Remote blowers to handle high static pressure in kitchen exhaust?
The kitchen ventilation system in our new build is the bane of my existence. Our engineer specified a 10” duct for the exhaust in order for the system to meet my requirements:
1. Range hood exhaust airflow of ~ 400-450 CFM on high speed (actual, not rated at zero SP)
2. Extremely quiet, ideally 46-49 dBA on high speed (definitely < 55).
Turns out we only have a 6” duct installed because bigger could not fit, despite this being the first thing I ever specified to the architect long before any of the “conflicts” were designed or built. Drywall is done and the kitchen is now installed. We purchased a Ventahood liner insert for custom wood cover but it does not work with the ducting: BH240PSLD https://www.ventahood.com/index.php?option=com_content&Itemid=333&id=361&view=article&modelsku=BH240PSLD
So the Ventahood insert requires an 8” duct for obvious reasons. My guess is SP might be about 0.2” to 0.3” in WC with the 8” duct so the “600” CFM would end up right around 450 CFM according to their chart — perfect! However, with a 6” duct the pressure is probably over 1” in WC and that airflow will be reduced I’m guessing to like 200-300 which is not only unacceptable for cooking based on my research but also it will be loud and destroy the motor which is simply not built for that kind of pressure.
No matter what at this point I think it will be an expensive fix and I intend to fight my GC on this. If we can’t rip out the duct to replace with 8” (I think it does not fit) then we need to put a different range hood and possibly a 3rd party exterior remote blower.
The other range hood insert I saved is the Wolf Pro insert which can be purchased with or without the motor – https://ca.subzero-wolf.com/en/wolf/range-hood/42-inch-pro-hood-liner-22-inch-depth
Now you have all the facts, is there a blower we could use on the outside of the house with the existing 6” ducting to withstand that extreme pressure and achieve 400 CFM of exhaust while still being super quiet?
Our installer left us a spec sheet for the “300 CFM” version of the same Ventahood insert, which would require no cutting open drywall and no modifications to the cabinetry. Of course, I am the difficult who client who does not want that. I’m not sure how I could explain to them that with a 6” duct and maybe 1” in WC of pressure, I don’t want a range hood performing at maybe like 200 CFM for very well researched and proven reasons, especially considering we are paying several thousand $ for the insert and the custom cover – the whole point of doing this and having an engineer specify the duct size was because we wanted it to actually work.
Little did she know… asking for kitchen ventilation that actually works is totally radical 🤣
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Centrifugal blowers such as are typically used in kitchen range hoods actually work LESS when the output flow is restricted, because they are actually moving LESS air. They tend to speed up when the flow is restricted because of this. This is also why your vacuum cleaner's sound goes up higher in pitch when you put your hand over the suction hose. Same basic principle. You may well get more noise with higher flow restriction, but you will not overload and destroy the motor.
You're not going to get around the noise issue forcing lots of airflow through a too-small duct. While I'm not entirely sure you need such high airflow for cooking, I can tell you that you can get beefy blowers that can mount outdoors. Such beefy blowers are commonly used for dust collectors.
There is a potential third option though, if you could manage to open things up enough to do it. It's possible to run two smaller ducts to achieve the same airflow as a single larger duct could handle. You just work things out by total cross sectional area of the inside of the ducts, which gets you close enough. I did something like this for my radon system here, where a single 4" PVC "duct" transitions to a pair of 3" schedule 30 PVC pipes to be able to fit inside of an existing 2x4 studwall. The two smaller pipes transition back to a single larger 4" pipe at the opposite end of the run. This will work with metal ductwork too, and can sometimes let you accomplish things like this (such as running a duct in each of two stud bays of a wall, then connecting them together at the floor and ceiling so that they act as a single duct of larger cross sectional area).
Bill
You can put 400CFM through a 6" duct but it won't be quiet. At 2000FPM, that is way too high of a velocity, you would need probably something in the range of 1.5" WG or more to do it.
If 6" pipe fits, there is a good chance a 6x8 or 6x10 would as well. This would get the velocity down and within the range of standard blower.
An in-line duct silencer is still a good idea between the blower and the hood even with a remote blower if you can fit it somewhere. These are pretty cheap and make a big difference in sound.
Also a small bulkhead to hide some slightly larger ducting would barley be noticeable and much better option in the long run than listening to a loud underperforming fume hood.
Thanks! It looks like we will have no option to open up the drywall so I think it’s going to be pricey to fix this and I am really annoyed with everyone working for me right now as this is much easier to figure out without finished painted drywall and cabinetry. It is really hard to see any world in which a 6” duct is ever enough for a *functional* kitchen fan that *actually works*, everything I see online requires 8” or 10”. Apparently the HVAC installer told us on site that only 6” could fit but I agree @Akos a bulkhead would have been fine as it’s just the mudroom (or even an entire dropped ceiling since it’s 9ft tall in there!)
