Chimney caps or crowns can stop damage from happening to your chimney. These chimney caps / crowns are simply roofs for your chimney. Water will be kept from entering the chimney and causing deterioration of the brick and masonry. Chimney flashing must also be installed to stop any water.
Hot water heaters and other fuel venting appliances must have vents sized according to code. Chimney venting may not be the right way to vent these appliances. Building inspectors can check your masonry chimney to make sure that none of the fuel exhaust will spill back into your home.