Water heater venting is important to remove the exhaust from combustion of hydrocarbons, including natural gas, fuel oil and propane. One of the byproducts of incomplete combustion is carbon monoxide, a deadly gas that often goes undetected until it is too late. Electric water heaters do not require vents.
Burning wood or coal produces smoke, a visible mixture of gases and water vapor that escapes up a chimney or flue. Hydrocarbon fuels burn cleaner and with less water vapor, so they are less visible and have fewer odors than smoke. However, these fumes are detrimental to the health of people living in the home. The vent draws these fumes to the outside of the home, where large volumes of air dilute them to protect home occupants.
Most water heaters use a vertical vent. These vents use the fact that hot air is less dense than cool air to create a vacuum that removes the fumes from the home. In some locations, vertical vents are not practical. Water heater installers sometimes choose horizontal power vents. Horizontal vents cannot move the fumes as well because they do not exhibit the chimney effect. Instead, the manufacturer installs a fan to remove the dangerous fumes from inside the home.