Range Hood Ducting

[zinfamous]

I'm going to be installing a new range hood soon and will almost certainly order this unit later today:

http://www.prolinerangehoods.com/index.php?main_page=product_info&cPath=32_45&products_id=176

(side-question: anyone familiar with Proline?) I've chatted with some local commercial distributors, gotten some quotes, spent the last couple of months online doing some shopping, and this pretty much satisfies all my needs for capacity, features, styling, etc. I've gone back and forth with inline vs remote vs internal blowers, and settled on internal. Reviews for Proline are...hit and miss it seems but I don't mind rolling dice.

The house has an outlet vent, ducted horizontally to the outside, in the kitchen about 60" above the surface of the range. It has been boarded up for who-knows-how long, but I've now uncovered it. After installing this hood, that would leave me about 12-14" of vertical duct length before I need to bend out to the outside duct. The recommendation seems to be minimum of 18" vertical before the first elbow. Added to that, the existing duct is about 4-5 inches off-center of where the vertical duct will exit from the blower.

I think the best option for me is to duct vertical with proper rigid ducting (hood and external duct is 8" diameter, so that's what I'm going with), then use flex duct for the elbow portion, such that I can snake up with the horizontal connection using two "softer" angles rather than two abrupt 90 degree angles. My main concern about this is reduced efficiency (inevitable?) and noise...but I'm guessing it won't be that big a deal.

Does that sound reasonable or anyone have a better idea? I don't want to go up through the roof, another 20 feet, or make another hole next to the current one (plaster, brick); and the current hole has a pretty nice cast iron fixture on the outside that I want to preserve, hoping that the outside duct/backflow shield will be able to poke through without having to replace the fixture.

I'm also avoiding installing an air-exchange baffle and sensor because: 1)I'm doing this install myself, and no one can stop me. 2) I don't think I have the space to work with one through my duct run and 3)I don't have forced air, anyway...so it wouldn't really work, right? ...but I do want to install a simple, inline spring-loaded baffle that adds the backflow barrier when the hood is not blowing out...this is pretty standard, right?

TIA

 

[Humpy]

Seems the only other option would be to tilt the vertical section right from the hood towards outlet. I suspect this would be more efficient and you could use all rigid duct. Such a short run with 8" duct there won't be much difference either way.

 

[herm0016]

can you use an adjustable at the bottom and tilt it at the angle to the hole, then a 90? like humpy is suggesting.
our hood is an island. i had to use about 6 inches of flex to get it connected, as the roofers put the roof vent in a couple inches off, and its vaulted ceilings so i had to do the correction in a small vertical space.

ill have to look at these for the next remodel, going to need a 48 island model soon.

 

[zinfamous]

Ah right, I was considering tilting first at the hood with a straight run, but then forgot about that. You can only get rigid elbows at 90 degrees, right? ...I was also considering chatting up a local welder/metal forger to see if they could custom make a run for me...Those adjustable's are rigid, but you can select a range of angles, right? Are they ridged on the inside? I was looking to keep a sheer surface all the way through the interior (though I guess the flex duct won't allow that, anyway)

My one concern with doing it that way was that I wasn't sure if an angle right out of the hood, even if ~30 degrees, would seriously wreck efficiency and generate more noise.

This blower has a 4 stage from 400 up to 1k CFM. For the most part, my more normal heavy needs would be around 600 CFM, (going by maximum BTU of range) but I can see myself demanding up to 1k when I have my cast Iron scorching at 600F and another pot or two boiling next to it...and maybe the Wok at the same time. I'm crazy sometimes.

 

[herm0016]

The adjustable ones are rigid inside and out, they have a couple collars at an angle, you turn them till you get what you want.

similar:

https://www.grainger.com/product/3F...3026!&ef_id=WbFZoQAAAT7jGTDh:20180307193203:s

tape the seams and wrap it in some car sound deadener if you are worried about noise. I think you will take less hit in cfm / noise with this than with any flex in the run.

 

[zinfamous]

The adjustable ones are rigid inside and out, they have a couple collars at an angle, you turn them till you get what you want.

similar:

https://www.grainger.com/product/3F...3026!&ef_id=WbFZoQAAAT7jGTDh:20180307193203:s

tape the seams and wrap it in some car sound deadener if you are worried about noise. I think you will take less hit in cfm / noise with this than with any flex in the run.

Ah awesome. I'll take a look at those. Thanks! ...hopefully they can be found in 8" diameter.

 

[Scarpozzi]

I think you should remember that venting requirements definitely have peaks and valleys. Even if you don't max your efficiency, it'll still move ample air and clear the room.

Depending on how long the run is, you can get 8" flexible ducts...if you're worried about noise in the line, you could go for insulated 8" line, as used with HVAC systems. If this were a dryer vent, I wouldn't recommend flexible lines because they typically get lint build-up and you want the straightest, cleanest walls you can get in those cases....for bathroom and kitchen vents, you're find with flex pipe.

