Direct, indirect or gas
A direct heater, like most gas models, burns its fuel in the airflow, so every kilowatt (kW) of heat reaches the space along with the fumes and moisture from the flame. That suits building sites and well-ventilated workshops, but not closed rooms or anywhere people sleep, and the moisture can condense on stored goods. An indirect heater sends its exhaust out through a flue, so its clean, dry air suits marquees, occupied buildings and livestock.
Sizing by kW and by volume
Multiply length, width and height to get the volume, since a high roof holds air that needs heating too. As a rough guide, a draughty building needs around 50 watts per cubic metre and an insulated one about half that, which puts most workshops at 10 to 30 kW and barns or small warehouses at 50 to 100 kW. Allow more for open doors and the fresh air a direct heater needs, as every change of air has to be heated again.
Fuel, running time and safety
A diesel heater burns roughly 1 litre an hour for every 10 kW, so a 20 kW model empties a 20 litre tank in around 10 hours, and a thermostat makes each tank last longer by switching the burner off once the space is warm. Direct diesel and gas heaters release combustion fumes into the space, so keep fresh air coming in, and never use a gas cylinder below ground. Where no fumes are acceptable, choose an indirect or electric heater.
