Wall mounted vent versus vent with external blower in the attic

14 years ago

I am thinking of using my current rangehood as a liner (the color matches the range) and putting in an external vent in the attic (or the roof) with a duct silencer. Currently, my squirrel cage fan is rated 450CFM. It is not pulling enough air for my greasy cooking and I smell the lingering odor of the night before's burito, stir fry etc. I don't grill in the house since I have a built-in outdoor grill right outside of kitchen.

It's a Viking hood. We actually took the entire thing apart today and it was very easy to get the motor/fan out. (and degreased 10 years' worth of grease residue....) The motor has an easy slide pin electrical connection for a fan and I just need to run the fan wiring on the side of the duct. (is that how it is done?)

If I replace the fan with say 600 CFM or even 1000 CFM, will I notice a significant improvement in the performance? Will the smell from the night before not be a problem? I have a very open floor plan where there are no walls between living/dining/kitchen. Will the open floor plan be an issue regarding odor spread? What are your collective thoughts?

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Comments (5)

John Liu
14 years ago
450 CFM is not huge, but does not seem clearly inadequate, if you aren't grilling or woking. Is the hood wide/deep enough, mounted far enough forward (toward the cook), or mounted at the right height? Perhaps you could keep the existing hood and fan, but mount it a little further forward and a little lower, so that the hood captures vapors better.

If you do upsize the fan, then . . .

What is the size of the existing duct (diameter if round, each side if square/rectangular). How long (roughly) is the duct run to the roof? If the duct is too small, a higher-CFM fan may not be able to perform as rated, because it will be too hard to pull air through the too-small duct.

How leaky or air-tight is your house? You might need to bring air to come in from outside (open window, vent, or even blower) to replace the air being exhausted by a very powerful range hood. (Called ''makeup air''). The risk is causing fireplaces, gas heaters, water heaters, etc to ''backdraft''. If you are in a cold climate, this can be an issue.
wa8b
14 years ago
How big is your range hood in relation to your range or cooktop and at what height is it mounted above the cooking surface? Generally speaking, the larger the capture area of your hood and the closer to the cooktop, the fewer CFM's needed to effectively exhaust cooking fumes.

An inline blower might allow you to increase the CFM's and provide quieter operation, but you'll be limited in choosing a blower size by the duct size you have in place.

In-line blowers usually require an electrical junction box near the location in which it's being installed. Your idea of running a wire along the duct and connecting it to the existing electrical connection for hood mounted blower might technically work, but it would not be a legal installation. It shouldn't be difficult to get an electrician to run a new cable from the existing junction box that provides power to your existing set-up to another location and install a proper junction box there.
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We did as kas mentioned. You gain the most, as far as lessening sound, by turning the fan motor down. Even with ours all the way down, and you really have to listen hard to hear it, it will still hold a paper towel on the intake grate---this is enough for pulling off steam when boiling water--our is 24 inches above the inducttion cooktop. When you really crank the thing up, and I rarely do as it sucked one of my cats up (just kidding) but it does really evacuate fully up, just about all the noise you hear is from the air rushing in the grate. You can isolate the outside unit (Mechanically) from the wall (Rubber mounts) and you could even do so with a space between the duct to the blower using duct tape or whatever. That way you minimze any mechanical vibration between the blower and the duct. We (sometimes) not always , can hear a slight 60hz (low frequency) hum when the fan is at it's lowest setting--I did not bother to Mechanically Isolate mine--but at least to us it is a minor noise and as I said it comes and goes, if it did it all the time, I could find out what causes it. If you run your mixer inside the house, then you put it outside even by the window, it will be quieter as when it is inside all the noise from the mixer goes inside, when it's outside much of the noise is just disappating into the surrounding air, cept that which comes thru the window. As the blower is enclosed, and connected to the duct work, the difference would not be quite that as noted above, but it would be substantial. Good Luck. Gary
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My $.02 your looking at a minimum capability at 600 cfm. I would bump it up to 900-1200 cfm and make sure I have a variable speed fan control. I prefer external blowers or inline blowers. I put in an abbaka liner and a 1200 cfm external blower in our vacation house and love it. Bumping up the blower size is relatively inexpensive and may be quieter. (The fan blades are probably larger and they spin slower.) Ideally get a hood that is 6" wider than the range top.
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The best cooktop ventilation depends on your priorities and constraints (more or less like everything else). In commercial practice, it is common to use large capture areas with apertures at about seven feet, but there are many possible approaches, including perforated ceilings leading to exhaust blowers. Commercial practice is heavily biased toward effective and efficient (power wise) capture and containment. Residential practice can make use of commercial approaches, constrained by architecture, aesthetics, cost, and noise. Sometimes, however, the constraints force significant deviation from the most effective approach to capture and containment, considered to be a hood over the cooktop that has an aperture large enough to encompass the rising and expanding cooking plumes from the burners, and with enough air flow to ensure that what is captured is expelled outside. Side draft ventilation will usually have limited ability to capture rising effluent from burners distant from the aperture without having a flow rate that is impractical and/or noisy. (The plume can have an upward velocity of 3 ft/s; the side draft is unlikely to "bend" that plume completely toward itself unless very tall and/or very strong.) A side draft can at least reduce the grease load on the house surfaces and cooking odors distributed beyond the cooktop. Ceiling capture areas have to be large enough to not only encompass most of the expanding cooking plumes, but also the ever easier effects of cross drafts in deviating the plume direction. One would want a capture area of the cooktop size expanded by, say, 0.3 times the distance between the cooktop and the ceiling just for plume expansion without considering unknown to this writer cross draft conditions. So a 3 by 2 foot cooktop could at 5 feet above the cooktop require an added 1.5 feet of width and length. This now 4.5 by 3.5 ft aperture will need perhaps a bit lower velocity of blower air flow than the desirable 90 ft/min velocity at the normal hood height due to the plume cooling off some by the time it gets that high. Lets assume an average of 70 ft/min over the collection area is needed (this is a guess; I have no data for that distance above the cooking surface). This velocity times the area required computes to 1100 CFM. This will require at least a 1500 CFM zero-static-pressure rated blower in most circumstances, and a deliberate MUA system. While roof hugging centrifugal blowers such as Abbaka, Broan, and Wolf sell can somehow operate in snow (I have a Wolf and it stays clear in southern NH), a more commercial style up-blast configuration blower would be better for deep snow conditions. Some upblast blowers at Greenheck kas
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Differential means the difference between the outside temperature and the temperature you want the MUA to be (warmer or colder than outside). CFM means actual hood CFM, and the actual hood CFM will never exceed that provided by all the sources of MUA CFM available at the kitchen. If the differential pressure is zero, then one has perfect MUA (pressure balance) and the hood will only be "gagged" by its own baffles, transitions, and ducting. The achieved indoor-outdoor differential pressure won't generally be zero, but it can be kept modest relative to changes on the fan curve. Aim for 90 hood CFM per square foot of hood intake aperture. This should provide complete containment for most cooking for captured plumes using residential hoods with baffles. (Plumes are initially captured if the hood aperture overlaps them.) So, for a desired/required 800 CFM, say, one finds 800 on the abscissa and goes up to a differential temperature curve representative of an outdoor and indoor temperature difference, and thence left to the ordinate where the BTU/hr needed is listed, based on the specific heat of air and the mass of air being moved at that CFM. BTU/hr has a counterpart watts value for electric heating, and also a tons of air conditioning value if one intends to cool. Conversion factors may be found by use of an Internet search engine.
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kaseki
14 years ago
Think of the effluent rising from a pan in a conical shape. The angle of the side of the cone is about 22.5 degrees, and thus the overall cone expands at about 45 degrees. The effluent that is intercepted by the hood (and doesn't curl back out of the hood) is exhausted, the rest distributes itself throughout the house and is exhausted mainly by air replacements. This may take a long time and waste a lot of heated air.

