If it is a gas water heater, do not vent through this space!
Never combine exhaust with combustion air exhaust.
Don’t combine two exhausts together either as one could blow back into the other.
Always have a unique exhaust vent for each type of exhaust.
If you are venting into a closet then you may not have enough force to push the all of the air out of the closet. Once a certain pressure inside of the closet is reached all of the smoke will just blow back.
You need to have a vent all the way to the exterior.
If it is more than 20 feet of effective length then you will need to add a booster fan. You can calculate effective length from this link. With a booster fan you might be able to double this but don’t push it too far. Some booster fans will come on automatically when they detect an increase in pressure but not all. Some people have made them work with a switch. Add the booster near one end or the other but it should be somewhere where you can access it for maintenance.
If you have a permanent or hidden vent-run, I would recommend using a smooth metal vent pipe with the largest radius sweep for bends that you can fit. Liquid mastic on all joints and seams seals best, but at least tape them.
Avoid excessive kinks or bends.
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