The main goal for MUA placement (given that there is enough flow rate and minimal house pressure drop) is to minimize disturbing the uprising effluent flow. From this point of view, and given that some commercial systems do this, air release under the oven or thereabouts should be acceptable. It is necessary for the MUA to flow upward without too much turbulence, so it is necessary to release the MUA without too much turbulence. This may require some experimentation. The acrobat file CKV_Design_Guide_2_031504.pdf that should still be accessible on the Internet may provide some insights, even if not directly applicable.

Whether the cook will tolerate 30F toe cooling is another issue.

Last, while I don't have a large range to dump air under, someone on this forum once noted that range Manufacturer X's installation instructions prohibited the practice. I don't recall who X was. It may have been a gas flame disturbance issue or oven cooling issue. Perhaps someone remembers.

kas

The first link's content looks the same as my link, even though the documents' titles are different. At present I don't have time to closely compare them.

Greenheck's guide is always useful.

If your hood is actually 4 ft by 4 ft, then the aperture is roughly 16 square feet, requiring (in my view) 16 x 90* = 1440 cfm actual maximum flow rate capability. Perhaps you made a typo and intended a more typical four ft by 2 ft, requiring 8 x 90 = 720 cfm. Achieving either of these flow rates will require a blower with a zero static pressure flow rate significantly higher due to the pressure losses through baffles, ducting, and (unless the MUA is boosted) the MUA path. This could be as high as a factor of 1.5X. In all cases, rounding to a near blower capability is sufficient given that these rules are approximations.

*Greenheck recommends 85 cfm/sq. ft., as I recall, for one of the rules of thumb that they address.

Fashionable MUA-resistant cook's footwear:

The calculation I did was (48" * 48")/144 * 50 = 800 cfm. Greenheck uses 50 cfm for gas ranges and ovens. But I understand I wouldn't want to undersize the system.

And I was thinking more of

Consideration should be given to the next three columns if any grilling is to be done (even with a cast iron grill pan rather than a wire grill over flames) or wokking (modest though it might be relative to what is assumed in the r.h. column). [Readers, this is in reference to Figure 4, page 9 of the Greenheck guide.]

You will have an advantage by using such a deep hood because much of the un-immediately-captured effluent will have reduced reflection momentum and might be expected to drift into the larger collection area where additional 50 ft/min (~100 ft/min at the baffle gaps) is active.

I guess with respect to this collection area assumption I need to ask: Does your 4 ft by 4 ft hood have low baffles covering that entire area,
or is it more commercial in style, with a single baffle row high up in a
tapering hood? "Pro style" residential hoods such as Wolf, Independent, and
others sell have have low baffles and other internal differences relative to true commercial hoods that might affect what average velocities are needed from the air flow, but documentation of that impact on average velocity sizing is slim to none, and I am forced to extrapolate from my Wolf hood's behavior.

The underlying fact to be kept in mind is that a gas flame heated pan, according to the Finnish researchers I have referenced here in the past, produces up to a 1.2 m/s uprising plume peak velocity, which is over 200 ft/min. I suspect it is the failure to immediately contain the peak velocity components that leads to the schlieren photographs of plume spillage from commercial hoods, such as Greenheck [Figure 3A, page 8] and other sources have documented.

Ideally, one wants the plume to be contained (removed) when it hits the baffles. If any of it reflects then the next line of defense is the capture volume below the baffles providing a means for a second chance. When that doesn't suffice then one gets spillage (grease and odor in the kitchen).

Do I recall correctly, John, that you are a professional cook? If so, then you will be the best judge of what differences in cooking plume characteristics may occur between your commercial experience and your expectations for your new residential range.

kas

Thanks for all your thoughts and knowledge. I'm not a professional cook, just an ambitious home cook.

The hood I am thinking about is like the one shown below. I need about 12"-18" of soffit above the hood, for the duct. There will be a single elbow then a 6' horizontal run to the exterior wall. Thus I may not have enough headroom to use a hood that comes down lower in front. But if I do, then I will use one of those.

These hoods only have baffles at the rear of the hood, at a 45 deg angle. I assume this is to make grease collection easier.

