In this response, I am interpreting your term efficiency to mean plume capture and containment efficiency by the hood. Initially we shall assume that the flow rate at the hood in the presence of all pressure losses is adequate, and then revisit that issue.

1. Establish a perimeter at the cooktop level that roughly represents the locus of all pan bases that might be used.

2. For computational convenience, attempt to eyeball smooth this contour into an equivalent rectangular contour. (If you have access to a planimeter, or have a suitable drawing program, the area calculations can be performed more exactly without smoothing.)

3. Draw this on a piece of paper to scale

4. Multiply the height of the range hood by the tangent of 10 degrees.

5. Extend the step 2 shape by this value on the sides and front. (Extend the back also if an island hood is being analyzed.)

6. Draw this extended contour onto the paper over the initial contour.

7. Now draw the hood aperture to scale over this expanded contour, with hood back fitted to contour back. Note that the aperture may be smaller than the external size of the hood, however, at the wall the hood aperture may be assumed to extend to the back of the hood exterior. Also, if cabinets are in use at the sides, the hood aperture (at the cabinets) may be effectively extended, but this depends on the cabinet base height vs. the expanding plume size at that height -- for part of the side capture.

8. Calculate the fraction of the expanded contour that the hood aperture contour overlaps. This is a measure of the capture efficiency of the hood aperture being analyzed.

9. Redo for the other aperture.

10. The loss or gain in efficiency is the difference between these two measurements.

For the listed hoods, the efficiency of the 20-inch unit should be less than that of the 24-inch unit, noting that it is the internal dimensions of the apertures that have to be used for analysis.

Assumed above is that the effluent entering the hood aperture is contained by successfully moving past the baffles or mesh filter. For this to occur for the hottest plumes, the air velocity averaged over the hood aperture (ideally uniformly) should be about 90 ft/min. (A bit less can be tolerated for induction cooktops due to lack of a hot gas combustion plume mixing with the hot greasy pan plume and raising its temperature and velocity.)

90 ft/min is equivalent to 90 CFM per square foot of aperture. For a six square foot aperture, for example, the requirement for actual flow at full cooking heat is 90 x 6 = 540 CFM.

Blowers only achieve their touted flow rate at zero static pressure; i.e., hanging in the air. With baffle or mesh pressure losses, duct losses, hood interior transition losses, losses in the MUA system, the pressure across the blower will not be zero. It may be a few tenths of an inch of water column. Without a careful analysis using a lot of guesswork, along with the fan curve of the blower, or otherwise performing a house experiment with each hood, it is not possible to know the achieved flow rate. Instead, it is probably safe to assume that a rated zero static pressure flow rate of 1.5X the required actual flow rate will achieve the required actual flow rate. In the case of the six square foot aperture example above, this takes us from 540 CFM required to around 810 CFM rated.

Calculating the requirements for your two candidate hoods based on their interior apertures is left as an exercise for the student.

kas

Thank you kaseki. I have read many of your posts here. I need the dictionary to decipher your posts at times, lol - but I get the jest!

Based on your example, I am going with the24 inch depth Zephyr. @ 715 cfms, mu air and at 30 inches above the cooktop, I should ok. From what I researched , the Zephyr should be quieter than the bosh 6 vs 10 sones at max.

Ric, did you end up going with the Zephyr Venezia? If so, are you happy with the brand, effectiveness and quietness?

We've had the Zephry for 3 years and are quite happy with it