A vented gas space heater is an indoor heater that burns natural gas or propane to produce heat, requiring an exhaust vent connected to the outside air. This is because when fuel is burned, it can produce carbon monoxide. This colorless, odorless gas can be extremely dangerous in large amounts.
When a fuel is burned, it rarely burns completely, leaving certain by-products behind in addition to the expected carbon dioxide and water vapor. Carbon monoxide is one of these by-products, a colorless, odorless gas that can displace oxygen, if it builds up to sufficient levels. In high enough concentrations, it can cause headaches, dizziness, loss of consciousness and asphyxiation. Running a gas-powered heater indoors without proper venting can drastically increase the amount of carbon dioxide in a room, posing a serious health threat.
Vented gas space heaters are designed to be operated indoors safely. A built-in exhaust hood collects the carbon monoxide and other gases created by the burning fuel, and an exhaust chimney channels these potentially dangerous gases outdoors. If the space heater is a permanent installation, a dedicated permanent vent should be installed. Portable gas space heaters often have an adapter that connects to the closest window, in order to vent the heater safely.