Commercial hoods require steep baffle angles to ensure good grease drainage per NFPA 96. Is that a grease drain at lower right?

Defining the actual aperture area that should be used to calculate total CFM may be unclear with this configuration, calling into question my CFM recommendations in your related threads. What is the actual size of the baffle area, measured in the plane of the baffles?

Can those baffle sections fit into a residential dishwasher, at least one at a time?

The baffles will be about 20" x 48" in area. They will fit in a standard dishwater, laid flat. That is a grease drain at lower right.

My first thought is that if this hood is centered over a 30 to 36 inch range, then the total air flow rate, calculated by multiplying the baffle area length by the necessary front to back overhang (24 to 27 inches, say) for capture, and then by the desired specific flow rate (50, 90, or whatever cfm/sq.ft.), might be sufficient. However, if the entire six feet of length is to be able to capture and contain the rising and expanding effluent from both a range and another cooking device, then an excess flow rate is needed to make the foot at each end effective in at least trapping rising effluent for eventual baffle capture. (I'm unsure just how much of that zone would be useful if a higher flow rate at the baffles is used. Also, this higher flow rate will potentially be noisier given that the velocity at the baffles will be higher.

My second thought is that this hood's manufacturer may have a suggested flow rate based on observed performance in the commercial settings for which it is designed.

To put it another way, this hood neither fills its aperture with baffles, nor is tall and internally tapered such that rising effluent passing its aperture is reflected upward until it all reaches a smaller baffle set. Instead, the aperture is larger than the baffles but seemingly not designed to assure that a plume impinging at any point within the hood canopy is reflected toward the baffles or directly impinges on them. As a result, my computational fluid dynamics intuition is not in my comfort zone. :)

kas

My husband wants to do what you are suggesting -- the MUA vent underneath the range. It will let cold air in, but ideally, much/most of it will be sucked up into the vent instead of dispersing into the kitchen...

Consider even a large for most of us 6000 square foot house with ten-foot ceilings. The volume is 60,000 cubic feet. A blower pulling an actual 1000 cfm, replaced by cold MUA, would cause a thermal cooling time constant of 60 minutes if the MUA were introduced into all rooms uniformly. (The time constant is where the temperature drops 63% from the higher toward lower temperature.) However, if the MUA is only introduced uniformly into a 2000 cubic foot kitchen, then this time constant is only 2 minutes or a bit more if there is some mixing between the kitchen and other rooms. If directed upward toward the hood from the floor under the range, one might guess that there could be 50% mixing with room air as it rises, increasing the time constant to 4 minutes or so.

In other words, depending on how cold the cold air is and what the mixing ratio is, for normal cooking times where full hood power might be used the kitchen will cool off. Cooling would be unpleasant in NH in the winter; intolerable in Minnesota in the winter; but probably tolerable in San Diego year round.

kas

I once imagined a system in which unheated MUA is piped directly to the burners and used in combustion. But it doesn't work.

(Burning 1 cubic meter of air generates 3 MJ of heat energy which is 2,843 BTU. So a 25,000 BTU burner on full blast burns 8.8 cubic meters of air. Per hour, since "BTU" is actually BTU/hour when talking about burners. 8.8 cubic meters/hour is only 4 cubic feet/minute or 4 cfm. My range has 2 x 25K BTU + 2 x 18K BTU + 2 x 15K BTU burners for a total of 116K BTU. So with all burners on full, 19 cfm of room air is being burned - the oxygen in that air, actually. So the volume of air used in combustion is only a tiny fraction of the cfm required for venting.)

So it seems like for unconditioned MUA to work in a cold or hot climate, either (1) the vent hood has to be very effective so that the least possible cfm of airflow is required, or (2) the MUA has to flow directly to the vent hood with as little mixing with the temperate air in the house as possible, or both.

Kas, on (1) you have me doubting the effectiveness of the low profile commercial hood I showed. I thought it would be a great hood because it is so large (will extend horizontally at least 1 foot past the cooktop on sides and front). But I now see the baffles only cover a small part of the hood, the hood doesn't extend vertically down very far in front, and the hood isn't shaped to funnel rising air into the baffles.

