PS; this source says 16 guage carbon steel or 18 guage stainless. So there is conflicting information.

"For the duct system to be code compliant, officials require either 16 gauge carbon steel or 18 gauge stainless steel construction. Along with that, the best products have an interior liner and the right exterior surface to make cleaning grease and other common contaminants easy."

https://www.durasystems.com/5-points-to-consider-when-choosing-a-kitchen-exhaust-duct-system/

I have no idea since you are limited to ductsize you will need to find a hood that complies to that. Everytime there is a 90 degree turn in ducting you will require higher CFM and 2 turns is IMO pretty poor design . Maybe post the plan for the kitchen Ideally either out the exterior wall or straight up throuh the roof are the right plans.As for the actual ducting I see no reason for stainless but that I guess depends on code where you live. BTW that length of run is also a bit issue

Here is a picture of the setup that I'm building, as found in the user manual for the Zline 696-RD-36.

https://www.zlinekitchen.com/manuals-1 > Range Hood

On that note, the conflicting information is abundant! Every person I talk to has conflicting answers. Some more casual, some conservative, and some pessimistic. Some say "eh just do 6" flex pipe" and others say, "It must be 12 inches with no bends... and it must be 20 guage stainless steel". It's kinda frustrating to be honest. :-) I would like to know, what are the standards?

I called my local plumbing supply house and the thickest guage vent pipe they have is 26 guage. I don't know where to find the thicker guage duct.

Jay

I think you need to get a pro to help who understands the issues . To me that range on another wall would be the answer and I will say that layout for the ducting is horrible. Is this one side of a duplex ? Post the damn plan for the whole kitchen

First, before I forget: The MUA air vent on the ceiling should be a diffuser, not a register. Your image looks like a diffuser, and it is OK to spread the MUA out in different directions, as illustrated, but the 'bad' direction is towards the hood. This will interfere with plume rise. Hart & Cooley seem to make every possible diffuser configuration, so choose one that avoids putting the MUA toward the hood.

Second, your Broan kit seems to be only a controlled damper. Thus the pressure loss in the MUA path needs to be evaluated and compared to the max pressure differential than any 'exposed' combustion appliance can stand before back-drafting their exhausts. If you are marginal, make the MUA path diameter much larger than the hood path diameter. Else, you will need an MUA blower.

Third, while I don't know the gauge question answer, standard hood duct sold by Fantech and myriad others should be adequate for residential purposes. You do not have a requirement to weld together duct sections, put in access panels, clean the duct every year, have fire suppression in the hood, etc. as do commercial installations. Most residential cooking with a decent hood filter will leave a trace of grease on the ducting that will rancidify (harden) to a varnish and not be an egregious fire hazard.

The point of using steel duct is to keep any fire that might occur away from combustible materials. The filters (baffles, hopefully) are the fire block from the cooktop.

And I didn't see how you determined air flow requirements. We have addressed this many times, so it might help to reveal your thinking w.r.t. required CFM and blower-rated CFM.

1) Ideally, the MUA is provided as laminar flow toward the cooktop/hood interface area. That can only be achieved by a large perforated wall across from a cooktop/hood under test. In a real kitchen the goal is the same, minimal turbulence (other than the cook moving about) in the air being supplied to the area under the hood. Turbulence is generally high just past the diffuser fins. So the air path from the diffuser to the hood should be relatively long so the air flow can 'straighten out.' Air fed from the ceiling pointed away, from down the hall, and similar such configurations can approximate this goal. Some commercial designs emit the MUA from a feed just above the hood, but it is pointed toward the opposite wall. In restaurants there are all kinds of schemes, including some of the MUA introduced in the dining area to ensure kitchen odors are kept in the kitchen. Ultimately, slight turbulence will not be a problem if the air flow is high enough.

4) I prefer the Greenheck method as it is closer to the actual physics (fluid dynamics) of the plume capture and containment. So rather than CFM vs. potential heat generated by the burners, I recommend on the basis of hood air velocity needed for entrainment into the air flowing into the baffles. Greenheck did their own study, but there is data out there to used for estimation. (See the Greenheck KVS Guide -- https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf). One difference is that due to their large entrance volume below the baffles, commercial hoods can operate with a good bit of plume averaging.

Anyway, for hot cooking where the oil or grease temperature is at the smoke point and the hood is residential type, I suggest 90 ft/min = 90 CFM/sq. ft. of hood entry aperture. When multiplied by the aperture area the product is the desired CFM . The blower has to achieve this in the context of pressure losses at the baffles (varies with CFM), in the MUA system (varies with CFM and the MUA restrictions), and in the ducting (varies with CFM), so the exact blower fan curve needed to match the air resistance curve is a recursive exercise based on often unavailable data. I suggest just approximating by taking the desired CFM and multiplying by 1.5 to get a rated CFM. All of this is rough guessing in any case.

