Does range hood ventilation need to have a straight vertical duct through the roof or can I use a couple of 90's to re-rout the existing ducting? The ducting will go through a single story.

Every elbow will act as the equivalent of a certain length (~4 ft. depending) of straight duct, reducing the total length the vent can be. You can read the effect on dryer ducting at the following link:

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.

Dan O.
www.Appliance411.com
The Appliance Information Site

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Can I vent my range hood up through my chimney if nothing else will be using that chimney? My furnace doens't use it, nor does my hot water heater and I don't have a fireplace, so the only thing remaining vented up through the chimney is the range hood.

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.

You can run your vent pipe up through the chimney (as long as nothing else is using it), but you MUST run the vent to the exterior. Venting into the attic (even temporarily) is an extreme fire hazard from heat and grease build up.

Bob