Photograph of an outdoor heating oil tank Heating Oil & Fuel Oil Properties Guide
A Description of Petroleum-based Fuels & Their Characteristics

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Properties of heating oil & related petroleum-based fuels:

Types of heating fuels & other petroleum based fuels such as kerosene, diesel fuel, home heating oil: This article describes the different petroleum-based puels and their characteristics.

Are some of these petroleum based fuels interchangeable? We compare the specific gravity and pour point of various oil fuels, kerosene, heating oil. We explain how to use a hydrometer or the Friedman freeze test to distinguish kerosene from heating oil.

We also describe problems with heating system reliability when certain heating oil additives are used or when sediment is stirred as empty or very-low-level oil tanks are re-filled.

This article series answers most questions about all types of heating systems and gives important inspection, safety, and repair advice.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

What are the Different Petroleum-based Fuels and What are their Characteristics?

Oil delivery in process (C) Daniel Friedman The article below describes the differences among these various diesel fuels, oil fuels & heating oil fuels, arranged in our list from "lightest" to "heaviest" are in the types of hydrocarbon chains that are distilled out of the crude oil during refining (and also that some of these fuels may contain other additives.

Article Contents

Definitions of Types of Fuel Oils

Oil burner schematic (C) Carson Dunlop Associates

What this all means is that the heavier petroleum based fuels (higher numbers) have longer hydrocarbon chains than the lower number fuels, they have more BTUs per gallon, they will be more viscous (and often dirtier or will contain more contaminants including environment-polluting sulphur).

It is not helpful to order and burn Kerosene #1 over #2 fuel oil except in outdoor aboveground oil tanks in areas subject to temperatures below 16 °F.

Improved Properties of Modern Low-Sulphur Home Heating Oil

Now that home heating oil is required to be low in sulphur, heating service technicians report the following observations

Dyes used in oil fuels

Research on dyes in oil fuels and in biodiesel:

Definitions of Octane and Septane

It is helpful to understand why we talk about septane ratings for diesel fuel rather than diesel fuel octane ratings. Octane ratings for fuels focus on how smoothly the fuel burns, not its energy content.

Octane ratings are important for high compression engines such as many modern gasoline engine-driven automobiles because under high compression a lower-octane fuel will not burn evenly, causing knocking (or detonation, knocking means that there is partial and uneven "premature-explosion" of parts of the fuel charge) that can actually damage an engine.

Common octane boosters include MTBE, ETBE, isooctane, toluene, and previously, lead.

Octane: eight carbon atoms in molecular structure

Septane: seven carbon atoms in molecular structure - see Heptane just below.

Heptane: is the zero point of the octane rating scale.

Heptane is "any one of several isometric hydrocarbons, C7H16, of the paraffin series (nine are possible, four are known); - so called because the molecule has seven carbon atoms. Specifically, a colorless liquid, found as a constituent of petroleum, in the tar oil of cannel coal, etc." - Websters Dictionary

Iso-Octane: is the 100 point on the octane rating scale

A simplified explanation of the difference in these measures is that octane rating describes how evenly a fuel burns, not how easily it ignites, or according to some sources (since a more even burning fuel may be harder to ignite), octane measures how hard it is to ignite a fuel while septane rating measures how easy it is to ignite a hydrocarbon fuel.

In other words, a higher octane fuel is harder to ignite, but burns more evenly than a fuel that is similar but lower in octane.

Note: measuring fuel oil viscosity is generally based on ASTM D396, measurement with SVM 3000

How to distinguish between K1 kerosene and #2 No2 home heating oil

Reader Question: I have a question I just can't find the answer to anywhere. I work for an oil company that delivers home heating oil. We usually haul K1 kerosene and #2 heating oil at the same time. And we run several trucks.

Both of these petroleum fuels are dyed red. And we are "often" in a situation where we don't know if a truck compartment has K1 or #2 oil in it. They look virtually the same. How can you tell one from the other? Please respond to this question. Thank you, L.M.

Reply: Identifying petroleum fuels by density or specific gravity or by their waxing point at cold temperatures: using a hydrometer

K1 (kerosene) and No.2 home heating oil look the same, and indeed a mix of K1 and No.2 is sold by some oil companies for use in outside home heating oil tanks to avoid waxing or jelling problems in cold weather.

These two fuels look the same to the naked eye, they smell about the same, and are from a very similar origin in the oil refining process (K1 is lighter and comes off before No. 2 heating oil during the refining process). And as we explain in the oil fuels article above, the I.R.S. requires a red dye in tax-exempt diesel fuel such as that used for home heating.

With that background, I can think of two ways that one can, without very sophisticated analysis, distinguish between kerosene and No. 2 home heating oil.

Fuel density or specific gravity: Kerosene vs. home heating oil

Density or specific gravity measurements are made on a scale that uses the density of water as a comparison point. Water has a density of 1 gram/cm3 at 4 °F. That's our yardstick. A suitable hydrometer can measure a liquid’s specific gravity. A hydrometer is a hollow, sealed, calibrated glass tube, a float, and a scale.

