Yet another hood, CFM, and MUA question
srseverance
6 years ago
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Alan & Xiaoxiao
6 years agokaseki
6 years agolast modified: 6 years agoRelated Discussions
hood questions 101--not cfm or mua
Comments (10)The OP wrote: "Kaseki--So, perhaps the appropriate thing to say is that mesh filters are not as effective as baffles but can work if kept clean? Noise is a concern of mine. I don't want something that sounds like a jet on a runway (exaggerating a bit here). Perhaps baffles are the ideal compromise. " I thought what I said was they can be equal if clean, without explicitly noting that clean is a condition that may be brief. For the same flow rate, I would expect mesh filters to generate a bit more hiss. Fan blade turbulence noise will depend on the fan and other factors, but whatever acoustic level the tip noise is, I would expect little perceived difference to the cook between baffles and mesh filters, everything else being equal. "I know VAH markets itself as much more efficient--e.g. their 300 CFM will actually perform the same as someone else's 450CFM. Are people here in agreement with that statement? I guess if that were true, I might be willing to put up with a bit of extra noise and cleaning hassle. " This has been commented on here in many threads, including a recent one by me. It is unlikely the motors are more efficient (air moved per hour per kw-hr used). Grease collection fraction over the particle spectrum has never been published, to my knowledge, so comparative collection efficiency is unknown. The claim of being equivalent to a 50% greater cfm standard configuration is, in my view, likely to be a comparison of "apples to oranges." Normally, fans are specified as having a particular flow rate at zero static pressure, and as the pressure difference across them increases, the flow decreases. The relationship is typically plotted in a "fan curve." Respectable fan manufacturers will publish the curves for their fans. Broan has fan curves on their web site. For other examples, see Rotron and Fantech (published as tables). What I believe VAH are saying is that in their fan housing, their fan moves X cfm, but if it were in the open as other fans are rated it would move 1.5X cfm. Unfortunately, that is not the whole story. There are other pressure losses in a system, such as duct friction loss, turning loss, transition loss, filter loss, and, sometimes the largest, the lack of adequate make-up air loss (the house pressure goes negative). So, without a fan curve one cannot determine how much lower the cfm will be for a VAH in situ. Not that the situation is all that much better for the fans with fan curves, because the hood and baffle losses have to be guessed at for most hoods (commercial being an exception). And house pressure has to be estimated depending on what MUA system, if any, will be in use. As a gross rule of thumb, I assume that baffle hood systems normally configured without more than 0.03 inches of MUA loss will meet the VAH factor; that is, the actual cfm will be 2/3 the zero static pressure cfm rating of the fan. (Note that more negative house pressure will backdraft some combustion appliances and should be avoided.) kas...See MoreAnother vent hood mua question: please help
Comments (2)In San Jose, the link below applies. See C.1. Edit: Also, this link http://resaveguide.lbl.gov/step-1-assess-the-home Here is a link that might be useful: SJ ventilation requirements This post was edited by attofarad on Mon, Apr 22, 13 at 18:30...See Morehvac guy suggested 300 cfm vent a hood to avoid mua--thoughts?
Comments (31)It is hard to know where to start here. The flow rate (cfm) is determined by the uprising velocity of the cooking plume effluent and the aperture area of the hood, and not to first order by the BTUs. I would start at 90 times the area in square feet. Baffles will not separate grease from the air at really slow air speeds, so if one intends to go as low as possible, then a mesh that is routinely cleaned is probably better. However, mesh hoods typically have undersized apertures, so capture is degraded at the hood periphery. In other words, the hood is smaller than listed. Baffles will at any speed provide fire blocking, their other purpose. All fans have fan curves, including those made from magic lungs. The fan curve plots flow rate versus pressure drop across the fan, which results from duct friction, duct transition flow disruptions, mesh or baffle restriction, lack of MUA, etc. Typical fan curves are slightly convex, with cfm on the abscissa and pressure on the ordinate. When the pressure drop reaches some maximum, such as an inch or two of water column, the flow reaches zero. At zero pressure drop, the flow is (should be) the rated flow. The pressure drop is never zero in situ. VAH may be counting fan and hood, which can also be the rating used by some others at some times. It depends on whether the rating is for the hood with fan or for the fan only. The VAH rated flow certainly does not include the losses from the ducting and duct transition to the cap at the outside. Unfortunately, unless susceptible to a calibrated measurement, code enforcers will look at the fan rating and not actual flow for enforcing MUA rules. Ideally, they would test for negative house pressure vs. what combustion appliances present are not allowed to exceed without risking back-drafting. The relative loudness and ugliness of outside fans has to be compared to the relative social ugliness of loud inside fans. YMMV. I would not, however, duct to my neighbor's door. Some other path should be adopted. kas...See MoreStovetop wall hood - CFM and MUA advice
Comments (12)Thank you so much, kaseki,colbran, and weedmeister for your very helpful responses. Kaseki, the education you provided was phenomenal, and I really appreciate the time you took to explain the science behind the decisions. I had read the forums fairly extensively, but you have added new depth to the discussions. It is interesting - came home to read these responses after a 2 hour meeting with our contractor. One member of the team is urging me to get a hood that is 450 CFM and is dubious about the need for MUA. They also indicated that I didn't need the 36" hood for the 30" cooktop. (This is a very good company - considered to be one the best in town (I won't say where) and I know we don't have a commercial kitchen. On the other hand, we do use high heat to sear meats and to stir-fry. The person doing the construction diagrams did indicate that the information I presented and the questions I was asking were in fact consistent with safety standards (why you want a wider hood, MUA concerns etc). . It is an induction cooktop, which does reduce combustibles compared to our current gas stove, but I still want to make sure that we remove odors and smoke adequately. Because we live in a cold climate, we would need to address heating the air from MUA. From reading other forums, I get the sense that if I am running the a lower CFM hood at its highest setting, noise is likely to be greater than if I am running a more powerful unit at a lower setting. (Don't know if that's correct or just someone's opinion) I'll continue to read the sources you suggested to see if that is true. So far I can't find any dB information on spec sheets. Again, thanks to all of you for your responses....See Morefriedajune
6 years agolast modified: 6 years agoathomesewing
6 years agosrseverance
6 years agosrseverance
6 years ago
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