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Appliances Remodeling Kitchens

Duct size for Ranged Hood 6" or 8"?

5 years ago

In the process of remodeling my kitchen and was wondering what size duct would be adequate for my range. Would the 6" suffice or would going larger to the 8" duct be recommended even though I might not use that much cfm?

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Comments (5)

Buehl
5 years ago
last modified: 5 years ago
Do you mean the duct for your rangehood? If so, what do the specs say? Most likely you will need 8", but some call for 10". The wider it is, the better the air flow -- and the quieter the fan!
Jim
Original Author
5 years ago
Yup Range Hood, the ones I have been looking at were Samsung and Broan which max go to about 600 cfm and recommends 6". But like you said, bigger might be better.
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vinmarks No, in your case I was tempted to write that the VaH can make use of that size duct. I think VaH may be negligent if they don't point out that that duct size needs to be matched on the MUA side. As pointed out earlier, few in the kitchen hood sales chain want to raise the issue of make-up air and the horror it can be to retrofit into a renovation.* But lest others think that they need 12-inch duct for 1200 CFM with conventional blowers in lieu of using 10-inch for typical lengths, I wanted to point out that that would be only a modest improvement relative to getting the MUA in with low pressure loss. Pressure loss is counted from the cooktop to outside and then from outside to inside back to the cooktop. ------- *An example MERV7 high flow 4-inch deep pleated filter, 2 ft x 2 ft in area used to catch the big particles and bugs that the MUA air could pass into the house, used at 1200 CFM (300 ft/min) would have a pressure loss of 0.1 inches w.c. (See table from https://www.texairfilters.com/comparative-tests-on-pressure-drop-of-hvac-filters/ If one looks into VaH data, for example see below, 0.1 inches w.c. drops the flow rate of a dual blower (1200 CFM) hood to 1060 CFM. At 1060 CFM, the filter pressure loss is a bit less, so one really needs to plot filter loss vs flow rate against the fan curve implied by the data below to obtain the expected flow rates (excluding ducts, MUA register, and exhaust duct cap pressure losses which also need to be counted) . And for comparison, using an equation I found on the Internet at the HVAC engineering forum referencing the 2003 ASHRAE Handbook at Page 52.5 (confirmed in my copy) and adapting it with some trepidation to screening as was done at the forum, and assuming a screen of 70% openness across a 12-inch duct at 1200 CFM, the pressure loss is even higher than the pleated filter, reaching 0.7 inches, w.c. With a 15-inch screened duct, 0.1 inches w.c. at 1000 CFM could be achieved. The moral is, use a lot of screen area, or low loss filtering, or let the sparrows in.
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Thanks for the response. I would think the CFM has just as much to do with the motor / blower as it does with the diameter of the duct. It's not like you can't push a larger volume of air though a smaller opening given the right amount of pressure. Anyways, here is the 900 CFM insert we are considering with recommended 6" ducting: https://www.prolinerangehoods.com/46-proline-pljl-wall-range-hood-insert-15-deep
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Yes, I meant 8-inch will be better, in general. Grease collection at various points is a complex matter. For example, one reason simmering bacon, in spite of its low upward plume velocity, needs a higher than minimum air flow is that the baffle centrifugal extraction process needs enough velocity to work. In general, however, hotter cooking generates faster plumes and needs higher hood intake air velocities. If the duct is warm enough and the flow rate not too low, the grease collection via condensation will be low. If the velocity in the duct is not too high, the grease collection via impingement will be low. Most home cooking levels are such that what does collect in the duct rancidifies (hardens to a varnish) and doesn't become a nuisance or fire hazard. Commercial ducts, however, need yearly cleaning and inspection. If performance is your priority, then review of hood threads here will provide insights into the requirements needed to achieve good capture and containment.
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The answer to the question is use the larger duct for as much length as possible. From the description, it seems that it could be 6-inches over most of the length, raising the question of why the last bit couldn't be cut larger. Note that duct transitions should be gradual and not abrupt. Your 400 CFM rated blower can only do some lesser flow rate when subjected to pressure loss, some of which is in the hood itself, some of which is in the make-up air path, and the rest in the ducting. Suppose you could actually achieve 200 CFM. Then 4-inch duct would have to be flowing 2300 ft/min, a higher value than desirable and likely to have more pressure loss than the 200 CFM assumption might have assumed. So maybe 100 CFM. You will be in the average condition of 1950s hoods, so all you need is a larger duct to move into th '90s. :)
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kaseki
5 years ago
There are (at least) two options -- (a) blindly following the hood manufacturer's recommendations and (b) deriving from basic principles. The latter approach follows:

Flow velocity at the hood entry aperture should be around 90 ft/min for good containment with baffles. I have no data for mesh filters. Ninety ft/min is equivalent to 90 CFM/sq. ft. of aperture area. Ninety times the aperture area is the total desired actual full power CFM.

The desired duct velocity is 1000 ft/min or more (but not over 2000 ft/min). A six-inch round duct has an area of 0.2 sq. ft. and an eight-inch round duct has an area of 0.35 sq. ft. So divide the result of paragraph 2 by each of these values and see which is closest to your goal.

Note that the blower rating has to be significantly higher than the result of paragraph 2 because it has to work against the pressure losses of (mainly) the hood filter, ducting, and make-up air. A factor of 1.5 may be adequate.

Given an already selected hood with a specified blower, I would take the 600 CFM specification and divide by 1.5 to get 400 CFM hoped-for actual flow rate, then divide by the two duct sectional areas to compare 8-inch with 6-inch duct diameters. The results suggest the 8-inch duct would be better.
Miranda33
5 years ago
last modified: 5 years ago
Install the 8" duct. No question. While you're at it, look at other hoods that spec an 8" duct. As far as I know, Samsung not known for making hoods, is it. Broan is, and offers many choices. A hood with an 8" duct means you will get a quieter more efficient hood, regardless of the cfms. Look for baffle filters, not mesh filters, and a canopy shape.
Jim thanked Miranda33
Milo Pompeii
5 years ago
I just told my HVAC contractor what I was getting and he did the math. The 10” duct is installed as well as the make up air system. In the next two months, they will install the cabinets and appliances. There will be a 36” Wolf rangetop with a 43” cabinet housing for a 40” x 22” hood liner with 900 CFM fan. Hoping it all works and they know what they are doing!

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