29 thoughts on “Ventilation Primer

  1. Hi, I tried to send you an email with some ventilation questions to the address listed on your questions page but it bounced with a User unknown error. What is your correct address?

    Thanks Liz

  2. Dear Sir,

    I have been looking at the exhaust system design (Ventilation Basics, Part II), and am still a little stumped.

    Could you possibly crunch these numbers for me? Here is what I have: (I am setting-up a range hood with rear and side baffles).

    Elecient AXC 200B centrifugal fan (636 CFM’s)

    Hood is mounted 30″ inches high. (from top of table to bottom of hood)

    Hood is 17.5″ inches wide.

    Hood is 12″ inches deep (from rear of hood to front edge of side baffles).

    Duct tubing is 8″ inch smooth-walled steel.

    Duct length is 18′ feet, with two 90-degree bends.

    If you could do this for me, I’d appreciate it!


    In Christ: Raymond

  3. Raymond — are you sure of those measurements? Most are usually wider — in the 30″ range. What you are proposing to use sounds like a flame plume capture hood and I don’t recommend that type of hood.

  4. Mike,

    go to this site (http://www.lampworketc.com/forums/showthread.php?p=1908119#post1908119) to see what I am doing.

    I also have another question for you.

    The duct from the rear of the range hood is rectangular, but opens to a 7″ inch round tube. Then I have a transition that opens it to the 8″ inch that I will be using for the rest of the set-up. So here’s my query…

    Should I keep the 7″ inch opening at the rear of the hood (which would increase the air speed – suction – at the hood)? Or should I open-up the back of the hood and attach an 8″ inch duct tube from the “gitgo”? (Hence, reducing the air speed – suction – at the hood).

    You can email me personally if you’d like.



    In Christ: Raymond

  5. Whoops!

    8″ inch round duct, with 18′ feet of duct and two 90-degree bends.

    Hood BOTTOM is 23″ from table top

    Hood width is 30″ inches

    Hood depth is 17.5″ inches. (From back of hood to front edge of side baffles).

    In Christ: Raymond

  6. My gosh!!!!!!!!!

    Hood depth is 12″ inches, from rear to front edge of baffles.

    I’ll get this right sooner or later.

    In Christ: Raymond

  7. Michael, since you didn’t answer back, I assume you are kinda busy. If you followed the link I gave you above, you would have seen the project I am making for my wife. The main thing I wanted to know was the size tubing I should have used to get the best air velocity. Everyone states that the larger the duct tube size, the better. But yours was the first article I read that mentioned air speed, and how it can be adjusted by the duct tube size. Anyhoo…I wanted to go kinda overkill, and so decided to go with the 125 CFM flow rate. (Even though this is for the ceiling hung, or island type hood set-up). Like I mentioned above, the hood area is 23″ x 12″ / 144 x 125 = 239.58 CFM needed. So what would be the best diameter duct tubing if I were to have 18′ feet of tubing with two 90-degree bends? We do frit work, so we’d like to keep the air speed at approximately 3000 to 3500 FPM. From what I gathered after reading your article, I would probably need to go with a 6″ or 7″ inch duct tubing. Is this correct? I will be making the final touches today, Saturday the 14th of June. If you could post a response for me today, it wold be really appreciated…as the tubing will be the very last thing that will be done. Thanks, and have a great weekend. In Christ: Raymond

  8. Actually, use the measurement of 30″ (hood width) – since you don’t have solid sides and back on the hood (those are the baffles). The measurement then becomes ((30 x 12) / 144) x 125 = 312.5 CFM. You have to go with 4″ in order to get 3500 FPM — that’s the main problem with working with such a small hood – you can’t get the needed velocity to keep dust and debris flowing through the system. Additionally, with the 4″ ducting, you are pushing against about 2.16″ of static pressure — it becomes a damned if you do and damned if you don’t sort of situation.

