I am considering getting a downdraft cooktop and I'm wondering if a over-the-range hood fan is required also, even though there is a fan in the cooktop? I was thinking all I need is the cooktop, right? Has anyone had experience with these options?

The only advantage to the range hood is the light. Both work just as well.

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Let me start off with a small disclaimer: I am not a professional installer or repairman; the following comments are based on my own experience with shopping for a ventilation solution, consulting with (over a dozen vendors), and reading quite a lot of materials.

That being said...

The biggest difference between a range hood (mounted ~30" above the range) and a downdraft (mounted on the back of the range) is the overall performance:

A downdraft only provides enough suction to capture a part of the "pollution stream", and only from the back burners. A range hood covers the entire width of the range, and captures all the smoke/humidity/grease that floats up. (Remember high-school physics - hot air tends to rise.)

Even if a range hood is a few inches less in depth than the stove, the airflow's conical shape pulls the smoke/odor/grease droplets into the hood's filters anyway. Meanwhile, anything that rises above a downdraft's effective-suction area is free to float around (and damage your cabinets over time).

Besides the difference in placement, there's the issue of total airflow. A 300-400 CFM downdraft is not going to be nearly as effective as a 440-CFM Thermador - or a 625-CFM Miele - or an 800-CFM Futuro Futuro range hood. In other words, with a range hood, you have a much larger selection of airflow/power, than with a downdraft.

Last but not least, there's the "looks" issue. Personally, after spending a fortune on high-end appliances and custom cabinets, the last thing I want is a simple rectangular box. If price is not an issue, go with a nice German or Italian-made range hood!

My huge complaint about downdraft vents, particularly those between the burners similar to Jennair and Amana cooktops, is the difficulty they cause when a gas burner is set very low. The draft draws the gas flame away from the center of the pan, and can even put out the flame. If you don't have automatic ignition this can be dangerous. I suspect as a result cooktops with built-in downdraft vents have a very high "low" setting, resulting in burned chocolate, etc.

The ductwork also takes up room in the cabinet below the cooktop, better served by drawers or shelves.

Never again downdraft for me. Fortunately my kitchen also has a cheapie hood so I never turn on the downdraft vent. All it does is take up space, fortunately it is level with the burner grates so I can rest pans on it. I can't imagine what the original owner was thinking when this kitchen was designed.