myths about cfm's and mua
musky-hunter
11 years ago
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cooksnsews
11 years agodeeageaux
11 years agoRelated Discussions
Approach To Calculating Vent Hood and MUA Required
Comments (7)Once you decide on a cfm requirement (I won't address that in this message), you need to determine the pressure losses from the hood transitions in and out, its baffles, the ducting AND the residual negative pressure that you will have in the house with the intended MUA. These pressure losses are then used to look up on the fan curve the actual cfm. For this you need to choose candidate fans and get their fan curves (from Broan's website, Wolf's customer service, Fantech's web site, etc.). Hood and baffles may be worth a tenth of an inch of water by itself. The house may be a half a tenth negative. The duct losses will vary with length and bends. For example, achieving 1260 cfm will likely require a fan that, at zero static pressure, is rated somewhere between 1500 and 2000 cfm. As you add MUA (accounting for house leakage when there is residual negative pressure), the pressure loss in the ventilation fan decreases raising the cfm exhausted. This is clearly a recursive analysis unless you separate the functions by establishing a specific household pressure you are aiming for. At a specific pressure and outward flow rate, and given an estimate or measurement of the house leakage at that pressure, the MUA can be separately engineered. Don't forget the effects of any other household exhaust fans, dryers, furnaces, etc. As I recall noting before, household MUA is more difficult to engineer than restaurant MUA, and is more like large building MUA where the inlet and outlet flow rates, and even wind flow pressure effects, are always changing. Likely points of control are the MUA motor or a bypass flow path motorized damper. If the furnace has a separate (passive) MUA and is installed in a sealed room, then a passive MUA system for the rest of the house may be acceptable. Unlike a building, you probably won't have a problem opening a large exterior door against a negative pressure. kas...See Morehood 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 Morerange hood, cfms, muas -- what would you do?
Comments (3)Have you asked your GC or HVAC sub if MUA is actually enforced in your area? I ask because my municipality's code has the same requirement, but my former GC, who had never heard of the code despite installing many high CFM hoods, checked with our city's inspection office and was told that they have never enforced the MUA code. That doesn't mean that one should ignore the possible need for MUA, especially in a new build where homes are much tighter than older ones. I installed a 42" 600cfm hood over a 6 burner, 36" CC rangetop despite the usual btu-cfm formula showing that I needed a more powerful blower because I found a great deal on a gorgeous custom hood with blower already installed. I've been cooking up a storm with rangetop and hood for more than nine months now and have done fine. We entertain often; I cook daily from scratch; and I often use a few burners or more at once. Now, its not been long enough to notice any possible greasy build up on walls and cabs if that was going to happen. But on a daily basis, I'm doing well with fewer cfms than suggested. The fact that my hood is 6" wider than the cooking surface, as often recommended here, probably helps. If your hood will also be wider, I think you could get by with a 400cfm hood. I'm only a lay person, not a venting guru. :)...See MoreHigh Cfm Range Hood Make Up Air MUA
Comments (4)I suspect the 0.4 factor is intended to account for the fan/blower performance in realistic conditions, particularly conditions that correspond to the small amount of MUA that the formula requires. My comments here have usually assumed a factor of 0.67, but that approximation assumes that imperfect MUA won't lower the interior pressure more than 0.1 inch w.c., there is another tenth lost in the baffles, and a commensurate amount in the hood ducting. One-tenth negative pressure, however, is too much for combustion appliances with air intakes that aren't isolated from the kitchen, or for fireplaces. Note that the Wisconsin formula listed by the OP is for cases without such combustion appliances. If there is an intent to filter the MUA duct for anything smaller than a bat, I would at least taper out to a largish filter so the filter pressure loss will be minimal. A smaller sized furnace filter would do, or a window screen if large enough (perhaps two square feet). When arguing for a four-inch duct because it can move X CFM, one has to consider what pressure loss will occur at that CFM. I think six-inch would be better in this case unless the duct length will be very short. kas...See MoreNateman72
11 years agocooksnsews
11 years agoTrevor Lawson (Eurostoves Inc)
11 years agofoodonastump
11 years agodavidro1
11 years agomarcolo
11 years agokaseki
11 years agoSparklingWater
11 years agoNateman72
11 years agokaseki
11 years agomarcolo
11 years agoSparklingWater
11 years agoNateman72
11 years agokaseki
11 years ago
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