A portable indirect fired heater is a mobile furnace with a fan forcing air over a heat exchanger where combustion occurs internally and fumes are exhausted through a stack. Heated  air is delivered to heat areas where clean breathable air is needed. Indirect fired heaters are typically installed outside with warm air ducted to interior area/s to be heated. Alternatively, they technically may be installed inside with the exhaust ported outdoors using flue materials available at the local hardware store; however indirect fired heaters can be loud when used in interior spaces and safety, regulations and fueling are potential issues of the user's responsibility. Smaller indirect fired heaters typically have an on-board fuel tank. Larger models often utilize fuel from a separate fuel tank plumbed to the heater.