My guess is that an elbow on the output side of the blower has less of an aerodynamic interaction with the blower blades than one on the intake side, but overall likely about the same pressure loss as anywhere else in the duct. Of course, if the offset is acceptable, two 45-degree bends would be gentler.

Turbulence in the bend will add some noise, but mostly in the higher frequencies where the silencer should be able to deal with them.

Any beating between the blades and the possibly varying air resistance each sees as it rotates across the intake face of the elbow should be controllable with duct dampening material. (The air pressure on the sharper curvature inside of the duct elbow should be expected to be slightly lower than on the other side of the elbow by Bernoulli's principle.)

Can't speak to the theory or numbers but most free standing (island) hood arrangements have the blower immediately below the ceiling and a 90 elbow above that in the joist channel. So, yes, a 90 immediately after the blower and they typically have much more ductwork going to the outside.

At 10 feet above the hood, I doubt you're going to hear anything.

I think generally you want some bit of straight duct feeding in to any elbow. This allows the air to achieve a more laminar flow and reduces the static pressure losses of the elbow. I think that there is also less pressure loss for an elbow on the suction side. Elbows can also help with noise reduction. So I would put the elbow at the top of of your stack and then put the blower somewhere in the middle of the run to the outside.

If you do the elbow after the blower then I think you'd want a bit of straight duct between the two.

I would verify this with someone who knows this stuff better though. If you're using a fan tech blower I think they will give you good advice.

Here's a note from the installation instructions for a Broan hood with an external blower:

"Note: To achieve reduced sound levels at the range hood, the external blower should be installed at least 10 feet away from the hood location and include one 90 degree elbow in the duct run between the hood and blower." Source.

So based on this, you've got the 10 feet and should just put the blower after the 90 elbow.

I don't think from what he wrote that the OP has enough run after an elbow for the blower, but I'll leave that point to him to confirm or not.

While Broan makes a good point of turning the duct on the way to the hood, sound-reduction-wise, in this case the silencer should out-perform the "missing" elbow.

great comments/discussion everyone

Additional food for thought. I just remembered (dont know how this slipped my mind earlier) its actually not a straight shot out of the hood, the hood "flange" itself attaches at the ceiling to 2 reversed 45s. This jogs the duct to the side just enough to fit into the chase we built above. So its almost perfectly straight but not quite.

Also, I might have enough room in my attic to put the blower after the 90, but I am so close to the wall I would have to add a second 90....so the duct is running up along the "outside" wall...and when it gets to the attic its right there, just a 2x6 away from the sheathing....so if it were beneficial acoustically or functionally I could probably put a 90 on top of the stack to the blower, and then it would have another 90 on the other end of the blower to the outside.

One question I have about the "use 2 45s instead of a 90". Is there a difference I am not realizing between using 2 45s attached to one another instead of a 90? Or when you say use 2 45s instead of a 90, is the implication that there is *some* length of straight duct between the 45s? If the implication as I am assuming is that there is some length of straight pipe I assume any length is good? like 6-8"?

Notwithstanding the last bullet, two 45s connected to form a 90 degree bend have a larger radius of curvature and hence lower pressure loss. They do take up more space. They also allow slight angular and offset adjustments due to the extra number of rotational interfaces. (Note in this latter case I am referring to adjustable 45s.)

are the 45s you are referring to 45s....or 90s adjusted to be 45s? I went to the HVAC supply today and they didnt stock 10" 45s so I grabbed a couple 90s...which can be adjusted to be 45. This will definitely be a more gradual sweeping corner (but take up a lot more space). I think I can do one of these corners (2 90s adjusted to be 45 each forming one big 90 corner) from the vertical duct to the blower, but I wont have enough room to do a big sweeping corner to to the wall vent...ill prob wind up putting a single 90 there right out the wall.

I think that will be fine...minimal resistance in the (relatively) longer run from the hood to the blower, and then a tight 90 after the blower but that 90 is the *only* duct between the blower and the sweet open air.

