30 inch hood for 6 inch ducting
Comments (12)
Related Discussions
6 inch to 4 inch Reduce - 1.5ft duct
Comments (1)If you are happy with something under 300cfm, and more noise, you can use the 4 inch. You aren't going to 600cfm, even with 6 inch. If you have a fan curve (cfm vs. static pressure), a calculation can be made. Do you have a make/model for your hood/blower?...See More36 inch range hood 6 vs. 7 vs. 8 inch duct?
Comments (16)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....See More10 inch duct through 7 inch hole?
Comments (34)Thank you @kaseki and @robertgoulet for your insights and experience. I did some sound test last night (when it's super quiet). At maximum blower speed, I measured 65dB near the control knob both with and without the baffle. That suggests I have more than just baffle hiss at play here. I had a HVAC contractor do the 10'' ducting to roof, but the connection to the hood was made by another appliance installer, which, from what I can tell, did a sloppy job -- I can visually see kinks in the connection point when I look up from under the hood. The LD10 silencer just arrived and I'll have it installed in next few days. Will report back whether/how much it helps....See MoreHi Folks; What are the high CFM 30 inch hoods that are 27 inches deep?
Comments (6)Out in the open, the effluent plume from a pan (aided by gas combustion products around the pan, where applicable) rises and expands. Most of the plume (for hot plumes) lies within a 10-degree from vertical cone extending upward from the pan edges. As the hood/cooktop configuration becomes enclosed by a back wall, and partially by cabinets or side skirts (primarily commercial), the hood effective aperture is partially extended downward and forward. This brings some of the entry air due to the hood system blower to be somewhat brought down and forward. With sufficient enclosure all of the hood air will be pulled from the cook's position. But also to be considered is that air velocity away from a hood entrance falls rapidly. So when a hood just overlaps a front burner pan, even with partial cabinet enclosure, plume capture is chancy. If unsuccessful, there will be some uncaptured plume. This is not the same as hood spillage, where capture is initially successful, but plume momentum causes some plume reflection out of the hood due to insufficient CFM for containment. So if forced for some reason to use a hood that isn't ideal in front burner overhang, one has to compensate by cooking the greasiest stuff on the back burners. Of possible value would be generating a side view of the configuration with burner locations marked and various pan sizes marked, with 10-degree off-vertical lines intersecting or not the hood aperture. This may inform you of what pans to use and what you can cook in them. Ultimately, though, experience with the hood will provide a refined understanding of capture performance, because gedanken experiments with nominal plume characteristics will not address every fluid dynamical effect that occurs in practice....See More
Buehl
3 years agolast modified: 3 years agokaseki
3 years agoPlethora of prolix postings kaseki here. We note that 600 CFM through a 6-inch duct is 50 ft/s air speed. One would need a three-phase 400-Hz aerospace ducted blower to achieve this (depending on duct length). (See Rotron.) And the whine of this type blower would exceed even the noise of the duct flowing air at this velocity.
So, as usual, we recommend starting at the requirements and working toward hood specs, then toward a hood. For wok cooking, even on induction, a hood intake air speed of 90 ft/min is desirable. This is 90 CFM per square foot of hood aperture, which itself needs to overlap the pans side-to-side and front-to-back by enough to encompass the rising and expanding cooking plumes.
For example, a two x three foot hood aperture would require an actual 90 x 6 = 540 CFM. But wait, there's more -- a lot more. The hood or in-line or roof blower CFM rating needed for this varies with the pressure losses (air friction) that the exhausting airflow meets leaving the house, and which its replacement air meets getting back into the house. Without analyzing these, a guess at a needed blower rating is 1.5 times the required air flow rate, which gets us to 540 x 1.5 = 810 CFM. Call it 800 CFM blower rating (zero static pressure).
The duct velocity should be in the 1000 to 2000 ft/min range, not the 3000 ft/min one might hope to get in a 6-inch duct. Go for 8-inches of steel duct for 1550 ft/min. (Recall that we are making an assumption about the hood, so correct these numbers when the hood is selected.) 10-inch duct is even less restrictive, but more difficult to work with and it takes up more attic room.
Consider a external (e.g., roof) blower with an intermediate silencer (Fantech) for a reasonably quiet full power hood.
Hood should have baffles for filtering at these velocities, and these will function even when dirty.
A means of providing make-up air is necessary to even achieve the 1.5 rated flow rate factor. Screened open windows will probably have low enough pressure loss, where such is allowed. However, for those reading this that have combustion appliances in use, sufficient MUA is mandatory for safety against back-drafting carbon monoxide.
It is helpful, particularly with wok cooking, that the hood have some reservoir volume under the baffles. But this is usually slight for residential hood offerings. The attached image shows the underneath volume of a Wolf Pro Island hood as an example. (This works for me. Under the distant end of the hood in the image there is a Cooktek induction wok hob.) A completely flat array of baffles is not desirable, but may be needed if there is some reason the hood has to be "thin."
Other aspects of residential hood selection involve aesthetics and price, but I won't delve into these. Hoods are fairly simple things, as are conventional blower motors and fans. One can add electronics in proportion to how much one wants to degrade the mean time to failure. I can note from experience that if the hood has halogen lights that will be over one's head, choose a hood that allows replacement with available LED lamps.
Kay Thompson thanked kasekiKay Thompson
3 years agolast modified: 3 years ago
opaone
3 years agoopaone
3 years agoKay Thompson
3 years ago
Charles Ross Homes