I could possibly use a full-height commercial hood (that extends vertically down 2 feet on all sides) if I can put the ducting up in the joists (which do run the right direction). I could use "skirts" to extend the hood lower on the sides if I'm okay with a rather bizarre looking kitchen.

Or I can go looking for a different hood.

As for (2), the whole point of introducing MUA under or behind the range was, I figured, to have less mixing of MUA with tempered room air. Although I an reading that the other benefit is to reduce turbulence that would blow the dirty air out of the hood. So I'm not sure I can do any better.

Hmm. Heating and cooling the MUA will get expensive. I'm trying awfully hard to avoid that, as you can tell.

Oregon is not MN or NH, but isn't SoCal either. Winter temps are often 30-40F high / 15-25F low.

http://cfbt-us.com/wordpress/?tag=oxygen-consumption-principle

Obviously, you see the problem that we have created for ourselves by opening the kitchen to the house and wanting to keep the house clean. An example Qing dynasty Chinese rural house (built by someone with with sufficient income) had an attached kitchen that could be as messy, air wise, as needed for cooking without bringing the cooking effluent into the sleeping and eating areas. There was no electricity to run ventilation. (Such a house has been reconstructed at the Peabody Essex Museum in Salem, Massachusetts. https://en.wikipedia.org/wiki/Yin_Yu_Tang_House

One historical New England farm house I looked at closely had a kitchen that was between the living quarters and the enclosed connection to the barn. Its stoves were cast iron, wood fired, and not insulated. Any effluent generation from cooking could be easily cleared out without leaking significantly into the rest of the house, and the radiating cast iron would keep the kitchen warm. Also, persons living in close proximity with livestock might be assumed to be less particular about odor than those of us who demand good ventilation, preferably tempered.

With respect to your candidate hood, it presents two difficulties. First, it is probably not designed for best air use efficiency for capture and containment of high plume velocity cooking. I think what we might call a typical commercial hood as you describe would be best for efficiency. It is made deliberately so for commercial purposes because ventilation power is one of the highest overhead costs.

Second, it seems to be large for your application. For air flow rate minimization, it has to, er, not be bigger than necessary. So, a hood with a capture aperture that on front and sides can intercept at least ten-degree expansion from the locus of all likely pan sizes on the range burners should be sufficient. In your case that would be at a height of seven feet, and may include extra width for your future additional cooking appliance. There are sources of custom commercial hoods, and you could probably get one without the otherwise required fire extinguishing means. Because they internally taper to a single line of baffles, they tend to be tall. They also are not as shiny inside and out as a pro-style residential hood.

I know my Wolf Pro-Island hood is sufficiently deep (front-to-back) for operation at close to six feet, but I"m not sure I would be satisfied at seven feet. Using a room fan or air conditioner that blows toward the hood is disruptive of the plume. ModernAire or other sources of custom residential hoods could be consulted on what they can do in your case. Going with commercial is a ceiling height issue, with beauty relegated to a lower priority; going with custom residential is a cost issue. Going with non-custom residential, like the Wolf hoods, may be a head clearance issue if your goal of seven feet clearance was for that purpose.

(Hood skirts will do more to say Pro to those in the know than anything else you could do.)

My understanding is that MUA from the floor reduces turbulence relative to certain other MUA at the range or at the hood approaches. However test kitchens addressing plume measurements and hood performance generally introduce the MUA from an opposite perforated wall area. Mine is introduced into a hallway ceiling roughly 25 feet from the hood. A thousand cfm or so expanding into a hall-way and thence into an open concept kitchen/living room greatly reduces the velocity such that there is little bending of rising effluent. MUA source apertures some distance from the hood should be fine if the
configuration of the path from diffuser to the hood expands and lowers
the MUA velocity.

Residence configurations are all different and selecting an MUA approach can be very difficult. One can go with in-line heating, or introduce the MUA into a basement with a sufficiently powerful space heater to preheat it before it rises up through the floor. When cooling is also required, then complexity abounds. The Chinese attached kitchen didn't have to deal with either heating or cooling, the cook did.

kas