Designing Kitchens and Bathrooms - Downdraft Vent Systems?

We are installing a new kitchen in a one-story addition. There will be a gas cooktop on a peninsula and we are trying to figure how to vent the vapors. My wife cans tomatoes and such, so the majority of the vapor will be steam. If it is at all possible, we don't really want a vent hood in the kitchen, but I am dubious about whether a downdraft system will really work, especially with a gas stove. The addition has a low-slope metal roof, so there is additional motivation to run the duct-work in the crawl space rather than to the roof. There is no soffit in the kitchen, so making it to the wall would also be "challenging".

Does anyone have experience with downdraft hoods?
Used with Gas cooktops?
Performance with big pots of boiling water?

Any real world experience would be appreciated.

Thank you,

Steve

There are several appliance mfg's who also offer downdraft venting systems and while they all recomend that they be install as an integral unit with their appliances, they can all be installed as "stand alone" appliances.
The efficiency is directly related to the CFM, length and configuration of the ducting.
A "remote blower" configuration will be quieter.