My engineer just suggested this mixed flow inline fan for the 6” ducting: https://www.ruck-airmovement.com/index.php/fans/mixed-flow-fans/s-prime-evo-ec/sprime6evoec?productFrequency=60&filterFrequency=60
I am not sure what the static pressure would be so I don’t know where we’d fall on the fan curve, under 400 for sure but I guess it could be usable. The noise, however, is absurd so we would need a silencer and even then I’m not sure how quiet it would be. If it’s like 60 dBA for inline, how loud would it be at the actual range hood? This also involves opening up the ceiling to put the inline fan in and the silencer. And a different hood such as the Wolf one, plus modifying the custom wood cover. Yikes.
I haven’t seen any remote exterior blowers that can handle the high pressure and still deliver close to 400 CFM, this would be nice since it would keep the noise mostly outside. Seems like we’d still need a silencer though so would be opening the ceiling regardless.
The issue with the exterior vent part fitting only 6” I think had to do with the framing/sill plate and maybe also the siding. As you can see here, it is high enough in the joist so that it remains in the metal siding portion rather than on the wood.
Ultimately this seems fairly straight forward as you guys have pointed out, it’s a lot of air and a small duct — so I’m not sure what I expect anyone to say, other than “sorry for your (pressure) loss.” I need some sort of alien technology that defies our current understanding of velocity 🤣
Those open web trusses look like they had plenty of room for a 10" duct.
Don't think of opening drywall as expensive. It's the cheapest material in the whole building. It's about $3 per square foot all in. So redoing 30 square feet of drywall is like $90. And it's something contractors do all the time.
Ahh interesting! I recall pointing out the duct looked too small and my PM didn’t know what it was, figured it was 8” and it never came up again. There is a supply duct running perpendicular across though so I guess they had to stack them maybe that’s why 10” didn’t fit. Or my other theory is that only 6” fit for the exterior vent and they just decided to do the whole duct 6” since it sounds like the typical “it is standard to use 6” for range hoods” 🙃
I guess opening the drywall seems like a big production to me but you’re totally right. I think it’s more the cleanup and other stuff that’s annoying. Like having to rebuild or adjust the wood panelled cover for the hood insert. And really the time for my project manager to coordinate this, we’re talking at least a thousand $ just in his time I’m certain as he would need to go to site multiple times, talk to the drywaller, painter, etc.
I imagine when all is said and done, switching the hood and adding the inline fan will still cost $2-$3k and then we will have somewhere under 400 cfm (not sure where we would be on that fan curve with the 6” duct, surely not 1.5” WC?!?!) and it’ll still be loud as hell.
Opening drywall is really not a big deal. About the biggest issue is dust but that can be easily controlled.
You mention mud room. Another option is to leave that 6" duct and run a new duct through the mudroom. Most hoods can be rear discharge, so you can attach an 8" duct to the back of the hood, go through the wall and than along the top corner of the mudroom to the outside. This would also give you space for the remote blower and an in-line silencer.
Sometimes for these things, it is easier to build the bulkhead out of MDF. More expensive than drywall but doesn't need as much finishing plus since it is structural it doesn't need any framing so can be smaller.
That blower type your engineer suggest would work. The 8" one would easily do 400CFM with reasonable ducting and not too many elbows. The silencer could be a Fantech LD8 which is about the same size as the blower so should not make any bulkheads larger.
The drywall stuff is easy. Any contractor can open it up. They probably all have decent patching skills too. Back when I was starting out as a low voltage cable tech, I probably patched several thousand holes in a typical summer after we finished fishing wire. I got very good at it. It's fast, and it's pretty easy, just using some scrap drywall, a plunge saw, maybe some scrap furring strips and screws for larger holes, and mud and mesh tape. The hole can be closed back up AND MUDDED in one day, then prime/paint the following day. The biggest time consumer is just waiting for stuff to dry between coats. Don't let anyone try to make a big deal out of some drywall repair work.
I agree a larger duct looks like it would have fit. You probably ran into some of the "we always do it THIS way" attitude with someone. That's BS, but it happens. If the prints show an 8 or 10 inch duct, I'd lean on the GC a little to get this done, ideally on their dime. You'll be a lot happier in the long run, and it's not going to be a huge deal to fix it right from the beginning.
Bill
I wanted to relocate thermostats which are in the middle of the wall at eye level and preventing hanging art in spots, apparently it is nearly impossible due to drywall. Haha.
