there is no ventless hood that I know of that will work even remotely as well as a vented hood. By any chance is your stove along an outside wall? Then you could vent out the wall instead of the roof. Your Thermador salesperson can also direct you to the number of CFMs needed for your stove so that you have optimal venting. Believe me, as someone who has used both--a vented hood is definitely the way to go.

A ventless hood is an oxymoron. All it does is blow hot greasy air back in your face and on your cabinets. Leaving you with poor indoor air quality.

Venting externally, through the roof or wall, is necessary only if you cook. And then there is the Make Up Air that will be needed to resupply the air. You cannot separate the range purchase from all of the required accessory components. They cost as much, or more, as the range does. Going Pro Style costs much more than people think it does.