All downdraft venting is useless. That just sounds more useless than most. Capture area is THE most important aspect to ventilation. Downdrafts have zero capture area. To stick with a useless system is the very definition of a sunk costs fallacy. https://thedecisionlab.com/biases/the-sunk-cost-fallacy/

The only solution that will be effective is overhead venting. If the health of your family and cleanliness of your home isn’t worth doing that, then I hope you have good health insurance, and paid cleaning help.

https://newscenter.lbl.gov/2013/07/23/kitchens-can-produce-hazardous-levels-of-indoor-pollutants/

https://www.theforumjournal.org/2003/01/04/use-of-kitchen-ventilation-impact-on-indoor-air-quality/?fbclid=IwAR3gI5UDKFDRd4r1EygoXg-3hC07BdD6yCtAtQOlBFb9YnEe7R6aRPNyvdY

https://ww2.arb.ca.gov/resources/documents/indoor-air-pollution-cooking

How to design an effective venting system. https://www.yumpu.com/en/document/read/46472115/kitchen-ventilation-systems-application-design-guide-greenheck

And don’t forget about the required Make Up Air. https://www.fantech.net/fileadmin/user_upload/fantech/Support/Media_Center_USA_Files/e1574-makeup-air-system.pdf

@User Thank you for your comment. I could have done without the snark, but I imagine this is related to years of experience dealing with downdraft hvac systems that just aren't very effective. My post was intended to simply gather information to see if there are ways to use the existing ducting and blower in a more effective way, as a new system would be costly. It sounds like your advice is to abandon the existing system and to figure out an overhead solution. Thanks

Thanks @Shannon_WI - yes, we are planning to slowly renovate the kitchen - starting with the countertops, which are original and in rough shape. We may do the kitchen island at the same time, which is why I was thinking about venting possibilities that we should incorporate into the countertop plans. Totally agree about consulting with an hvac specialist. Thank you for your comment

I know I am butting in a bit here, but wanted to let you know that renovating a kitchen piecemeal like that is usually not a good idea. For example, when you put new counters on 40-year-old cabinets, the cabinets will look all the more shabby and dated, and then, when you do decide to re-do the cabinets, you will have to lift the fairly new counters and hope they don't crack (they often do). In addition, older cabinets sometimes are not strong enough to support the heavier weight of new counters if they are granite or Quartz. Older cabinets are also often out of level, and to bring them level to support properly the new counters, well, perhaps it would be better just to have new cabinets. Similarly, a new island while leaving the rest of the cabinets original will just highlight how old they are and how discordant the appearance is with the new island and counters.

I am not trying to be Debbie Downer, just wanting you not to throw good money after bad.

Thanks for your advice @Shannon_WI - it's not a major reno - we like the layout and don't intend on making any major changes - so I guess it's more of a refresh. The cabinets are solid and are in good shape, but will be cleaned up and refinished. We will use Corian for the counters. We have been replacing and upgrading the appliances as they die, and will replace the floors next summer.

Why is overhead venting impossible? I have managed to get overhead ventilation in a number of kitchens in old houses.

@palimpsest Not impossible, but a costly prospect to set-up new venting for the island. There is existing ductwork and an exterior blower for the weird DIY downdraft, so I was looking for guidance about downdraft options that could use the existing ducting. @Shannon_WI is correct that I should consult with an hvac specialist to get guidance about possibilities vs abandoning the system and starting fresh with overhead

You might be surprised about the cost. One of the places I did this was to prep a house for sale where the owner had a non-functional downdraft range. So he needed a new range and a new hood and new ductwork. He had a very low budget available to put the house on the market. I believe it was more or less similar in cost to a new downdraft all told.

"There is existing ductwork and an exterior blower for the weird DIY downdraft, so I was looking for guidance about downdraft options that could use the existing ducting."

The issue is effectiveness of any possible venting scheme (not necessarily a venting solution) and whether the ductwork and/or exhaust blower are adequate even if a pop-up downdraft is to be used. You didn't report the blower rated CFM or the duct diameter and length.

Further, one might want to examine the ducting, at least at the island, for grease. Lack of same implies that the register scheme is as ineffective as some of us might predict. On the other hand, if there is grease that isn't just varnish now, there is a fire hazard without a metallic filter of some sort playing the role of fire block.

@kaseki thank you for your comment. I'm not sure about the duct diameter (can only see the outlet with the register, which I assume is stepped down in size).

If there is a 90 deg corner, for the length I would estimate about 5ft for the vertical run, and maybe 8ft for the horizontal run, but this is a rough estimate.

The marking on the exterior exhaust fan housing is "NuTone RF/WF-35". I'm not sure what the cfm rating is for this old fan motor - I don't see anything online, but I can contact Broan/NuTone to try to get that info.

A little bit of old, hardened grease is visible near the register, but not very much. I don't use the vent system, and I doubt it was used very much by the previous owners. Definitely want to avoid a fire!

Verbo, I will have to disagree with you I have a JennAire downdraft that sucks harder than a hooker on Hollywood and Vine..

You cannot claim a contravention of the laws of physics. No matter how strong any fan may be, it cannot make up for the lack of capture ability. That’s immutable. Or delusional.