Indirect diesel heaters keep combustion in a sealed chamber and flue the exhaust outside, so only clean dry air reaches the space. That is what makes them the class hired for occupied floors, drying out plaster and curing concrete. The question on most jobs is not which single heater, it is how many, because a wide floor is warmed by two ducted units far more evenly than by one big one.
Size the set to the volume, not the floor area
Heat fills a space, so a building with 20 ft ceilings needs far more output than the same footprint at 8 ft. Give the yard the length, width and height, the wall build up, how open the doors will be and the temperature you have to hold. They size from that, and a specification desk will do it while you are on the phone.
Why yards send two units instead of one
One large heater in a corner makes a hot end and a cold end, and the trades in the cold end open a door and undo the job. Two mid sized units ducted to opposite parts of the floor even the temperature out and give you something left if one drops off. Splitting the load also cuts the size of the electrical supply each one needs.
Every heater still needs a power lead
Indirect units burn diesel, but the blower, the pump and the controls are electric, and the fan on a large unit is not a trivial load. On a site with no permanent supply that means a generator sized for the heater set as well as everything else. Get the running amps for each unit at the point you agree the sizes.
One fuel plan for the whole set
Three heaters running through a cold week empty tanks three times as fast, and nobody wants to be filling jerry cans at 6am. Ask the yard about a shared bulk tank with feed lines, or a scheduled delivery, and confirm who owns the refill. Running a heater dry usually means bleeding the line before it will relight.
Questions people ask about indirect diesel heaters
How is an indirect unit different from a direct fired one?
A direct fired heater sends the products of combustion into the room along with the heat, which adds moisture and needs a lot of fresh air. An indirect heater keeps them apart and flues the exhaust outdoors, so the air stays dry and clean enough for occupied and finishing work.
Can I duct one heater into several rooms?
Yes, with a splitter or a lay flat duct run, provided you accept that the far branch delivers less. Yards stock duct in standard lengths and will tell you how much run a given fan will push. Long runs and tight bends both cost you temperature at the outlet.
Do these need to be outside the building?
Normally yes. The unit sits outside with the flue clear of doors and openings, and the warm air is ducted in. Keeping the machine outdoors also keeps the fuel, the noise and the refuelling out of the space the trades are working in.