thoughts are that the MAU entry must be far enough away from the exit point that the rest of the cubic air in the kitchen would get removed first.

Another aspect of that is that the air flow patterns from having both an entry and an exit so close to each other may create some funkiness. That's a technical term, of course. ;)

    Bookmark   January 27, 2011 at 9:52AM
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scottvd - you wouldn't be looking at the Broan system, would you? I have been investigating MUA and am intrigued by the Broan Automatic Make-Up Air Damper...

    Bookmark   January 27, 2011 at 12:45PM
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breezygirl - I did not pull permits, I'm sure a dog door would not qualify as an acceptable MUA system. (Yes, I was fully aware of all ramifications of my decision and I sleep well at night.)

Scott - warmfridge brings up a good point. Air flow moving up and around your range may not be a great idea in all cases. And I was somewhat serious about the dust bunny bit - are you sure you are OK with the probably air flow path and the things that may be riding the wind as you cook?

    Bookmark   January 27, 2011 at 3:49PM
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I think that the OP is on the right track. The make-up air should be as close to the appliance as reasonably possible. That is the way MUA should work for other combustion appliances like water heaters, furnaces, boilers and fireplaces. You don't want unnecessary air changes in the rest of the house or even in the rest of the kitchen just to remove smoke, combustion gas and aerosols from the cook surface. That is just wrong-minded. Opening a window or door is not acceptable because it is too dependent on which way the wind is blowing at that moment.

In laboratory design, a number of years ago, the design of fume hoods took a major turn. They used to just suck ambient interior air, a lot of it, up the stack. 20-30 years ago, for a spell, they just started incorporating raw MUA into the hood design. When working in those wonders you are definitely aware of the outside weather because outside air is moving directly over the person working in front of the sash and everything in the hood was exposed to outdoor air conditions. It was a big energy savings, but not so good for the laboratory scientists and their work in some cases. Now there are other solutions. If you are having trouble envisioning what a fume hood looks like, just google it. You will get lots of hits. I sometimes wish I had one at home for cutting onions ;-)

Rather than putting the MUA far away from the range it should be as close to the range as possible. The heck with MUA under the range. From my point of view, a lab guy, the most efficient way to build a MUA system would be for the range manufacturers to incorporate MUA into the range design like those now-outdated fume hoods. MUA should probably be supplied all the way around the perimeter of the horizontal surface. As a cook, I can say that I would not want it to be a simple opening around the range surface because that would be a maintenance nightmare. I expect that it might be best located somewhat lower than the horizontal surface in a way that it is easy to clean and does not get spilled into. A system like that would make for a comfortable cook and energy efficiency.

The drawbacks of the now-outdated fume hood designs really would not apply to a range/hood system. In those, what you were working on could be very toxic and the process temperature-sensitive. In the case of a kitchen, the design can be a little more loose as far as allowing trace "fumes" from the stove to escape. What you are working on is not temperature-sensitive in the same way as a laboratory procedure.

    Bookmark   January 28, 2011 at 12:19PM
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I installed a MUA system very similar to the OP. Duct under the rangetop cabinet w/ a motorized damper opened when the fan is turned on. Since I made my own cabinet boxes, I dadoed channels on the sides and back of the rangetop cabinets to allow flow all the way around the rangetop as ionized points out. Seems to work well.

    Bookmark   January 28, 2011 at 2:39PM
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I would have thought that the problem with MUA in the hood itself is that it would negatively impact the hood's ability capture ... because all the hood would do is suck up the MUA and blow it outside, without necessarily sucking up the smoke. There was a great link on this subject in an earlier thread which had pictures of airflow for various hood configurations...

    Bookmark   January 28, 2011 at 2:59PM
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fritzp, can you say a little more about your MUA system? It sounds really interesting but I can't really visualize it. Oh, what CFM is the vent hood?

    Bookmark   January 29, 2011 at 10:47AM
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One important factor for MUA is to NOT disturb the rising effluent that you want to capture with the hood. Cross drafts, even from people moving about, cause some effluent to be lost from immediate capture.

In commercial hood tests I've read about, introduction in the hood wasn't too successful. Introduction via a down pipe behind the stove to the area below the stove was better. Best results were obtained by using a large diffuser panel in a wall in the test laboratory. If the air flow is kept laminar it tends to disturb the effluent the least.

I agonized for a very long time over where to introduce my make-up air, and under the cooktop was not a good option for my layout. I ended up with a 3 ft x 3 ft diffuser at the other end of a hall that abuts the kitchen. In this location it can be minimally inconveniently (on the vast scale of reno inconvenience) ducted to a roof intake via fan, filters, heat exchanger, etc.


    Bookmark   January 29, 2011 at 3:20PM
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