    Now, if you close in the hood completely (put back and sides on the enclosure — use galvanize sheet metal), you then need ((30 x 23) /144) * 100 = 599 CFM, and then with 6″ ducting you get 3050 FPM, but a static pressure of .91 inches. If you use 7″ ducting, your flow is 2241 FPM with a system pressure of .27. There are trade off’s here as with any design. Personally, I’d go with the 7″ duct design model with a closed-in workstation.

  9. Mike,

    thanks for the response…albite a tad too late…maybe.

    Let me explain: I ended up using the rectangular 3 1/4″ x 10″ inch hole in the back of the hood (which came with a 7″ inch round tube on the other end), and used a transition to an 8″ inch duct tube. Then I used 8″ inch duct tubing for the rest of the set-up. The distance was also reduced to 13″ feet of total duct tubing. (Three feet fron hood to fan, and 10″ feet from exhaust side of fan to end of duct tubing). My range hood is also enclosed, with a rear and side baffles.

    Buuuuuuut…I may still make a quick change. Here’s what I’m thinking…

    The actual distance from the range hood to the fan is a tad over three feet. (We can safely say it’s 3″ feet). So I am thinking of swapping this out with 7″ inch duct tubing, and keeping the rest of the duct tubing (from the exhaust side of the fan fan to the end of the tubing) with 8″ inch tubing.

    Here’s my reasoning/madness…

    I figure if the suction end is using a 7″ inch duct tube, the static pressure will be greater, hence increasing the air speed at the hood intake. But by using 8″ inch tubing on the exhaust side of the fan, the static pressure will DECREASE, hence evening out the workload of the fan. (It may even INCREASE, even more, the air speed at the hood).

    Does that sound right to you?

    I’ll post pictures of the finished product (as well as any specifics), on LAMPWORKS.ETC.

    So in closing, if it’s possible, could you work out the actual air speeds for me, using the above formula? (7″ inch duct tubing from hood to fan intake [3″ feet], and 8″ inch duct tubing from fan outlet to end of tubing [10″ feet]).

    Or can I keep it the way it is, with the total tubing distance now being only 13″ feet?

    Thanks a bunch!

    I’ll send you a link to the finished product. (Or detailed pictures of my set up, if you wish).

    In Christ: Raymond

  10. The 3 foot run of 7″ will have a velocity of 2380 FPM. The 10 foot run of 8″ will have a velocity of 1822 FPM.

    The problem is that by increasing the duct size, you lose velocity, and you will have drop out in the airstream of any suspended particulates. This is why it is so important to maintain the exact same duct size through out the system.

    Static pressure is calculated as a system TOTAL, not as separate before/after — you will have higher suction (negative) pressure (0.10) in the short section, but lower blowing (positive) pressure in the long section (0.05), for a total pressure of 0.15 inches. (Signs are not used as part of the math, they only indicate suction or blowing pressure.)

    Because of the volocity drop (558 FPM) due to the change in duct sizes, any “large” particulates that are in the system from the hood pickup will settle out in the longer larger diameter section, and every time your wife disconnects the long section, it will leave a trail of dust and debris all over the floor. I’m also concerned about how you are exhausting the duct itself. The article mentions you are placing the duct in an open door. Will the rest of the opening be blocked off so the exhausted air cannot re-enter the room? Makeup air will always take the least restrictive way, and even if you have another open window or door to outside, some air is going to get sucked back in through the open door that you are using for exhaust unless you block it off some how. I’d recommend a piece of plywood specially cut for the opening, then a hole for the duct to go through, and then close door on the plywood so that it closes off the rest of the space. This will also keep insects from entering the room.

    It would be my recommendation to stick with 7″ ducting throughout the entire system to keep the flow rates identical on the suction and blowing sides of the duct.

    What I meant about “enclosing” was to enclose the total workstation, around the wooden supports that you used. As it stands, the only suction area is going to be directly UNDER the suction port opening in the top of the hood. It is going to draw general room air from around the rim, front, back and sides. If you totally enclose the workstation around the sides and back with sheet metal, you will only draw from the front face of the hood.