I’m getting the sense we need to find a better solution that actually provides the 400 CFM and is reasonably quiet. My conclusion is that this is simply not possible with the 6” duct so we either find a way to make a bigger duct fit or perhaps your idea Bill of running another small line.
I don’t really want to settle so I will instead “lean on” my GC now. I just know everyone will make it seem like I’m so unreasonable because we could just pop the “300” CFM Ventahood (exact same physical unit with different blower inside) and call it a day. I am just a girl, after all, what do I know about basic physics 🤣
That's pretty lame on the part of the GC. The hole in drywall for a thermostat is about the size of a dime, maybe a nickel.
I get that there's a balancing act around keeping the project moving and redoing work that's already been done, but that's a miniscule change to make the client happy.
Moving the thermostat is not a big deal. Drill the new hole wherever you want it to go, ideally in the same stud bay (so mostly move up or down vertically, but you have about a 15" wide horizontal cavity too in many/most cases). You can splice the wires with the little gray or blue size wire nuts and since it's low voltage (24 volts is standard for thermostat wiring), you don't even need a box. You can patch the old hole with one of the mesh patch kits, or put a piece of furring strip in and secure it with a screw on either side and one screw for a drywall circle to fill the hole, sometimes mud alone is enough. Mud the gaps -- tape isn't usually even needed for small patches like this. Paint.
The part of moving a thermostat like this that takes the longest is (1) waiting for the mud to dry (use the "EasySand 5" for this and that problem goes away), then (2) waiting between coats of paint. I personally have done this sort of thing so many times that I can do the entire thing, AND both coats of paint in ONE day -- and that's while doing other stuff between waiting for things to dry. You can use the cheap disposable foam paint brushes to feather the paint around the patch too -- use it like a brush in the larger areas, and as a light blotter on the edges. With practice, you can make a "perfect patch" like this that is nearly impossible to see.
Don't let the GC BS you. Remember that the GC works for you. It's the GC's job to keep the project moving, to coordinate between trades, and to make sure things are BUILT TO PLAN.
Bill
Interesting. In the time it takes for me to complain I could probably just move the thermostats myself and repair the drywall.
Back to my range hood crisis - the primary issues I’m thinking of right now are:
1. What is the most efficient way to reduce the velocity? > 2,000 fpm is absurd so we know more ducting is required. I like the idea of adding another line with a Y and maybe an additional 6” exterior vent.
2. Do we keep the Ventahood BH240PSLD (“600” CFM) or do we switch to a Wolf hood with a Ruck inline fan and silencer?
My engineer agreed adding another 6” line could work. He recommends: “ I would run 10" duct from the range hood to the 10" silencer then 10" down to 6" for the Ruck fan and then Y into (2) 6" duct and have two 6" outdoor hoods if there is room. The 10" silencer is 36" long. Could use an 8" if the 10" will not fit.”
Given the terrible pic of framing and duct it’s hard to know, but my sketch below shows where I think the duct currently is in PINK and then the RED is a rough idea of where it could maybe come through as 10” before transitioning to 2 smaller 6” ducts (including 1 of them ending where it already exits the wall).
We have some extra space in the mudroom side directly behind the range hood where we could add a bulkhead of sorts (and access panel). The tall cabinet is already on site but the wood bench/shelves are not yet built. Maybe this does nothing, but it’s a big area available so I thought it could either be some kind of plenum space to slow air or could at least house the in line blower and silencer since they are large. This space is about 72”W x 20”H x 24”D.
Noise tends to come from restrictions to flow. You want to minimize restrictions, that means keep the run as straight as possible and minimize bends/transitions/etc. Also, two 45 degreen fittings is usually better than one 90 degree fitting in terms of flow restriction and noise.
If you want to parallel two 6" ducts, you want to keep the two parallel runs as close to identical as possible. That means the same length, and the same number and type of bends. Ideally you run the two runs right next to each other along the same path. The idea is to make each duct present the same backpressure so that they share the flow equally. For areas, a 10" duct is about 78 square inches, an 8" duct is about 50 square inches, and a 6" duct is about 28 square inches. That means two parallel 6" ducts is pretty close to the same effective cross sectional area (and the same resistance to flow as a result) as one 8" duct. There is a little more to this than that, but it's close enough for our purposes here.
I'd skip that Ruck fan and just go from the 10" or 8" duct directly to the Y fitting, then keep the two 6" runs parallel to each other as much as possible. You should be OK using two seperate 6" vents at the exterior wall too, one for each duct run.
