@kaseki any thoughts as you seem to really know what youre talking about :)

@opaone any thoughts you have would also be appreciated aa you ade another expert

@Alaur87 i know you had similar issues with not being able to find a HVAC company to help. any guidance? im in NJ and am struggling to figure this out. thanks

Thanks! What calculation did you use to determine the number of cfm @User

My suggestion (for residential style hoods) is to achieve a minimum at full power of 90 CFM per square foot of hood entry aperture. So first decide on the hood size that overlaps the cooking zone by enough to capture most rising and expanding cooking effluent plumes, say 6-inches each way. Then calculate the area. Then multiply by 90. At this point you have the desired/required air flow rate, but have to account for pressure losses from various causes and compare them to the blower's fan curve for your candidate blowers. Or just multiply by 1.5, it should be close enough. MUA should be the same values and duct sizes. All air that leaves the kitchen has to be made up somehow. Pressure loss in the MUA filter and heater may be more or less that of the hood loop but for guessing purposes, equivalence should do. Read the vast number of hood and MUA threads here for many details and perspectives.

For example, if you actually had a liner with a 54 x 27 opening, the area would be about 10 sq. feet. This calls for 900 actual CFM, which might be supplied by a blower rated at 1350 CFM. Abbaka has one in this region, as do Wolf and others. I am writing here of roof blowers, which, when provided by enough duct distance, allow addition of a Fantech silencer to keep the blower's noise level at the cook down to conversational level. If you go out the wall expect more noise unless you build a chase for a duct and silencer. An oversized blower run a partial power may be relatively quiet, but you are already near the limit for commonly available residential blowers. There are always commercial blowers available for any need.

Note that liners for liner type hoods are designed to fit standard cabinet sizes, so the liner itself may be an odd size. Just be sure that it is still big enough in opening area for capture. This is nominally 10 degrees from vertical away from pan edges of any pan that might be used on any burner. So you may also note that as the hood base is raised, the desired size grows. And if it is raised above 36 inches, then even more hood area is recommended to account for spreading with temperature drop and cross drafts.

Thank you so much! are there any relatively affordable make air solutions (other than opening a window)? Since i would need heated air it sounds like it would be b about 2500 in materials alone from fantech. also what type of professional should be able to help me? Thank you @kaseki

I can't address what is affordable. Even DIY has significant parts costs and probably wiring/ducting labor costs.

For HVAC issues, a HVAC business. In some areas this may require one that also deals with restaurants so they understand MUA.

Read MUA threads here.

Review @opaone's text at https://bamasotan.us/range-exhaust-hood-faq/

Here is a link to a Fantech competitor that opaone likes: https://www.ecomfort.com/Electro-Industries-EM-MA20-240-1-14/p58515.html. They may have various models I haven't looked into.

Opening a window might not work for a vent hood of this capacity. You probably do need a heated and actively operated MUA. That's one of the reasons why this forum tries to steer people away from buying very large stoves. There are just too many ripple effects. It's all doable, but it does drive up the cost. Frequently, the cost of the range is the smallest issue.

If you need customized vent solutions, I would recommend reaching out to ModernAire. We are very happy with our hood from them, although we didn't order just the liner. And I like that all their hoods are built to order, so they can easily accommodate special requests for custom sizes.

@kaseki already hinted at it, but I would strongly encourage you to mount your hood above head height. With smaller hoods, people frequently mount them closer to the cooking surface, as that can make a smaller aperture work OK. But with a big hood I don't think you'll like the experience. The hood will always be in your sight lines while cooking.

