I love my vent-a-hood but I do not like inserts for venting I find the cabinets get grubby and if a white kitchen those overhead cabinets also discolor over time. I also find 300CFM is the bare minimum ever for venting.

This device also seems undersized for good capture, although the volumetric flow rate of 300 CFM (very likely not achieved in real life) might be appropriate for the undersized capture area.

Without getting into the usual "how to get venting system requirements from user intent" process, the OP might want to provide data on what ducting is available or planned, cooking style, nature of the cooktop, how the air that leaves the kitchen is replaced, etc.

@dunavon I’m considering the same tlh model and wondering where/how you netted out?

@kaseki When you said that this is undersized, were you factoring for the 8.25” extension, giving it a capture area of 20”?

@Patricia Colwell Consulting Regarding your comment about inserts and grubbiness…would you include undercabinet hoods with that? Or only inserts without a front extension? I’m wondering because this one extends out like an under-cabinet, possibly mitigating the grub factor?

[In parts due to some issue with Houzz or me]

First, the VaH " magic multiplying" factor has been addressed here multiple times. En brief, blowers have what is called a fan curve (see figure). In free air, the flow rate is higher than when restricted by ducting, lack of make-up air (MUA), and, where present baffle of mesh filters.

Because the VaH "Magic Lung" versions of their hood product lines lack filters, the hood restrictions are less. However, ducting and MUA losses are still present and care has to be taken to account for these when determining the actual flow rate that can be achieved.

The Magic Lung squirrel-cage blowers have a shallow fan curve, and can be significantly restricted by additional pressure loss elements of the kitchen ventilation system. Conventional blowers rated for 450 CFM (for example) in free air (zero static pressure) can usually achieve 300 CFM with all of the pressure loss elements I listed. The VaH hood/blower assembly achieves 300 CFM only when hanging in free air.

Second, the process here is backwards. Establish a hood height and then entry area such that the rising and expanding cooking plumes are encompassed (captured). At a minimum, a wall hood should extend 3 inches beyond the cooktop on the sides, and to the front edge of the counter at the front.

Once the entry area is known, then the requirement for containment is set by the needed flow rate into the hood. I recommend 90 ft/min, equivalent to 90 CFM/sq. ft. If your cooking is benign, with no hot oil/grease action above a simmer (as with bacon) then a lower value may be adequate.

Once the actual (required) flow rate is established, then the blower can be specified. Also, the duct to the outside (unless very short) should be sized to minimize both impact and condensation grease accumulation, which calls for full power operation duct velocity in the 1000 to 2000 ft/min range.

Thank you very much @kaseki. Only after I posted did I see the wealth of advice you've offered over the years. Thank you so much for your help. I did the calculations and it looks like I need about 600 CFM for a 36" x 23" entry area. Maybe slightly less due to the use of induction (30"). Although the TLH-136 discussed above only has a 19.5" depth with panel expanded, I was going to mount it 3" off the wall (and seal up the 3" gap) which i assume would count toward the entry area, putting it at a 22.5" x 36" entry area. Either way, the TLH-136 does not cut it CFM-wise, even at their equivalency rating, and since I only have a 6" duct, I cannot move to their higher model. I was looking for retractability, in order to mount flush with a 15" D cabinet, which is how I landed on this model. Back to the drawing board. I will look into the "6" duct" conversations and see what I can turn up.

How long is the duct?

Don't buy anything until your MUA approach is clear.

Thanks @kaseki. It's 3.5' High + 7.5' across with one 90 elbow.

Six-inch duct should be OK up to 400 CFM actual, or 600 CFM rated. Wrapping with some vibration absorbing material might help. Turbulence noise will be inevitable. Usually, one won't have to operate at that level. If you need 600 CFM actual, then you need to expand the duct one way or another.