very interesting articles on CFM and makeup air; 250 cfm suffices
scrappy25
10 years ago
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rovo
10 years agopalimpsest
10 years agoRelated Discussions
how many cfms for small kitchen but large area
Comments (11)VAH squirrel cage design blowers use the blower to impel grease particles onto the local blower surround instead of letting them pass by and condense on the ducting, where they could be a fire hazard. Hence they act as both a blower and a filter of sorts. Baffle designs separate the filtering function from the air movement function. Baffles allow the blower to be roof or exterior wall mounted, thereby reducing noise, among other modest advantages. My view, occasionally expressed here, is that the most efficient and effective hood designs are by necessity those used in commercial kitchens, which have a very high electricity bill due to ventilation needs. Residential hoods that emulate commercial designs are deemed to have some of their advantages, while usually providing tolerable aesthetics. It is of course always possible to made A look like B without B's benefits, so some care in selecting A is needed. I believe hood systems made by reputable sources will all work as described. Unfortunately, the descriptions are never complete. I am certain that my Wolf 1500 cfm rated roof blower with Fantech in-line silencer is quieter under my Wolf Pro-Island baffled hood at fullpower than a similar rated flow rate VAH would be at full power in the same sized hood. Clinresga on these pages a few years ago reported measuring two similar hood systems, one VAH and one non-VAH in two houses he owned, and found the then VAH noisier. This is because fan blade tip turbulence noise dominates, and can be suppressed when the fan is roof mounted, but can't when the fan is in the hood. Once that is suppressed, then the next dominant noise is baffle noise, and at 90 cfm per square foot of aperture using reasonably designed baffles this is only a hiss lower in amplitude than a wok doing its thing. kas...See MoreWhat CFM is your Range Hood ?
Comments (34)I had a 600 cfm downdraft installed behind a 30 inch gas cooktop & it seems to work fine. The HVAC guy that did the install raised his eyebrows at 600cfm as it really is quite a bit of air moving. (as a comparison a 1200cfm fan is adequate as a "whole house fan" in a 1500 square foot home). The HVAC guy's first concern was the house (1700 sqft) going to negative pressure when the fan is used. Negative pressure in the house is why local coding kicks in...with negative pressure your gas water heater exhaust gases flow into the house rather than out. If you have a fireplace then you could reverse the flow of any smoke/CO/CO2 from the chimney and pump the gases into your home. I had an outside makeup air vent with a barometric controller installed as a part of the DD vent install(an extra $280). I also had the 6 inch connection to the vent increased to 8 inch pipe for the entire run through the basement (8 inch moves twice as much air as 6 inch pipe) & this helps retain (not reduce) the cfm rating of your blower. The 8 inch was reduced to 6 inch just before we vented to the outside. Once the vent was in I tested whether the house went negative pressure with the vent on...even with the external makeup air installed the house is just barely positive pressure. Had I not installed the external makeup air the vent would have taken me VERY negative in pressure. I have a gas hot water heater, furnace, and run a woodstove(burn air is external) for winter heat. Positive pressure in your house is good...it'll keep you alive. You may want to discuss your vent cfm sizing & it's impact on the whole house with an HVAC person....See MoreVent hood question - make-up air turbulence, acceptable neg pressure?
