Are all above options dependent on the garage ceiling height? I'm not sure...what if the garage ceiling is higher than the interior.

I think if the ceiling in the garage is higher than the ceiling on the first floor, it's still possible, but you'll need to box in the vent pipe where it comes through the adjoining wall and you'll need a 90 in there to go up into the garage attic space. Basically it will look uglier, but it still should be possible. Generally the garage floor will be lower than the first floor and the ceiling will be the same height so the roof line matches. Elevations should give you some idea.

Thank you for your replies - my understanding is outside venting is a no brainer and really should be done.

My cooktop has a down draft and goes under the floor to the outside. It does have a 90 degree turn...

My next few questions would be...

1. does having one 90 degree turn eliminate all usefulness of venting to the atmosphere? (how many 90 degree turns will there have to be to render venting to the atmosphere useless)?

2. What other specific factors would come into play to make venting to the atmosphere useless or have the same effect as not venting at all?

3. If venting to the atmosphere is not an option at all - what are the best "choices"/alternatives? If you can't vent to the atmosphere - I assume there is no advantage of having a hood vs microwave?

So it's been a while since I've needed to calculate head pressure losses in pipes, but you have to consider how many CFM your desired vent system is moving before you can calculate how many 90 degree bends will negate. That's why I suggested a remote fan setup since you can get higher CFMS that one that resides in the hood itself.

2. You'll need some kind of port to vent out of and they'll have a spring or gravity baffle to prevent critters from getting back up in. That's some static head pressure too. Piping diameter and type (flex, corrugated, solid) will affect airflow velocities as well. Flex pipes can do angle change sweeps (gradual) for a better flow than solid pipes.

3. You can get a hood microwave too. Even if you can't vent to the outside, a hood can be useful since they have grease traps. Sure you won't be venting heat or smell, but the hood will capture grease particulate and help contain it, rather than having it disperse all over your kitchen.

Great, helpful comments. Thank you very much.

lets say a hood was 430cfm...then how would you calculate?

You might be better off asking this question to the people in the heating and cooling forum. As far as I can remember, it's recommended that you have around 6 cfm per 1000 btu for your cooktop and 430 cfm is probably going use a 8" pipe. Run length and number of bends can be used to calculate if the velocity will drop below optimal for the given cfm.

Why not run it in a soffit straight across the top of the foyer, laundry and closet? Access to the attic shouldn't be in a closet anyway.

The IRC requires a required attic access to be located in a hallway or other readily accessible location. Even if your building department will allow it in a walk-in closet, you don't want the fire department to waste time searching for it and there should not be anything below it.

thank you for more suggestions. JDS - there is access to attic space via the garage as well.