 

[Greenman]

Don't use flex duct, it catches grease and dirt. Use the adjustable elbows, they make them in 8". There should be a back draft damper on the hood already. A second one may be of benefit if you live in a frozen wasteland (that's anywhere that it snows). Be sure the wall outlet is screened or you'll have vermin coming in your hood. Once you have the duct through the wall, spray foam does a bang up job of making it air tight. Also, be sure you have a good seal at the edges of the wall outlet or you'll get water dripping in behind it causing rot.
Post a pic if you get stuck on something.

 

[zinfamous]

Ah spray foam. Another product that I love, yet for whatever reason I didn't think about here. I had to pry out some fiberglass insulation behind the plaster wall and the little drywall sheet that was screwed over the wall duct, and was just going to shove it back down there.

Ordered the hood, and it should be here some time next week. With my efficiency, I will probably get it up by June!

 

[Greenman]

Spray foam is a gift from the Gods, and a pretty serious disaster if you make a mess with it.

 

[NetWareHead]

You generally want the straightest line. There is a formula used to calculate the length of a duct run and every elbow or other fitting adds an X amount of equivalent inches. Which makes sense as elbows represent a flow restriction. Avoid flex duct as it catches dust, lint and especially in a range vent where the air will be grease laden and will slowly build an obstruction in the flex duct. Use less acute angles to duct to where you need to be, a 45 or 60 degree is arguably better than a right angle 90. Also, the more bends, the more noise the air will generate as it makes its way through the ducting.

What is the air exchange baffle used for? Didn't see any reference to it in the manual you linked to. Is this to prevent air infiltration when the unit is not running or to provide a makeup air function? That's a serious range hood you have there for a residential kitchen.

Spray foam is a gift from the Gods, and a pretty serious disaster if you make a mess with it.

Ugh, so much truth right here. Wear disposable gloves when working with it. And have acetone handy to clean up any spills before the stuff cures. The first time I used it, I made the mistake of using my fingers to trowel/shape it into gaps and it cured all over my hands. My hands had yellow cheesy looking hard foam bits all over (with embedded dust), it is tenacious and very persistent. The skin on my hands looked diseased for over a week while I carefully scraped it off every day with a utility knife.

 

[zinfamous]

You generally want the straightest line. There is a formula used to calculate the length of a duct run and every elbow or other fitting adds an X amount of equivalent inches. Which makes sense as elbows represent a flow restriction. Avoid flex duct as it catches dust, lint and especially in a range vent where the air will be grease laden and will slowly build an obstruction in the flex duct. Use less acute angles to duct to where you need to be, a 45 or 60 degree is arguably better than a right angle 90. Also, the more bends, the more noise the air will generate as it makes its way through the ducting.

What is the air exchange baffle used for? Didn't see any reference to it in the manual you linked to. Is this to prevent air infiltration when the unit is not running or to provide a makeup air function? That's a serious range hood you have there for a residential kitchen.

Yeah, I'm going to try and avoid 90 degrees if possible, not sure if I can though, because the outside duct is already horizontal. I'm going to see if I can make a run that tapers off earlier so that it can be more of a 45 or 60 degree. I was thinking of getting a simple spring-loaded, or whatever baffle in the line that simply creates a barrier to the outside for when the hood isn't running. If it does have one installed already, then I won't bother.

I was thinking about getting a make-up air exchange, but that involves math and county regs, investigating air flow in my house (old and porous like cheese...so I don't think I need one, right? lol).

I like the ss baffles because I don't want to deal with mesh carbon filters. I'm considering getting a wok mon so that I can create a 2 foot flame tunnel and stir fry at ~700F on wok, + all of the searing I do with my smokey cast iron. I think I will actually have some need for the 1000 CFM from time to time. Plus, this was a bit cheaper than the quotes from the local guy (A very similar Broan unit, but at max 600 CFM and $300 more and, well, a Wolf unit that is not in my market)

Ugh, so much truth right here. Wear disposable gloves when working with it. And have acetone handy to clean up any spills before the stuff cures. The first time I used it, I made the mistake of using my fingers to trowel/shape it into gaps and it cured all over my hands. My hands had yellow cheesy looking hard foam bits all over (with embedded dust), it is tenacious and very persistent. The skin on my hands looked diseased for over a week while I carefully scraped it off every day with a utility knife.

Yep, did the same thing. stupid hands, always doing their own thing.

 

[zinfamous]

Funny what you really end up with when you get things in place and see what you really have to work with.