I'm not a fan of Wolf kitchen stuff. Yeah, it looks nice, and BIG, but I think you spend an awful lot of money just for them to put that "WOLF" tag on their stuff...
Bill
Yes, I realize now the duct conflict with the supply line is probably because my GC put the joists backwards so they had to alter the HVAC design on site and we did not have this on paper on the drawing or approved by the engineer. Devil is in the details, the exhaust was just supposed to go straight out the wall and would have fit 10” no problem.
I also find Wolf to be a total rip off, I think the liner itself is like $2,600 maybe excluding the blower. I know the 600 CFM remote exterior blower is $1,250 and they don’t even provide a fan curve and for example the Ruck ones are like $500. I failed to find another hood insert well designed with a large capture area ~ 40x22 and then some containment volume like angled baffles or how Ventahood has done it. Even Miele looks wrong. It seems we can get away with a lot lower CFM (i.e. 300-400) with a large well designed + more ‘pro-style’ hood compared to a little dinky one where we would need much higher airflow for the same effectiveness. I’ll take another look, there was one someone recommended previously on here I lost track of (something ‘Connect II’ was the name).
Think I’ll just give up now as there have been complaints about my micromanaging on the project 🤣 A common construction trend it seems… everyone wants to do their job how they’ve always done it, without any standards or accountability. I find those who are beyond the point of learning are beyond the point of being employable, by me anyway. That is something I love about this site - so many of you with expertise in various areas and willingness to share your knowledge + experience, yet still with an obvious openness to further continued learning!
Thanks again everyone - I’ve really enjoyed learning a bit about this stuff even if it’s sort of for nothing because nobody will listen to me. Surely my new passion for HVAC and building science will come in handy one day, hah.
Have you looked at the Zline remote blower vents?
No!!! I don’t know how! These look great, as far as the actual hood design - especially the ones with baffles at front and lights at back. I will see if it is available here in Canada.
I don’t see any actual blower details on their site so I’ll have to dig, like for the fan curve. I wonder if it could be tweaked to reduce the blower as 700 is too much. That being said maybe 700 rated is going to still end up more like 400cfm anyway with all the static pressure.
The only way we can improve on building performance is by collaborating a bit and sharing ideas that work. My own philosophy here is that if I help people out and teach a little, we can all get better together. I don't like the engineers that like to lord over the contractors. I've helped a lot of my guys with license upgrades and things like that, and they all know they can always come to me and ask a question and get an answer. I've even incorporated some of their ideas into the plans before. Construction is really a team effort, and ultimately the job of EVERYONE involved in the project is to deliver a building to the owners that will perform as expected. Unfortunately, a lot of people don't seem to operate like that.
Anyway, it sounds like your GC played a bit fast and loose with the plans. They are not supposed to do that in ways that interfere with the engineered parts of the plan, since that stuff was DESIGNED and APPROVED a specific way. It's best to drop by the project periodically to try to catch that stuff early. I try to visit every job AT LEAST once a weak, ideally twice a week, to try to catch any potential problems early when it's still relatively easy to fix stuff. Sounds like you're past that point though so you're stuck.
That "joists installed backwards" could potentially be more of a problem than just ductwork. You've mentioned a few times that you have an engineer involved with the project. I'd ask your engineer to double check that the changes with the joist layout isn't going to introduce any structural problems. Not all structural problems are obvious (things like shear walls can be difficult to understand sometimes, for eaxmple). Best to play it safe here. The general rule in the trades is "don't F with the structure", which basically means don't mess up structural stuff that could potentially cause things to collapse.
I'd see if you could find a commercial style hood that has no integrated blower and use that remote blower you found. That might be your easiest option. You could also get a metal fab shop to fabricate a custom hood for you out of stainless sheet, which might not be as expensive as you think, but you will need to provide the fabricator with a design for what you want. A lot of that stuff is probably going to be made from 430 stainless (that "430" is a number specifying the alloy). 304 would be more long-term corrosion resistant, so it would stay nice and shiny more easily for longer, but it will cost more. I don't think 316 gains you much here, unless there is a lot of welding to do, and it costs even more than 304. You probably want to use 0.050" or 0.060" thick sheet if you go this route to have stay solid-feeling and not make noise while in use. Thinner grades are more prone to resonances, and bracing is more complicated when working with thinner material.
Bill
Thanks Bill, much appreciated 😊
I really like the idea of designing a commercial style hood, that’s what I originally wanted to do, but I’m just too exhausted at this point so I think it’s looking like adjusting the ducting however we can to minimize the velocity as much as possible and getting a different insert with separate blower will be our solution.
Clients: can’t live with ‘em but you sure can’t live without ‘em.