Range Hood Ventilation
LINK > How long can my dryer vent be?
Check the installation instructions for the hood you plan to install. If the ducting equivalent if longer than what is recommended for the hood, it will not vent properly once installed.
The Appliance Information Site
One other thing...the chimney was removed down to below roof level, so all range hood venting will go directly into the attic for now until I can get a vent cut out through the roof this spring.