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

Actually hood ventilation noise consists primarily of two components - blower or blower mechanical noise and airflow noise (ignoring vibration caused from a cheap and poorly constructed hoods). A low quality blower will introduce mechanical noise hence one should always strive to install the best quality blower you can - each of those blower options below have specific areas of use - an inline blower mounted remotely will reduce immediate mechanical noise versus an internal blower in the hood HOWEVER an inline blower brings its own risks in terms of resonance (air waves from blower oscillations and minute vibrations) in a confined area. An external blower mounted close to the hood is about the same as an internal blower in the hood - one typically uses an external blower if you have a difficult duct run - sucking air from the end of the duct run along a convoluted and long duct run is far more efficient than trying to PUSH air from inside the hood via an internal blower along the same duct run. At the end of the day noise has mainly two components - poor quality blower that introduces excessive mechanical noise (there will always be some but this can be mitigated by using a good quality blower) and most importantly the QUALITY of the duct run - a duct run that is too small or multiple 90 elbows or has been crimped(dented badly at installation) OR using the ribbed aluminum stuff versus smooth HVAC galvanized ducting introduces a lot of air flow noise - of course the type and design of the filtration system also introduces airflow noise in the hood. By far the biggest problem we have found in over 20 years of custom range hood sales and installations is ducts that are far to small for the airflow generated by the internal blower - modern blowers (good quality ones) sense duct restrictions and run faster (more revolutions and therefore more mechanical noise) to compensate and delivery the requested volume of airflow - a simple mathematical calculation will illustrate this - a 6 " duct which everyone believes is adequate provides 28.28 sq inches of airflow capacity whereas an 8" duct supports 50.28 sq inches - nearly double the volume of airflow - so of course an 8" duct is far more efficient and is why we recommend an 8" duct over a 6" for 600cfm and 10" duct over 8" for 900cfm and 1200cfm. An added tip - another benefit of external blowers is that because they are far more efficient at moving air over distances and bad duct runs is an 8" duct is sufficient for say a 900cfm or 1200cfm external blower - qualification of course being a high quality external blower system such as ours - cant comment on the others out in the market - as always you get what you pay for......at Custom Range Hoods we are always willing to help and will assist anyone with technical questions - placing an order with us will of course open the door to a mountain of experience and good advice - feel free to email us at allenl@customrangehoods.ca.

If the function of the hood were to air out the house, then the number of burners running at once might determine the desired flow rate. But that is not how hoods are intended to be used when cooking. The hood is intended to immediately capture the rising and expanding effluent (from any burners) and then contain the effluent using sufficient air velocity at every point under the baffles to both contain and partially degrease the rising effluent as it winds past the baffles. To do this the air velocity generated by the hood blower has to be high enough that smoke not "reflect" out of the hood aperture when it reaches it and finds a restriction. Uprising velocity can be 3 ft/s. At the very least the baffle gaps need this flow rate to assure containment. My rule of thumb is to take the capture aperture area in square feet and multiply by 90 cfm/sq.ft. This will be the desired actual achievable flow rate. No blower system will achieve its rated flow rate in a real environment. Even if we give VAH credit for an actual flow rate of an entire hood and blower assembly (in this case 600 cfm or 1200 cfm), once the ducts and make-up air (MUA) restrictions are included the actual flow rate reduces. And no information has ever crossed my attention horizon that specifies how VAH measure their purported flow rates, so we can't be sure that they are anything other than magic numbers. If I were to use a VAH in this context, I would buy the 1200 cfm rated unit and hope that I could run it at less than full speed for reduced noise and still achieve capture and containment of hot rising effluent from pans, griddles, or whatever. Boiling eggs will not require this level of flow rate (or any for that matter). But unless the 48-inch range is purely a poser installation, at some point you will use it to cook something that generates grease and smoke. Even capturing much of the oven output when it is opened will require a decent flow rate. Don't forget that for either blower there has to be a source of MUA. The actual flow rate out of the hood is always matched by the flow rate into the house, and if the latter is small, then so will the former. But worse, while that is happening the house pressure falls, and combustion appliances can back-draft, potentially inducing CO into the house. This is hazardous. So plan ahead how you intend to provide low pressure drop MUA. kas

An oversized outside blower run at reduced speed would be quieter due to lower blade tip turbulence. An up-blast commercial blower operated as a side-blast blower could be quieter if properly sized to provide the required CFM at modest RPM. Baffle hiss will be the same for the same volumetric air flow rate.