MUA - Make up air for range hood (cross post)

13 years ago

Cross post from kitchen forum - see original message here

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..

Scott

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Comments (5)

kaseki
13 years ago
Are you going to filter the air? If not, and the damper is mechanized so that flow doesn't open it, then the pressure losses should be slight and the negative house pressure that is established may be within desirable bounds.

If you do filter, even with a four-inch pleat, unless the filter is gigantic the pressure loss through the filter can be a tenth or two of an inch of water. This will reduce the actual fan cfm and possibly cause backdrafting of furnaces, hot water heaters, and especially fire places.

I would encourage you to continue pursuing the path you are on, with the caveat that you should plan a space in the MUA path to insert both a filter and a fan -- the filter in case you find you have to minimize household ingestion of outside dust, and the fan in case you detect any problems with backdrafting.

Barefoot cooking in the winter may be a challenge.

kas
John Liu
13 years ago
Scott, just to elaborate on the concern I mentioned in the KF thread - that fan is rated 2,000 CFM, I see you've sized the MUA ducting accordingly, so you must be planning to use all of that flow.

A 14 inch duct is about 1 sq foot, means at 2,000 CFM air will be moving through the MUA duct at 2,000 ft/min or 33 ft/sec. That seems very high, maybe too high to be realistic for passive airflow. There will be plenty of friction resistance in that 1 sq foot ductwork, and there are plenty of other ways air can flow into the kitchen through 18 sq foot openings (the doors to the kitchen), so why are you confident you'll pull air from the passive MUA rather than from the rest of the house?

Suppose you do get that much airflow through the MUA duct, say with a fan. That is 2,000 CFM entering the space under the range at initially 33 ft/sec. If the space is 6" high by 48" wide i.e. that is 2 sq feet, so you can figure the air has to exit at 16 ft/sec, even if you use baffles to slow down the direct blast. That might ''blow your socks off'' or at least give you frost-bitten toes. And, will air exiting from the under-range space at that velocity and in a horizontal direction, will it really turn sharply to head up into the vent hood? Perhaps it will simply spread a frigid floor-level layer of air throughout the kitchen, while warm room air gets sucked out the vent hood. 2,000 cubic feet per minute of it.

Finally, the lesson I got from that article I linked you to, in the other thread, is that turbulence and cross-flow make vent hoods less effective. That is, I think, why the article says to introduce MUA from various locations including some far away from the vent hood. So you get a smooth airflow into the hood. That's something to think about too.

I imagine that fan won't actually flow 2,000 CFM in a real world vent hood, so these numbers are a worst-case, you'd recalculate using realistic outflow rate - but anyway you see my concerns.
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I suspect the 0.4 factor is intended to account for the fan/blower performance in realistic conditions, particularly conditions that correspond to the small amount of MUA that the formula requires. My comments here have usually assumed a factor of 0.67, but that approximation assumes that imperfect MUA won't lower the interior pressure more than 0.1 inch w.c., there is another tenth lost in the baffles, and a commensurate amount in the hood ducting. One-tenth negative pressure, however, is too much for combustion appliances with air intakes that aren't isolated from the kitchen, or for fireplaces. Note that the Wisconsin formula listed by the OP is for cases without such combustion appliances. If there is an intent to filter the MUA duct for anything smaller than a bat, I would at least taper out to a largish filter so the filter pressure loss will be minimal. A smaller sized furnace filter would do, or a window screen if large enough (perhaps two square feet). When arguing for a four-inch duct because it can move X CFM, one has to consider what pressure loss will occur at that CFM. I think six-inch would be better in this case unless the duct length will be very short. kas
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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.
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The only air that will flow out of the hood is that air that can enter the kitchen, so MUA is always necessary. The question is whether accidental MUA is sufficient, or is deliberate MUA needed. Generally at the 300 CFM level (which is more likely a modest 200 CFM actual flow rate), house leakage is deemed adequate for MUA unless the house is very well sealed. I you have combustion appliances, I would keep an eye on back-drafting when the hood blower is running and the house is closed up. A CO sensor may be desirable. Your municipality code enforcement officer can provide information on what regulations are being imposed.
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In order to begin to understand it, you would need to buy ANSI Z21.1 for about $650. There is some very technical engineering, even though it doesn't look like it, in the ventilation between walls of the oven and cooktop sides that ensures that the cabinets up against the range will not get hotter than 194°F. If you block the airflow by lowering the range (some people try modifying the feet for thin countertops that make them less than 36" tall), or putting ducts in for MUA, the outside wall of the range will get hotter. If you force that much extra air through the range, (push or pull) you change the way the gas burns, and create CO problems. Even if you can adjust the air shutters to compensate, you would then have to turn on the entire system just to make a pot of tea, in order to get enough air to get a clean burn. With holes in the floor or back wall, the natural draft intended/engineered to cool the outer walls of the range can be disrupted or diverted, leaving the heat to build up around the range - nothing to do with heat in other areas of the home. What can't tolerate more heat than the wood cabinets around the range? Another separate reason for no holes is the possibility of a gas leak. How much gas do you think that duct will hold, just waiting for someone to create a spark? The testing standards do not say no holes "unless someone thinks of some unusual excuse".
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John Liu
13 years ago
You might do look at commercial vent hoods and MUA systems for ideas. Can you position the range away from the wall, to use a backwall supply? Plus a supply vent under the range, maybe plus a supply vent over the hood? I am thinking it will help to lower the velocity of the MUA. If you will have three rectangular ducts, maybe dedicate one to each vent. I linked a better copy of the article that has the diagrams.

Here is a link that might be useful: Article
John Liu
13 years ago
I found the site for the other design guides. See link. Poke around that site, there are quite a few other white papers, reports, and presentations. Such as
http://www.fishnick.com/publications/appliancereports/hoods/
http://www.fishnick.com/education/presentations/
http://www.fishnick.com/publications/ventilation/

See also http://www.energy.ca.gov/reports/2003-04-10_500-03-007F.PDF which appears to be the test report that much of the information in the previously linked article came from.

I wonder if you might be able to design such that you could use a lower-CFM fan, in additional to avoiding MUA heating/cooling. I appreciate you bringing this topic up as it has motivated me to think about my situation.

Finally, if you want to do some calculations on your duct design, try
Air Duct Calculator - Hart & Cooley Brand Website
www.hartandcooley.com/.../Product.../Air_Duct_Calculator.sflb.ashx

Here is a link that might be useful: More Articles
John Liu
13 years ago
One last resource - as before, very much a commercial kitchen context.

Here is a link that might be useful: Another Design Guide

MUA - Make up air for range hood (cross post)

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kaseki

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