An indirect fired diesel heater burns fuel in a sealed chamber and sends only clean warm air into the space, with the exhaust flued outside. Hire runs roughly $150 to $500 a week depending on output, and the fuel bill overtakes the rental quickly: at full fire a 400,000 BTU unit uses close to three gallons an hour.
You are paying for combustion air kept separate
A direct fired heater puts its combustion products into the room along with the heat, which brings moisture and requires ventilation air the standards set out. An indirect unit keeps them separate and vents them outside, so it suits closed buildings, drying work, finishing trades and anywhere people work all day. That separation is the whole price difference.
Fuel burn follows the BTU, and the arithmetic is simple
Diesel carries roughly 138,000 BTU in a gallon. Divide the heater's rated output by that and you have gallons an hour at full fire: about 1.5 for a 200,000 BTU unit, about 3 for a 400,000 BTU one, more again for the trailer sizes. Thermostats cut real consumption below that, but budget from the full figure.
The tank, the pump and who keeps it fed
Small units run from an internal tank measured in hours, so somebody is refilling overnight. Larger ones sit beside a bunded tank the yard delivers, and fuel is either billed on delivery or run on a scheduled top up. Agree the refuelling arrangement before the cold snap, because that is when fuel deliveries queue.
Where the duct runs decides where the heat lands
These units are designed to push heat down a duct, and the duct run is what places the warmth where the work is. Long runs, tight bends and uninsulated hose all cost temperature. Ask which duct diameter and length come with the unit, and whether a return duct is included, because a room with no return heats unevenly.
Questions people ask about indirect fired diesel heater
Does an indirect fired heater need to vent outside?
Yes. The burner exhaust has a flue that must reach outside air, which usually means routing it through a door, a window panel or a vent. That flue run is part of the setup and is one reason these units want a straightforward opening in the building.
How big a heater do I need for a building?
Size on volume and how leaky the shell is rather than floor area, and add for the temperature rise you want in the time you have. A yard will size it from cubic feet, target temperature and whether the building is closed in. Ordering one size up is cheaper than running two units.
Can I run one on kerosene or heating oil?
Most rental units run on diesel or kerosene, and the yard will state which fuels are permitted for that model. Do not substitute without asking, because the wrong fuel affects the burner and the hire terms. Fuel supplied by the yard removes the question entirely.