The existing vent outlet is 6" id. and center line is 60" from the floor. My question is can I run the new duct work downhill a short distance without interfering with the performance of the unit. With the stove top height of 37" and the minimum distance of 24"-30" to bottom of new hood fan,after installing the duct out of the top of the hood fan, you can see that my existing vent opening is too low. Cutting into it to raise the hole is not an option. It appears to be some sort of asbestos type pipe from the 50's. Also is it ok to use rigid flexible duct for this type of installation? Thank you for any knowledgeable replies.