Some info that might be useful: https://bamasotan.us/range-exhaust-hood-faq/

"Being that some kitchens are designed without any MUA at all - I figured this would be better than nothing."

That's kind of like saying that some houses have a lot of exposed asbestos so we're only going to have a little. :-) It is better than nothing but not by much and especially with flex. Your MUA should be 1:1 to your range hood so a 1200 CFM range hood requires a 1200 CFM MUA.

Where are you located? How often do you cook without many windows open? How old is your house and do you know how leaky it is? What gas devices do you have and how many are 100% sealed combustion?

Following!

W.r.t. the original question: Duct requirements are specified in Chapters 15 and 16 of the International Residential Code, copies of which may be found on the Internet. Notwithstanding the acceptability of thinner duct for shorter length runs, I think you will find that the duct suppliers will be using sheet metal thicknesses suitable for the longer lengths.

Now we are "cooking with gas". This information is very useful.

2. Make up Air Systems: Dampers? or Damper and Blower? The manufacturers of Vent Hoods are miseducating the consumers. Many of them (I've called them on the phone) do not encourage, nor discourage Make-Up-Air systems (MUA). They say "look to your local code book". Realistically, the hoods should have matching blowers for intake and exhaust and it's not optional. I believe the hood manufacturers should be requiring it, and designing hoods that accommodate for it. It doesn't seem that they are doing this.

I believe I have sealed burners on my hot water heater and furnace. My house is about 13 years old, and is sealed fairly well with some spray foam in the attic. While this might mean it's "safer" to operate without a MUA, it will not be up to code, and it will not operate properly because the MUA is needed to create a good flow.

3. Duct Guage and Materials. It appears that the residential exhaust hoods do not require the ultra thick gauges or materials. From the International Residential Code book here, it appears that minimum 28 guage is acceptable. But they also don't want it flush to the wood surrounding it - requiring a 1/4 inch clearance.

https://codes.iccsafe.org/content/IRC2015/chapter-15-exhaust-systems?site_type=public

I did not find any additional useful information on Chapter 16 regarding duct construction. Here is the link for other readers to have a look in case I missed something. https://codes.iccsafe.org/content/IBC2018/chapter-16-structural-design

4. I will be reading up on the Greenheck method of venting to further determine my CFM.

I realize that the code snippet I provided for the overhead exhaust hood is regarding the hood not the duct.... But the duct material thickness is not specified in the code. Am I missing something?

"The manufacturers of Vent Hoods are miseducating the consumers."

Yes. Somewhat. Not really. They are in business to make money and they do that by making hoods as inexpensively as possible and then selling as many as they can for as much money as they can. Their goal is NOT your health or the IAQ of your home - that is up to you. Telling consumers that they'll also need to spend money on MUA if they buy hood X will negatively impact their sales (ignorant consumer will just go buy a hood from brand Y instead because it doesn't say you need MUA even though it has an identical need for MUA) so they do not want to do that.

MUA is also not something that fits well within consumer hood manufacturers world except to the extent that they might offer a hood w/ integrated front curtain ducting. It is much more of a pure HVAC thing. It can be supplied to the return ducts of an HVAC system, the supply ducts or ducted directly to appropriate locations. Incoming air can be heated by gas, electric or hydronic. Incoming air may need to be humidified or dehumidified. It often needs to be electrically integrated with the HVAC system controllers to function properly.

Consumer hood manufacturers - ARE NOT IAQ or HVAC people or engineers - they make something decorative that by nature must include air movement so they reluctantly include air movement in their product.

Commercial hood manufacturers - ARE IAQ and HVAC people and engineers. Their customers are much more educated than consumers. The one singular purpose of the product they are selling is IAQ - so they design and sell systems that provide good IAQ. Aesthetics is quite secondary for them.

Residential HVAC companies in the U.S. - ARE NOT IAQ people (they should be though). A tiny few, less than about 1%, know it well and a few more know it a very little but the vast majority know just enough, based on what they've been told at a 4 hr CEU course, to sound like they know what they're talking about but they don't really understand it. Most residential HVAC people do not understand air movement or why things are done the way they are - they only know how to use tables that tell them to do this or that and as soon as something is outside of defined parameters they're totally lost (though sometimes don't even realize that they're lost). Licensed Professional Engineers do (or most do, some don't) understand air movement and other elements. They are the people who create the tables that HVAC people use. They can think independently. Defined parameters for them are not tables but the physics of air - it's movement, temperature and components.