    Overhead hoods without long baffles or total enclosures draw lots more general room air and not much “contaminated” air — which is why the CFM for overhead hoods need to be higher — but they are still not as efficient as an enclosed workstation which forces the hood to draw air from around only the torch area instead of around the whole hood.

  11. Thanks, Mike.

    I’m going to take heed of our conversation – and facts – and go with the 7″ inch tubing throughout the system.

    Go to this site:


    …to keep track of my table developements. As you will see, “I DO” have rear and side baffles.

    By the way, when you said,

    QUOTE: “The 3 foot run of 7″ will have a velocity of 2380 FPM. The 10 foot run of 8″ will have a velocity of 1822 FPM”.

    Where those the air speeds using TWO SIZES of duct tubes? And if it was, now that I am going with a 7″ inch set up, what will be the end calculations?

    Thank, Mike…on behalf of my wife and myself.

    In Christ: Raymond

  12. Yes. That was the velocity in the tube for the length and diameter quoted.

    Your new numbers are 2376 FPM, and a total system pressure of 0.26 inches.

    Looks like a good design.

  13. Mike,

    can you do me one more favor? Could you crunch the numbers for my system using the 8″ inch duct tubes.

    I have already finished the project using 7″ inch duct tubing throughout the system. But I would like to talk a little about ventilation and the importance of air speed. And in so doing, I would like to show a COMPARISON in air speed when using different duct tube sizes, using my system as an example.

    Using a wind speed gauge, I measured the intake suction speed with an 8″ inch duct, and with an 7″ inch duct tube. (Photos were taken).

    And in closing, you mentioned that you were worried that the fumes might be exhausted out of the door…and that it might get sucked back in.

    Well, the first 5″ foot tube goes from the fan and ends almost exactly at the foot of the sliding glass door. And when the second 5″ foot section is attached, it protrudes almost another 5″ feet out of the patio door, at a steep angle away from the door. I figure 5″ feet should be a safe enough distance for proper exhaust ventilation, especially since the living room has huge windows on the opposite side of the room…which will be open for air exchange.

    I also lit a piece of paper on fire, turned it off (so it smoked heavily) and placed it in a glass under the hood. Nothing could be smelled of the smoke. It was directed outward, and nary a scent of smoke was sucked back into the room via the sliding glass balcony doors.

    I think it’s safe to say that the ventilation is far away enough, and at a steep angle enough, to prevent any of the fumes from returning back to the room. (Unless Mother Nature decides to make me eat my words, and causes a strong breeze to redirect the fumes BACK into the room). Ha ha.

    Thanks again, Mike.

    In Christ: Raymond

  14. 1822 FPM with a static pressure of 0.12 inches.

    Your supposition is incorrect — you need 10 feet of separation between ANY fresh air source and the end of the exhaust duct. That’s code. You’d have to either extend the duct another 5 feet -or- block the door opening to prevent back drafting.

    The fact that you couldn’t smell the smoke doesn’t mean anything. You are moving 636 cubic feet of air per minute — that amount of air really dilutes the small amount of smoke you generated — the problem occurs when the torch is on and generating NOX and the fumes off the glass, the stuff that is really hazardous. And what happens if the wind happens to be blowing back towards the house from the duct end? All the fumes will be pushed back into the house. That’s why it is so important that the opening used to exhaust air be fully blocked so absolutely no backdrafting could occur under any circumstances.

    The codes exist for a reason, and they should be followed pretty much absolutely.

  15. Hmmmmmmmmmmmmm.


    Let me get my creative juices flowing and come up with something to keep us both safe. (Darn! Just when I thought I was done).

    I’ll keep you informed. And if I find your email addy, I’ll send you a few pix of the finished product…before I post them anywhere else.

