A fuel-fired air heater is a device that uses a fuel source, such as natural gas, diesel, petrol, propane, or oil, to generate heat. The fuel is burned in a combustion chamber, and the heat produced is used to heat the air as it passes through the chamber. The heated air is then circulated throughout the space being heated, either by natural convection or with the assistance of a fan. These heaters can be used in a variety of settings, including vans, semi tractors, industrial and commercial buildings, warehouses, and greenhouses, and are often utilized as a primary or supplemental heat source. Some specialized fuel-fired air heaters may also include additional features such as humidifiers, air filters, and temperature and humidity controls.