Alternative fuels, mainly generated from waste, constitute a possible option to reduce the demand for fossil fuels. For instance, biogas can be obtained from cow manure using anaerobic digesters, a technology that is currently being implemented in several dairy farms. Synthesis gas, also known as syngas, is a mixture of hydrogen and carbon monoxide that may have fractions of other components such as nitrogen, carbon dioxide, and methane, depending on the gasifying method used. However, before a change to alternative fuels can be made, the performance of common appliances and industrial equipment using these fuels needs to be investigated. In this paper, a computational analysis of the performance of an air heater using natural gas, biogas, and syngas is performed. The results show that alternative fuels with heat values almost an order of magnitude lower than natural gas can be used in commercial air heaters with only minor variations in thermal performance. The main drawback is the higher flow rates required for alternative fuels, specially for syngas.