Added. Oops, just noticed you said water heater vent. Don't use my method there.
I put dryer sheets wherever there is an opening. They are good for propane spiders & stink bugs as well. I have been told that pet flea collars work as well, but dryer sheets are a lot cheaper. You will get some dryer odor though.
If you get ants some Comet or Ajax sprinkled around the point of contact with the ground stops them until rain washes it away. A long term solution might be Talstar, just be very careful how you use it. It solved a long term ant problem by spraying around my house foundation. Nobody trying to help eat my dinner now.
2013 Rockwood Mini Lite 2109S
2010 Toyota Tundra 4WD