@kaseki @User @M ive continued to call many hvac companies and still cant find one that will take on helping with makeup air.

i will be getting a 48” XO brand hood insert with a 1,000 CFM motor that is built in the insert. the hood will be on an exterior wall and will therefore vent directly outside.

i spoke with XO and my kitchen installer and they both said that i could use the 8” makeup air damper system made by XO. it is basically just an 8” vent with a damper that communicates with the hood fan and that brings in unconditioned air (no fan to bring in the air). they said i could place this in my basement (by a basement window) directly below the kitchen.

i live in a house built in the 1930s so i know my house isnt air tight

any guidance on this? i have contacted so many hvac companies and am now at a loss because install is supposed to be next week but i have no idea if what i am doing is okay thank you

For innocent passers-by: Choose appliances, including their support systems (including electrical, plumbing, vent and MUA systems) first, design kitchen and cabinets second (including HVAC, wiring, and plumbing diagrams), order everything third, wait for delivery (at least to distributor) fourth, once received and interfaces checked against diagram data gut and rebuild kitchen. Or do everything out of order and enjoy new opportunities to panic.

HU-109832731

Passive (no-blower) MUA can work under these conditions:

• There are no combustion appliances taking their air from rooms connected to the kitchen (air flow wise)
• The MUA pressure drop will be small enough that the anticipated air flow from the vent hood blower is met without pulling air through house leakage, or equivalently, the duct for the MUA is large enough for the expected flow. Note that heating and filtering of MUA can change the pressure loss.
• The room the MUA dumps into is connected air-flow-wise to the kitchen.

I'm of the opinion that if all of these are met, an 8-inch duct is too small unless very short.

Note that you can dump cold air into the basement for this purpose and have over-sized heating installed in the basement to heat the MUA as part of the room air before it gets to the kitchen. Uses of the basement such as dust-generating carpentry, spray painting, skunk raising, and similar aerosol related activities are not recommended.

The basement is open to the kitchen? Or are you going to put some form of registers in the kitchen floor or toe kick areas of your counters?

Passive MUA requires all parts of the MUA path be low in air restriction, and that the hood system blower have a fan curve that gets you the required air flow when added to the pressure loss of the hood baffles and hood ducting.

(I don't have a warm and fuzzy feeling that my comment (#6) had any effect on your parts selection.)

Youve chosen an insufficient hood. It needs to be both bigger, and with more CFM. If you cannot vent the range correctly, you might as well not buy it. Its a health hazard. Either go smaller, with less firepower, or double your budget and do the whole package correctly. Cheap import junk aint gonna do it. You’re creating a noxious health hazard.

so according to the stove manufacturer i need 900 cfm. i got a 1000 cfm motor because it is just venting directly outside. the hood insert is 48” but the hood itself is 60”

does this now sound correct?

@User forgot to tag you in this reply. please let me know where i am incorrect

What brand of range? Because its irresponsible that they suggest something so underpowered and unhealthy for you. I want to read the actual specs.

@User XO brand. i spoke with Matt in their technical help department. even Broan has a similar solution. i dont under the technical aspects of this and i also cant find a hvac person to help so am at a loss

Calculating the correct hood size is not entirely trivial and can depend on a lot of different factors, but @kaseki has come up with general guidance that is frequently quoted in this forum. And from my own experience, I can confirm that it gives numbers that are a good first approximation for what you need.

The main insight is that cooking plumes rise with a speed of up to 90ft/min (with bursts exceeding this number). So, this is a good number to aim for, if you want to actually vent the majority of effluent; and everything else follows from this number.

Hot air rises in an expanding cone shape. As a good rule of thumb for a hood that is mounted at average height, attached to a back wall, and has upper kitchen cabinets at both sides, making the hood about 3" wider than your range would allow you to capture most of the cooking fumes. This means, for a 48" range you are looking at a hood that should be at least 54"x24", but increasing the depth to 27" would be preferable. You also should have some amount of "boxiness" instead of just a flat surface. The box helps deal with air that bounces off surface of the baffles and with bursts of super fast air (i.e. in excess of 90ft/min).

This is the aperture opening that has to be able to move air at 90ft/min over the entire area. You can make the insert a little smaller, but you shouldn't go much smaller than the aperture size.