    In Christ: Raymond

  16. Mike,

    one very last question: What is your opinion on quick disconnects for the fuel line to propane tank connection?

    Because of the low pressure (5 psi), I would think that it is safe…so long as I am using the proper gaskets for propane.

    In Christ: Raymond

  17. I am researching setting up studio in garage. I have been reading your Ventilation Basics: Part 1-3. In part 2, you mention a table for figuring out loss factor and static pressure. The table can be found in Ventilation 101:Static Pressure, Table 1. Can you give me a link to that site? Thank you.

  18. Hi Mike,

    I just had a ventilation system installed in my studio and THEN I read this page. If you have the opportunity, could you look at my numbers and let me know if this is adequate, and if I need to do anything else? I have a Mini CC and use soft glass with propane/oxygen concentrator.

    Hood height from bench to hole 36″ (with 3″ lip on all sides)
    Hood width 36″
    Hood depth 24″
    Dust diameter 8″
    Dust length to roof outlet 120″
    4 bends with following degrees: 90, 60, 22.5, 22.5
    Fan is 540 CFM and is installed in the run above the hole just after the 60 bend

    Thank you!

  19. Hi again.

    Btw, I wrote “dust” twice in the last reply, but meant “duct” – probably obvious.

    I wanted to also ask you about fresh air recommendations. My studio is a 2-car garage, sans cars. I have a rollup type door along one side, a regular door on the opposite wall and another door on one wall between them. My workstation is along the wall opposite THIS wall. I also have windows in all three non-rollup walls.

    I usually have all the windows partially open and like to work with the rollup open. However when there is wind, I usually close the rollup halfway. I do not have much of a weather/temperature issue where I live.

    With my new ventilation system I’m wondering if the windows/doors being open will move the air in the room too much and affect the pull of the ventilation.

    Any recommendations on what to open/close?

    Thanks again,

    • Hi Heather!! The only problem is that your fan is undersized. I recommend 125 CFM per square foot of hood, you should be using a minimum of 750 CFM.

      With a 750 CFM fan, your design is good, showing a 0.37″ static pressure, which in most ventilation fans is pretty good.

      BTW, on my eCommerce website, I’ve got a free (you still have to register) Static Pressure Calculator. http://www.auralens.net

    • No worries!! Your fresh air source is plenty sufficient. The only issue would be is if your part of the country gets cold in the winter, then you might want to consider ducting your fresh air direct to the bench. That way you won’t chill down the entire space. If you do that, use 1 duct size larger for incoming fresh air than you are using for your exhaust.

      • Thank you so much Mike. I’m looking for a 750 cfm fan, so if you have a recommendation of a vendor, part number, etc., please feel free to let me know. 😉

        Thanks again. I’ll feel much safer when this is all installed and running.


  20. Hey Mike,

    i have a 8′ x 10′ storage shed im currently converting into a studio, with an 8′ wide by 4′ deep work bench for two stations. would you recommend venting with two inline fans postioned behind the torches or designing a fume hood per station that can join together and connect to run off of one fan or will i need a fan per station?

    • In a situation like this, with such a small room, you are better off using one exhaust fan on the outside wall between the two workstations, with enough CFM to change the room air every 2-5 minutes.

  21. Hey Mike, thanks so much for all this info, it helps a ton. I just finished doing the calculations and the only thing I mainly have to ask is, the CFM per sq st of hood size is just a minimum, correct? And I would also like to hear your feedback on my calculations..

    4′ x 2.5′ wall mounted hood

    1200 cfm fan

    duct size = 8″ round (.349 sq ft)

    total run = 1 feet

    number of bends = 0

    Loss Factor = 0.02

    velocity = 3438

    VP (velocity pressure) = 0.74

    SP (static pressure) = 0.0148 inches

    I found the fan I’d use from Grainger already, thus why it’s designed around 1200 cfm. My bench is only 30″ deep which is why I’m limited the hood to 4′ x 2.5′

    Thanks a bunch for your time!

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