VAL6 infrared heaters are able to use diesel #1, diesel #2 or kerosene as fuel. If the heater is being used in a environment cold enough that diesel #2 begins to gel, the manufacturer recommends blending it with kerosene or diesel #1.
The two different grades of diesel fuel contain different amounts of combustible energy. Similar to how standard gasoline is rated by the amount of octane, diesel fuel is rated by the amount of cetane, which is usually between 40 and 55. Diesel #2 is less volatile than diesel #1, but is more susceptible to freezing. The method of heating used by infrared heaters is called radiant heat, which occurs when light in the infrared spectrum travels from the heater to cooler surfaces, warming them in the process.