For argument's sake, let's assume that your capture area is 10 sqft. Then you would need to move 90*10 = 900 CFM. So, your stove manufacturer agrees with us. Wonderful! All of these are rough estimates anyway, but if they are in the same ballpark we know that we are not completely confused.

Until this point, we have been talking about actual measured airflow that we have to be able to achieve. Unfortunately, the second part of this question is how do you actually go about achieving this airflow. And that's where you have to realize that hood manufacturers use very creative math, when advertising their products. They measure airflow through the hood, when there is zero back pressure. In other words, there is no duct attached to the hood, and the hood isn't actually venting to the outside; so, there also is no concern over make-up-air. In these idealized conditions, your hood very well might achieve 1000CFM at the max power setting.

But in practice, there are all sorts of performance losses, and even very minor back pressure typically results in quite a sharp drop off in CFM. It's rarely an easy linear relationship. A good HVAC contractor who installs an oversized, very short duct without any bends can probably minimize losses, especially if paired with an active MUA system that provides positive air pressure. In practice, these ideal conditions are never met. So, short of precisely measuring the physical properties of your specific system, @kaseki suggests upping the desired CFM by 50%. In many real-life scenarios, this rule of thumb has worked quite well.

And that means, you should aim for a hood that is rated for at least ~1400CFM, and that's assuming that you did everything else right (i.e. short directly venting 10" ducts and an appropriately sized MUA system). Of course, all of these interconnected parameters have some play. If you cut corners in one part of the system, you can sometimes compensate for it in a different part.

We have a 30" range and decided on a 1200CFM hood, and we are very happy we did. The duct is large and not extremely long, but it does have some bends that might affect performance. But most importantly, we rely on an open window for MUA. This is not ideal, but works as we live in a part of the country that "doesn't have weather". So, this window stays open year round, and it's a significantly large window. Also, we have no other gas burning appliances in the same envelope. So, building inspectors usually let things slide. And in practice, the fact that we oversized the hood for our stove does a great job making up for the compromises made in other design choices. Despite an open floor plan, our hood vents perfectly and there never are any lingering smells. This certainly wasn't true before the kitchen remodel. But it is important that the window stays open at all times when cooking. And if that's not an option, a more elaborate MUA system would have been required.

Thank you!

I asked about the range, not the cheap import hunk of metal that the salesman is trying to sell you to vent it. The range’s specs are what control the situation here.

it is a 48” all gas wolf range with 6 burners and a griddle. their guidance was to take the BTUs and diving by 100

@User

It is a wolf range. @User

Thank you "M" for fleshing out my normal explanations. In many cases, blower spec misstatements are even worse than you wrote. Most blowers are rated in free air (zero static pressure) outside of their hoods (even when used as internal blowers). Wolf has some fan curves that are clearly adjusted for internal hood conditions, but the blowers are still named for their free air flow rates, while VaH stated values are for in hood conditions but not their Magic Lung equivalent values. As you note, none of these are actual in situ flow rates because of the other pressure losses. And these other pressure losses entail the entire path from cooktop to outside and back to the cooktop.

After the pressure loss of the baffles, if not counted in a given flow rate value, the next typical pressure loss is that of the MUA, and this can be significant even when the only pressure loss is a window screen. Hence the 1.5X multiplier.

As for flow rates based on BTUh of a given cooktop, this approximation comes from the commercial kitchen hood industry, and applies to their style of hoods only. Even then the industry has some opinions on suitability, and I refer readers to the Greenheck Guide for a more thorough explanation. (https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf) I prefer Greenheck's more physical use of plume velocity based flow rates, but have to correct for the poor reservoir volume under residential hood baffles (necessitated by residential kitchens not usually having 10-ft ceilings for the hood assembly). Hence, my suggested actual flow rate is somewhat higher than the left-most column in Greenheck's figure 4, but also one must consider that commercial cooking often involves full power across the breadth of the appliance, while this would be rare in a residential setting.

Figure 4 shows actual flow rates and these are usually achieved with "perfect" MUA.