How To Calculate Oil Fired Heater Fuel Autonomy?

  Posted by: Chief Editor, Selmore - Marquee Hire, on October 21, 2017 1:00 pm Leave your thoughts
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Calculating Oil Fired Heater Fuel Autonomy

This is a quick example of how you would calculate oil fired heater fuel autonomy for a marquee event...

Estimating fuel autonomy for portable heater operators and event planners alike

So what's the fuss about oil fired heater fuel autonomy?

Fuel consumption and fuel autonomy of oil fired portable heaters are intrinsically linked and relevant metrics, which if overlooked can produce embarrassment. all-round.

For avoidance of running short of portable heater fuel in the middle of a winter marquee event, it is prudent to know - with precision - the fuel consumption in order to determine the fuel autonomy of your indirect oil-fired heater.

Firstly, what type of fuel does an indirect portable heater use?

Typically, indirect portable heaters are built to consume one of two fuel types:

  • LPG (Liquified Petroleum Gas),
  • Diesel

In the case of products termed 'indirect oil-fired heaters' - the oil they refer to is 'Diesel oil' - a term to be used interchangeably with 'Diesel'.

How does diesel consumption work in a indirect oil-fired heater chamber?

Diesel combustion, unlike gasoline or petrol combustion, requires no spark.

The heat generated actually originates from the combination of physical forces interacting with the chemical properties of the substrate fuel involved.

This is all that is required to bring about the ignition for the heater to operate.

The operational stages of an indirect oil-fired heater are as such:

  • Air intake
    • ...the heater unit has an electrically powered fan for forcibly drawing in the external air into the combustion chamber
  • Combustion 
    • ...fuel is injected into the combustion chamber as air is being sucked into the unit. The combination of air intake and fuel compression together, brings about the spontaneous chemical & physical reaction of fuel combustion within the chamber
  • Exhaust release
    • ...there is a flue for swift removal of dangerous monoxide exhaust gases after the combustion occurs
  • Hot air expulsion
    • ...the hot, clean air from the combustion process is forced out of the heater unit by fan propulsion in to the external environment

Here is a graphic of the entire process showing the continuous airflow from intake to outflow...

Combustion System Airflow

Combustion System Airflow - by Master Heaters (found at

This gives further illustration to the concept outlined above where a continual airflow is drawn into the portable heater and forced into the combustion chamber.

Because the heat generated from the combustion of the fuel is continually met with more air displacing it from the air intake fan - this produces a constant flow of hot air into the external environment of your marquee.

The rate at which the chamber performs this combustion and therefore consumes the diesel fuel varies, depending on make and model of heater.

Why should event planners and hire companies alike care about fuel consumption or fuel autonomy of oil-fired heaters?

The service of providing a heater for a marquee based event in winter is mission critical to the enjoyment of guests and therefore event success.

Fuel is of course a prerequisite for the heater to function. Where there is fuel, there is heat. The converse is also true of course.

The duty of equipment hire companies is to ensure agreed service levels are maintained throughout the duration of event planned - and also to ensure a little more remains in the tank for creep of project scope i.e. unplanned time extensions.

In relation to this commitment to provide a continuous service, the hire company must budget for all necessary provisions including fuel, by estimation.

Most indirect oil fired space heater manufacturers will include within their user manuals and instructions, an estimation of fuel autonomy (or continuous operation).

However, there may come a time where you need a handy reference for calculating fuel autonomy of an oil-fired heater yourself.

How do you calculate operational time (fuel autonomy) of a diesel oil-fired portable space heater?

The first place to begin is with your heater's official figure for fuel consumption. Fuel autonomy can be derived simply from this number.

Fuel consumption is typically measured in kg/h (kilograms per hour).

This figure would indicate the method by which fuel consumption is measured is to weigh the fuel tank continuously during the process of combustion whilst calculating the total loss of fuel by weight in kg (kilograms) as time elapses.

The resulting figure calculated by doing this experiment is a rate i.e. how 'something' changes plotted over a duration of time.

The definition here of kg/h is a measure of how much fuel by measure of weight is used over the fixed time duration of a single hour.

To arrive at the figure of how many continuous hours your oil-fired heater can go undisturbed or topped up without ceasing operation, you'd simply need to divide the total fuel capacity of your selected machine by the figure for fuel consumption.

Not so fast!

There is one small matter of unit conversion which needs to take place.

Fuel capacity for diesel fired indirect space heaters is usually measured in litres, a measure of physical volume occupied by the fuel, as opposed to weight of fuel.

This requires knowing the following conversion:

'what is the kilogram equivalent to the litre capacity of your chosen indirect oil-fired heater?'

Once you know the litre or volume capacity of your oil-fired heater, then this conversion is possible.

Let's say, the volume capacity of your heater fuel tank is: 36l.

To convert this volume measurement accurately into weight in equivalent kilograms (kg) we need one last important metric: the liquid density of the fuel.

Each liquid has it's own density, measured in kg/m3. Water has a density of  1kg/l (the same as 1,000 kg/m3), or in other words 1 kilogram of water will occupy 1 litre of physical volume.

The density of diesel is slightly less: 0.832 kg/l.

Diesel, as a liquid, has molecules which are heavier in weight than water molecules, but because the molecules of diesel pack less dense than water, the measure of density for diesel is actually less than the more compactly arranged water molecules.

This being said, we have all the necessary ingredients for having a go at calculating a real life example of fuel autonomy from a oil-fired heater's operator's manual fuel consumption measurement.

The real life example calculation of indirect oil fired heater fuel autonomy:

Portable Heater for Marquee Tent

Heater Type: Portable oil heater MASTER BV 77

Fuel Type: Diesel

Fuel Tank Capacity: 36 litres

Fuel Consumption: 1.67 kg/h

[Diesel Density: 0.832 kg/m3

...time for a quick calculation to try out what we just learned:

(36 [litres] / 1.67 [kg/h]) * 0.832 [kg/m3] = 17.9 hours (17 hours and 54 minutes) autonomous heating

That's your answer!

If the marquee event for which you had selected this particular indirect oil-fired portable heater is expected to go beyond 17 hours 54 minutes (or let's say 17 hours to be safe) then contingency should be made for refuelling.

In such cases, it will be necessary for a hire company to have on stand-by throughout the event's duration a staff member who can refuel the machine.

Useful sources for further reading:

Explain That Stuff - How Diesel Engines Work

Quora - Density of Diesel

Master Heaters - BV 77 Indirect Oil Fired Portable Space Heater Manual

Do you need help in deciding the necessary strength of heater for your marquee event?

This article will help you calculate the heating requirement for your planned marquee event.

Are planning a budget for marquee heating?

Contact us to receive a quote.

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