So with the hood in place and the outlet vent on the surface actually sits a few inches off the top, I didn't have any distance to make a single straight run, lol. the 8" duct is so large it just barely fits in my space with the ss chimney soffit. I decided to forego the sidewall exterior vent, because removing the cast iron fixture out there on the wall, about 20 feet off the ground, looked to be a task that was not going to happen. The fixture has a circular baffle that rotates on a pivot when air is blowing out, and does prevent backflow, but it is leaky around the sides. (I'm sure it is original to the house...so about 85 years old. mostly made of rust) I have a plan to shore that up with some weather stripping along the inside bottom, so that it rests against the stripping when sitting flat and is not restricted from venting (bottom swings to the outside), and some stripping along the top rim on the outside.

So, it vents straight out and into the world now. The entire duct run is composed of 2 8" adjustable elbows taped together, and angled at 45-60 degrees each towards the iron outer vent. I went a bit nuts and used spray foam from the outside and the inside to "mount" the final elbow within the exterior iron vent (that duct is about 8.5", so there is a bit of a gab between the two that needed to be sealed and stabilized. So...spray foam! I hope I didn't wreck things. I'm going to need to do some shaving on the outer bit to make sure it isn't restricting the baffle and not coming out and into the vent (it kinda is).

Even with the two elbows, there is a small angular gap going into the top of the hood. Flex duct wouldn't really work along that line, so I got one of these 8 inch starting collars

https://www.homedepot.com/p/8-in-Starting-Collar-Take-Off-Snap-Together-SCF8/100396968

the elbow into the hood sits on the surface of the outlet vent on one side, but it leaves a large, expanding circular gap. So that collar wraps around this gap and on the widest part of the gap, the collar just barely covers the gap from surface to surface of its height (the flappy tabs sit on the outside of the elbow vent.) We tapped the shit out of it, several times over. Every single junction. The elbow inside the wall and joining the second elbow is rather tough, because it is surrounded by brick and mortar and is difficult to maneuver.

Turned on the vent, and at low speed setting, there is a bit of knocking noise in the fan after about 10 seconds. Almost like something is clanging around in there, but it isn't because that would happen as soon as you turn it on, I think. When I first opened the box to inspect the hood and test the functions, we had it on it's side (back towards the floor, business end facing out), and we got the same noise. Shut it off, stood it up and above the floor surface, and it was fine. ...So I'm wondering if there is some sort of balance issue? The venting is still a bit leaky as I can feel a good bit of air leaking out, so we are going to go nuts with the tape again, but the final elbow that is now trapped in the spray foam, I don't know if anything can be done there. You can feel some air leaking out of the collars in the elbow pivots, but most of it is coming out of that transition piece. So I'm not sure if it is a balance/leveling issue (left side is ever so slightly higher, about 1/20th of an inch, maybe), or air leakage, or both. I'm inclined to go with the air leakage, because after placing the baffle filters and testing the hood and all settings, the knocking stopped, so I think it's an issue with creating the proper draft as it is designed to do...meaning, I really need straight runs and no air leakage.

Man, I need to find a way to post photos. Stupid death of bbzzdd has got me down.

 

[NutBucket]

Imgur is what i use now.

 

[zinfamous]

Imgur is what i use now.

Yes, I need to get around to that...because I can't imagine anyone wanting to read that dribble I posted. It makes sense to me, but I doubt it makes sense to anyone else.

Anyway, we figured it out! I think. After playing around with the exterior damper and discovering that we could use it to control the knocking noise in the hood, and the sound of the fan, we realized that the way that damper works was creating a vortex there at the exit, forcing air back into the vent if it wasn't strong enough to push out.

The knocking noise is the damper within the vent hood, basically flapping about as it battles with the backdraft created by that vortex, on lower fan speeds.

...so, realized that we could remove the plate including that rusted out pivot damper, and replaced it with the proper exterior "down vent" that I had purchased, with the typical top-mounted, springed damper flap. Damn thing works like a charm, now. we shaved away the foam around the final elbow duct, then shoved the sidewall vent cap into and around the elbow, using it to cut away the loose foam. sucked all the loose bits up with the shop vac, then used some of that watery duct sealant to seal the junction with the duct and the outlet cap.

...I think this stuff takes about 20 hours to cure, and so of course it is cold, rainy and snowing for the next three days. So I'll probably wait until Thursday before I turn the thing on again.

Once I get imgur set up, I will actually post pics, somewhere.

 

[Micrornd]

Ugh, so much truth right here. Wear disposable gloves when working with it. And have acetone handy to clean up any spills before the stuff cures. The first time I used it, I made the mistake of using my fingers to trowel/shape it into gaps and it cured all over my hands. My hands had yellow cheesy looking hard foam bits all over (with embedded dust), it is tenacious and very persistent. The skin on my hands looked diseased for over a week while I carefully scraped it off every day with a utility knife.

Just an FYI, but here in FL, spray foam is available in a latex base. Soap and water clean up.
It is not quite as rigid as the yellow stuff, so it does allow some movement, which could or could not be an advantage/disadvantage.
And I've only found it in the "std" expansion rate, no low or high rate.
But until it hardens, you can also dissolve it with water, if needed