Comments (13)This is an issue that requires some measurements and some analysis. To even know where you are you need to measure the pressure in the house relative to outside the house as a function of different settings of vent air flow and MUA air flow. Either hire someone to make the measurements or buy (cost is fairly modest, as I recall) a differential air pressure device. In test kitchens, the means of introducing MUA is via a perforated wall some distance from the test stove/vent air handling equipment (hood). What is desirable is that the MUA, when it gets to the air volume between the cooktop and the hood entry aperture, be relatively non-turbulent. Introducing MUA at one's feet next to the stove may or may not achieve this. Often the toe kick spaces there are too small, and may, depending on configuration, aim at some other structure (wall, island, whatever) that will force the air flow up. Now there is turbulent air all around the cook and the hood. I think the MUA exit area(s) should not be too much smaller than the hood entrance area unless the MUA injection point is fairly far from the hood. In a (completely burger odor free) burger joint in Concord NH there is a CaptiveAire system in which the MUA is expelled downward from the ceiling in front of the hood. (This aperture is actually part of the hood assembly.) I would estimate that the MUA aperture is roughly half that of the typical very large commercial hood aperture, although I cannot see all of the hood aperture from the customer seating area to be very exact about this. In my residential configuration, a 3 x 3 foot ceiling diffuser is used about 20 ft down a hall from my 10 sq. ft. aperture hood. This hall delivery should slow down the air velocity to about 37 ft/min, which is a slight breeze -- less than a half mph. I haven't pressurized the MUA yet, but I have seen the effect of an interior split cycle air conditioner head spilling air toward the hood, there is significant plume displacement. Is your MUA source via a basement, or via the roof? It might be better if you dumped the MUA at the ceiling directed away from the hood such that the air takes a longer path to the hood aperture. Or dump it into diffusers at various locations in ceilings connected to the kitchen. (This might be a casus belli if not heated.) Or use a lot more toe kick area. Besides measurement, I would check that with windows open, MUA off, and hood on, that you can cook something and see it fully captured and contained by the hood system. Then as you close up windows, and do whatever you are doing (manually?) to set MUA flow, does this start interrupting the capture efficiency? And yes, if you don't have a closed loop controller then both hood and MUA should have continuously variable controls. By analysis I meant making a gross estimation from the hood system fan curve and the properties of your vent system of what flow rate you may be achieving. (There are contractors who can measure this by temporarily replacing a door with a measurement device.) Ditto for the MUA system. What is its fan curve vs. ducting, filtering, diffuser pressure loss, and heating scheme, if present? If your actual replacement air is 200 CFM, then ideally you want the MUA to be 200 CFM less than the hood can pull when the house pressure relative to outside is zero. One expects that if the house pressure is positive, more air will flow out the hood system. But in your case I suspect, as did opaone above, that there is really a lot of turbulence around the hood and this turbulence is interfering with the cooking plumes attempting to rise to the capture area (the hood entry aperture). kas...See MoreNew Kitchenaid low profile OTR 500 cfm microwave
Comments (51)A major point raised often w.r.t. hoods but missing from this OTR microwave oven topic is blower performance vs. blower rating. Blowers are generally rated by the air flow that they can induce when hanging in free space. This is the zero static pressure volumetric flow rate (CFM). When one reads the rating of a Broan/Best/Abbaka/Fantech/etc. blower, this is the number given. In use with other equipment, whether hood, hood filters, duct, etc., or microwave oven passages, filters, duct or not, etc., the achieved volumetric flow rate is significantly lower. Blower manufacturers such as those listed above will provide graphs or tables of the effect of back pressure (pressure loss in HVAC terms) on achieved flow rate. Whereas a hood system can be built so that the filters (preferably baffles for the higher end units) are the dominant restriction, microwave ovens filters, particularly those used for recirculation, cause much more pressure loss for a given flow rate, and this is exacerbated by the narrow passages the air may have to flow through to get around the oven. As a result, unless the manufacturer actually specifies the flow rate of the microwave oven blower system when in place, one should hope that the resulting CFM is above 100 typical, and maybe approaching 200 for the 500 CFM rated devices. These rates over the plume expansion area at the OTR height are generally inadequate. While not a fan of VaH due to their years of claiming magic performance, their recirculating hood has been reported on here as providing good results. While the initial cost may be highish, I recall a discussion of what the actual consumables costs were over time. The link below is the one with this discussion, as I recall. https://www.houzz.com/discussions/2347276/vent-a-hood-ductless-ars-range-hood-update#n=65...See Moreck_squared
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