Hence, at least for burners on which one cooks smelly or greasy foods, the hood has to overhang the conically expanding effluent. Generally, one wants the air flow at the interception of the effluent with the hood to have a velocity at least as great as the rising effluent (about 3 ft/sec = 180 ft/min). (Otherwise it may escape the hood.) This velocity is lower than in the duct, but should cover the entire entrance area of the hood. (If the hood is itself pyramidal internally, some leeway is possible on entrance velocity because farther up where the effluent is potentially subject to reflection, the sectional area is less.) (Ignoring edge effects, the area of the duct times the air velocity in the duct equals the area of the hood times the air velocity of the air at the hood entrance.)

Inside the duct, the velocity of the air should be in the range of 1000 ft/min to 2000 ft/min to keep the grease particles not captured by the baffles or screens from precipitating onto the duct walls. Going up in blower flow rate capacity (cfm) usually requires larger ducting, but this should be checked by calculation.

For duct sizing, assume that an external blower with ducting, transitions, baffles, etc. will, at full power, pull about 2/3 its rated (zero static pressure) flow rate. For a more accurate value, one needs the blower curve (flow vs pressure), calculation of all the losses at every point in the path, and some knowledge of how make-up air and/or house leakage is establishing the negative pressure that the house will get to or that you will let it get to.

If Viking makes a similar hood with an external blower, you should consider downloading the installation manual and noting how Viking directs the connection between power source, fan, and fan control. In general, the power connection to the hood control should have some spacing from the duct to keep the wire temperature within specification.

kas
kaismom
Original Author
14 years ago
There is a rectangular cut out for the ducting on the top of the hood. Then you are supposed to use a transitional piece between the rectangular cutout (estimate 3x8 inches) and round metal duct. Currently, that transition takes out straight out to the back with a 90 degree elbow. It can go straight up. I do not have any ducting going to roof now, so I can put any size I need.

I think the biggest problem is that the hood is undersized. It is exactly the same width and slightly less deep (front to back) than the cooking area. Hmmmm

thanks all.

I will look up the spec for how external blower is hooked up...
kaseki
14 years ago
Maybe since you want to use the old hood as a liner, you can simply extend its sheetmetal to a wider area inside whatever it is you are lining.

kas

Wall mounted vent versus vent with external blower in the attic

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John Liu

wa8b

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Wall mounted vent versus vent with external blower in the attic

Comments (5)

John Liu

wa8b

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kaseki

kaismomOriginal Author

kaseki

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kaismom
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