An old hydrometer that was very common was one used to check the antifreeze in an automobile. The hydrometer had a rubber bulb on one end and intake on its open end. The bulb was squeezed to suck antifreeze mix into the tube where the float would rise to mark a point on a scale.

The scale in that hydrometer was printed to report common densities of the water-antifreeze mix that in turn would read out the lowest temperature at which the mix would remain unfrozen.

In sum, the depth to which the hydrometer sinks is inversely proportional to the liquid’s specific gravity. So all we need is help from an equipment supplier who can sell you a hydrometer with the proper weight and scale to read the specific gravity or density range of heating oils or kerosene. A quick sample and a quick read on the scale and you'll know what you've got.

You can see from our table below that with only a very small chance of overlap, the specific gravity or density of kerosene averages about 0.81 while No. 2 heating oil will be around 0.90.

There are two types of hydrometers: the rubber bulb and suction tube type I described above (used for antifreeze testing in my example) and a simpler float with a graduated scale, or a "float hydrometer".

To use a bulb hydrometer you simply use the suction bulb to draw a liquid sample up into the tube sufficient to float the float and read the scale.

To use a float hydrometer you put your liquid sample into a clean glass container (such as a beaker) and place the floating hydrometer into the liquid with its scale rod pointing down and its weighted bulb "down". Depending on the density of the liquid the hydrometer will sink to a particular depth. You will be able to read the specific gravity or density of your liquid sample right on the scale of the float hydrometer's rod.

Where to buy a hydrometer and what type of hydrometer to purchase

Hydrometers are not expensive: you'll find models under $10.00 U.S. You'll find them online from many suppliers as well as in stores such as WalMart and even some grocery stores. But see our warning just below.

Watch out: for testing petroleum products like kerosene or heating oil, when you buy a hydrometer you'll want one whose scale reads from one and below - since we are reading the density of liquids that will be less than water. (By the way, that's why water that leaks into an oil tank is found at the tanks' bottom - the water is heavier than the heating oil so sinks below it to rest on the tank bottom.)

You do not want a wine making hydrometer for oil product testing as the scale won't serve our purposes. A wine making hydrometer such as the nice Rite-Brew model has a scale of .990 - 1.160 - no good for our purposes. But some multi-scale hydrometers may indeed work fine for our needs.

For measuring oil products you will want to find a "Low-STK" lab-grade hydrometer that measures specific gravities less than 1, i.e. in the range of our chart below - noting that all of the measurements will be at numbers less than one and more than 0.6.

Suppliers of petroleum hydrometers useful for the petroleum industry include

Table of Specific Gravity of Common Petroleum-Based Fuels

Petroleum Product (or other) Specific Gravity Gravity API Viscosity, SSU @ 100° F. Cloud Point
0.71 - 0.79
0.78 - 0.81 [3]
-40 F
Diesel Fuel Oil
0.82 - 0.95
16 F to -15F
Home Heating Oil
0.82 - 0.95
16 F to -15F
Heavy Fuel Oil
0.92 - 0.99

Notes to the table above

If purchasing a hydrometer to test petroleum products be sure the unit you buy measures in the specific gravity ranges shown above.

Fuel waxing under cold conditions: kerosene vs. home heating oil identification by the friedman-freezer-test

The cloud point for a petroleum product or fuel is generally described as the temperature at which small solid crystals are first visually observed as the fuel cools. This is the point at which the flow of the fuel through piping and equipment begins to be affected.

"Cold filter plugging" is the point at which a fuel filter (or heating oil tubing or filter) will plug and fuel will cease to flow. This is a lower temperature than the cloud point. But here we just care about the visible cloud point in heating oil and kerosene.

Kerosene has a lower cloud point, waxing point or pour point than No. 2 home heating oil.

Laboratory testing to measure the cloud point for heating oil, diesel fuel, or kerosene typically uses very precise measurements of temperature and short-range infrared trans illumination. But we can take a more crude approach for distinguishing between kerosene and heating oil:

I'll bet that you could put a small container of kerosene and another of heating oil into a freezer and check them every fifteen minutes. The first one to become waxy and solid is the No. 2 home heating oil. I just made this up, so I've named it accordingly. [Experts' comments or suggestions are invited. CONTACT us.]

Details about waxing in home heating oil, the definition of pour point, and a discussion of oil waxing problems and solutions are found


Watch out: don't forget about your test subjects in the freezer or there is some chance that a container may break therein and make a mess. Or use a clear plastic container to hold each sample. You want to be able to easily see the wax forming.

Problems With Heating System Reliability When Heating Oil Additives are Used or Low-Level Oil Tanks are Filled

When we serviced and installed heating equipment we often recommended use of heating oil additives to remove small amounts of water or sludge in oil storage tanks, or to act as a pour point depressant for outdoor aboveground oil storage tanks.

But while these are good products, things didn't always go well. We discuss the problem of sludge in heating oil tanks, lines, filters, and oil burner nozzles in more detail



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