Unfortunately, most U.S. engineers do not have a good understanding of human physiology (except perhaps for biomedical engineers). They do not, for instance, understand how CO2 functions in our bodies and how high levels of CO2 due to poor ventilation affect us. Or how poor ventilation resulting in high levels of VOC's, PM, Carcinogens and Pathogens affect us.

Am I missing something?

https://codes.iccsafe.org/content/IRC2021P1/chapter-16-duct-systems#IRC2021P1_Pt05_Ch16_SecM1601

(The root index link)

https://codes.iccsafe.org/content/IRC2021P1

Good find. Is rectangular the longest side or longest plus shortest or ?? Either way I'm a bit surprised how thin that allows for rectangular as it doesn't take much negative pressure to pull the sides in with SP increasing exponentially. Then again, this only allows for 1" SP and the duct between a hood and remote blower could greatly exceed that.

Generally you need stronger material on the negative pressure side of a blower than on the positive pressure side and then you also need heavier material for a rectangular duct than a round duct. All depends on the pressures though.

For example, dust collection systems will often use round spiral pipe because round straight gauge can totally collapse unless it is extremely thick which is quite expensive. On the positive pressure side they can have much thinner material and can have rectangular.

Does Greenheck have anything on this for exhaust hoods?

I don't recall seeing duct fabrication being addressed. However, many commercial rigs have an up-blast blower on the roof not very far from the hood and MUA conditioner above the room feed so there isn't a lot of duct length involved. Also, I don't think many configurations would plan on pressures above a half-inch or so because that would be a large loss, fan curve wise. The designer would just increase the duct cross section to keep the pressure low.

I think at full flow my baffles are in the ballpark of 0.2 inches, so that would be most of the exterior pressure on my ducting to my roof blower (given adequate MUA). I think I would need to slap on a strain gauge to see any effect on the ducting. My reasoning is this:

If you (very roughly) consider a PSI to be 2.3 ft of water (34 ft/14.7 psi; vague memory from a diving exam), then 0.2 inches is 0.007 psi. Integrating this over a square foot of ducting surface yields 1 pound of pressure, or about 3 pounds around a 10-inch duct per foot of duct length. This isn't much for a cylindrical duct of any reasonable gauge, and if this would compress it significantly, it would be difficult to lift and transport the duct section without damaging it.

I see this thread is a couple years old, but im hipong for some input. Jay, what was yiur final install and any issues you encountered ? We are installing zline 698-rd-40 which is the insert version of the wall hung chimney you mentioned. what was your final gauge ducting and how has your MUA worked out? Our house is 50yo old so plenty leaky lol, but we are sealing up as we go. I know we will need MUA, but we are waiting to install once we have the rest done and see just how much ”suction” effect we get.. I know not the most scientific approach, but I can only handle so much research at once! Any input from yourself (or others) is appreciated :)

Well, for serial design engineering, i.e., waiting to finish the hood system before considering the MUA system, and assuming zero instrumentation available, I would turn on the hood full power and then partially open a window or door; if there is a strong breeze you are definitely in need of an MUA system sufficient to make the door or window breeze a mere zephyr.

However, if you have combustion appliances that can take air from any part of the house connected to the kitchen, then serious attention is needed to either provide MUA to the combustion appliances, or meet the standards of the submission below from another Houzz member (name forgotten) converted from image to text.

Mechanical-Draft Appliances

Heating systems with fan-powered exhaust systems can withstand higher negative pressures than natural-draft appliances. Some types of fan-powered systems are much better than others, however. In order of effectiveness, the choices are:

* Sealed-combustion. Also called “direct vent,” these appliances draw all combustion and dilution air from outside. These can typically tolerate negative pressures in the range from 25 to 50 Pa.

* Power-vented. These draw their makeup air from indoors and are also called fan-assisted, forced-draft, or mechanical-draft. These can typically tolerate up to 15 to 20 Pa of negative pressure.

* Induced-draft. These have a small fan added for energy performance, not to overcome house depressurization. These can typically tolerate 5 to 15 Pa of negative pressure.

By comparison, an atmospherically vented furnace can back-draft with as little as 5 Pa of negative pressure, and a gas water heater will have spillage at 2 or 3 Pa. Fireplaces can start having problems at about 3 Pa. Canadian codes limit negative pressures in homes with atmospherically vented equipment to 5 Pa. U.S. codes do not currently address the issue (in a plainly spelled out "prescriptive" number).

1 pascal = 0.00402 inches, water column

1 atmosphere = 407 inches, w.c.