How to seal this open duct?

Just because there is an induction stove does not mean ventilation is no longer needed. Cooking produces a lot of things that aren't necessarily good for you to inhale. Consider adding an appropriately sized hood. You may end up needing this duct.

That looks like a round pipe behind all of that insulation. If it's an old oven flue, you can use a flue plug to stop up/cover the hole. It's typically a galvanized or sheet metal cap that's shaped to fit the hole.

Something like this: [https://www.supplyhouse.com/Lennox-29K12-2-Flue-Plug?srsltid=ASuE1wSOMkHvs7qgTy0F7mz4fuFucFTCEovIBI9zlRtlGbCrw3MkuWdXqV4]

You may want to see if you can find the other end of the pipe and make sure it's blocked as well. The pipe would make a good entry way for small animals looking for a place out of the elements. We would often have birds and squirrels climb down our flue pipe in the winter. Made for some fun times getting them out of the house.

It’s not pretty but if that hole is the size of a single or two-gang switch you can get a blanking plate meant for sockets and put it over it.

If you need it to be pretty I guess you can square the hole and/or cut a piece of wood to fit over it. Attach it somehow to the hole then fill the gaps with wood filler (look up how to fill a big hole in drywall).

If there’s a deep hole you need to fill you can also use spray foam.

It looks like someone already stuffed it with insulation.

Assuming this is in an unseen location, you don't have to get fancy. You could put a square of plywood over the hole, screw it into place, then caulk the edges. That should cut down on drafts and keep creepy crawlies out.

Someone needs to create a r/dontputyourdickinthat for lemmy

The finished product. Thank you for all the advice!

It’s hard to see what this is but if it’s just ducting, they sell end caps for radial ducts at big box stores. You would stick that on the end and wrap it with some aluminum HVAC tape. But it’s not really hurting anything as it is. Probably just vents outside.

Source: General Contractor/Estimator