I guess when I take a step back, the only reason I am doing this is in hopes that the 90 between the blower and the hood will reduce noise a little (as opposed to having a straight duct between hood and blower). I suppose I have nothing to lose and its only an extra 20 bucks in parts.

One other question for anyone who has used the fantech blower....I bought the rubber clamps they sell for either side of it. Does the duct not attach to the blower in a male-female manner (pieces of duct do with one another)? does the rubber gasket clamp onto the blower, and then onto the duct so that the metal from the blower than the metal from the duct do not make contact (thus limiting vibrations being transferred to duct work)? I havent studied the clamps (or the blower) yet but in my mind that would make sense....and if its not setup like that, and there is a male-female screw+tape attachment between the blower and the duct...i dont know what the purpose of those rubber clamps would be :)

I have used these clamps, and their first advantage is that one can remove the motor without having to shift an entire stack of ducting connected male to female. I have also used them for silencer connections. Second, some vibration reduction is plausible, limited by the rubber thickness. So, bring the ducting to the motor so it is barely gapped on each side and clamp.

Duct taping can be used before a clamp is slid over the gap and tightened. Don't use cloth-based duct tape on a grease bearing duct. There is a proper tape, although I can't recall right now what it is called.

Be sure the duct male-female connections are oriented so that grease flowing downhill stays in the duct.

Late to the party.

It depends on the radius of the curve. If you have room to use two 45s a short distance apart, that would be better than a single 90.

I think I am going to grab a couple of these:

https://www.grainger.com/product/MASON-Hanger-Mount-Vibration-Isolator-2LVR8

and suspend the blower from the rafters on threaded rod.

I assembled/screwed/taped the 2 90s (turned into 2-45s) this weekend and its definitely a sweeping corner which I imagine will be higher flow than a single 90 would have been. And I am picking up 1 more 90 which will shoot outside. Probably installing most of this next weekend (except for punching through the exterior wall which we may not be ready for yet).

Also test-fit the clamps...and they will work well as you described Kaseki...put them together so they are juust not touching, maybe wrap some duct tape around and then clamp it.

appreciate all the help from this group

A bit of a funny follow-up to this. We just finally finished the kitchen reno and moved back in and just today hooked up the blower and punched it through the wall in the attic.

So my question is...how do I know if this is working properly? Its so quiet.... Not joking, I seriously want to make sure I didnt mess something up with the wiring resulting in the blower not getting enough power :)

The wolf hood I have has 2 outlets in it. It was my understanding since I just have one blower that I could plug it into either and it would work fine...is that the case?

Assuming an outlet that fits the duct, then one outlet will do with the other covered or otherwise closed off. If your duct is bigger than the outlets, then best would be to have a HVAC place build a Y adapter.

How to test? I think that depends on how well you want to know the actual flow rate. There are people who determine house tightness by pulling a door and measuring interior pressure vs. the flow rate through a pseudo door with a calibrated blower in it. Conceivably they can measure the flow rate of the air moving in with their blower off. Edit: Better conceptually, they run their blower to make the house differential pressure zero, at which point the blower flow equals the hood flow.

If you have a complex blower controller you want to test, you can compare the apparent flow into the hood with the controller on max vs. just temporarily wiring 240 or whatever directly to the motor as a test.

A wind sensor held under the hood at the plane of the aperture could be used to determine air velocity and, with some checks to ensure it is relatively constant across the aperture, infer CFM from velocity and area. Your goal is 90 ft//min. over the entire aperture.

The ultimate test is if it does what you want. When you pan fry stuff is all of the effluent captured and exhausted by your hood? Are there any unpleasant odors remaining in your kitchen?

Cheap tests include holding your hand up to see how much airflow you can feel. Wetting the back of your hand can help. You can also stick a paper towel up against the filter to see if it sticks. These are EXTREMELY limited benefit though as they simply say that there is airflow but don't tell you if you have enough airflow.

For ours they have a bit of duct w/ a pitot tube mounted in it that they use for measurements so you could talk to a knowledgable HVAC person about doing that.

As Kaseki said, a wind velocity meter might do the trick. I've purchased numerous over the years for sailing, gliding and drone operation and they're fairly inexpensive.