Vent hood ducting: Multiple air flow and noise related questions

2 years ago

last modified: 2 years ago

Hello! I am installing a new vent hood with an inline blower. Based on multiple factors, including aesthetics, the vent hood will have 8 inch ducting, but the inline blower is set up for 10 inch ducting. I have been mulling over multiple options for the ducting, with goals of achieving as good of air flow as possible and minimize noise as much as possible. The vent hood installs straight into the attic, makes one 90 degree turn and then out a side wall. Regardless of my choices below, I plan to try to minimize noise by hanging my inline blower from the rafters, using an inline duct silencer, and wrapping some of the ducting in noise suppression materials. Here are my questions:

1) Is it better to go the long way (30 feet), so the inline blower is as far away from hood as possible to reduce noise, or can I go the short way (10 feet) to improve airflow? I would prefer to go the short way, if the noise would be okay, because it is less obstructed (I might need to jog a little going the long direction) but both are doable. I have heard that the blower should be installed 3/4 of the way down the run, but have not found any reason for that. Particularly in the short direction I would want to put it as close to the outer wall as possible.

2) Duct silencer: should I use a straight silencer such as the Fantech LD10 or can I use this elbow duct silencer (https://www.hvacquick.com/products/residential/Kitchen-Exhaust/Silencers/HVACQuick-CSL-Series-Elbow-Duct-Silencer), which they claim is superior to the straight silencer for noise reduction. The elbow duct silencer only comes in 8". I would place it immediately after the hood and then use the reducer (upsizer?) to go to 10" ducting. My concern is whether I might be sacrificing too much airflow by reducing early. If I use the straight silencer, I would probably use two 45 degree angle pieces to make the 90 degree turn.

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Comments (4)

kaseki
2 years ago
First, given a normal configuration, the baffle assembly probably causes the most restriction (pressure loss). Next would be imperfect MUA, such as a screened window or other path not pressurized by a suitable blower. Ducting might be third.

To reduce duct turbulence noise the silencer should be closer to the hood than to the blower to deal with the intermediate duct turbulence noise.

Elbow silencer link doesn't seem to get to any actual data. Fantech provides attenuation values vs. frequency. If you can get some data, comparison may help with selection.

Ideally, you will want to expand to 10-inches as soon as possible.

You didn't report actual flow rate you are trying to achieve.
Jeremy K
Original Author
2 years ago
Thank you, @kaseki. In answer to your comments, I will be using a passive MUA system with 10” ducting. In addition, our home is not particularly airtight. Hopefully, that won’t restrict too much. The in-line blower generates a maximum 1000 cfm. The hood catchment area is 48x27”. although it looks like the baffles do not extend all the way to the end, so maybe 42x24. I was leaning towards using the straight silencer, and I could not find any additional data on the elbow silencer, so that answers that question. Any thoughts on distance for the placement of the in-line blower? 10 vs 25 feet? Will it make any difference if I am using a silencer?
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kaseki
2 years ago
last modified: 2 years ago
I have forgotten the exact pressure loss of the LD-10 silencer I was told when I asked Fantech for the value (at some flow rate). However, I do recall determining that it was too small to impact my overall system design.

For silencer placement, consider the overall duct configuration from hood to roof. Where on that path is there a place where the silencer can be hung from the rafters and allow access without needing the butterfly stroke swimming in fiberglass. Pick the part of the reasonable location zone closest to the hood.

The baffles need not fill the top of the reservoir under them so long as the reservoir depth is sufficient (and airflow rate sufficient) to keep the plumes entrained into the baffle gaps. Commercial hoods have baffle areas a lot smaller than their entry apertures, but have large reservoir volumes under them.

Assume an effective capture area of 24 x 48 or 8 sq. ft. I would aim for 8 x 90 = 720 CFM actual, or a blower rated around 1100 CFM (at zero static air pressure).

Whether passive is OK for MUA depends on whether there are any combustion appliances taking air from a space connected to the kitchen. In that case the pressure loss in your heated and filtered passive MUA has to be less than the back-draft pressure of the appliance.

Second, the pressure loss has to be in your budget for your blower via its fan curve to still achieve around 720 CFM including baffle and duct pressure losses at 720 CFM. In other words, my giant WAG for scaling the blower rating to 1.5X the calculated air flow need depends on typical conditions and not restrictive MUA. You really don't want to feel air flow through your light switch plate edges when running the hood; it will pull dust and stuff into the house.
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opaone
2 years ago
A few thoughts.

- 1000 CFM needs a matching powered 1000 CFM MUA, as does anything over 400-600 CFM. Passive is not sufficient.

- Powered MUA is as important or more important in a leaky house. You DO NOT want to pull outside air through leaks in wall cavities and especially if outside humidity is very high as the moisture buildup will create mold and rot problems in your walls.

- The Fantech silencers are 0"wg

- My GUESS is that this elbow silencer will increase SP much more than a wide 90° elbow. This elbow is interesting but I'd stick w/ the straight silencers.

More: https://bamasotan.us/range-exhaust-hood-faq/
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Vent hood ducting: Multiple air flow and noise related questions

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Jeremy K
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Vent hood ducting: Multiple air flow and noise related questions

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kaseki

Jeremy KOriginal Author

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opaone

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Jeremy K
Original Author