Home heating oil is a refined petroleum product made almost entirely of hydrocarbons, the hydrogen-and-carbon molecules that form the basis of all fossil fuels. Specifically, it is No. 2 fuel oil, a middle-weight distillate pulled from crude oil during the refining process. Its molecules typically contain between 11 and 20 carbon atoms per chain, placing it in the same general family as diesel fuel.
The Hydrocarbon Breakdown
No. 2 fuel oil is not a single chemical compound. It is a mixture of hundreds of different hydrocarbon molecules, broadly sorted into two categories. Paraffins, which include straight-chain, branched, and ring-shaped molecules, make up 69 to 79 percent of the fuel. The remaining 19 to 25 percent consists of aromatic hydrocarbons, a class of molecules built around stable carbon rings. Common aromatics in heating oil include alkylbenzenes and naphthalenes.
This blend gives heating oil its key properties: it flows easily through pipes at normal temperatures, ignites reliably in a burner, and releases a large amount of heat per gallon. The balance between paraffin and aromatic content also affects how cleanly the oil burns and how much residue it leaves behind in your furnace.
How It’s Made: From Crude Oil to Your Tank
Heating oil starts as crude oil pumped from underground reserves. At a refinery, the crude is heated in a distillation column, a tall tower where different products separate based on their boiling points. Lighter products like gasoline rise to the top, while heavier ones like asphalt settle at the bottom. Heating oil condenses in the middle range, roughly between 175°C and 325°C (about 350°F to 620°F). This process, called fractional distillation, is how refineries produce everything from jet fuel to motor oil from the same barrel of crude.
After distillation, the oil may go through additional processing steps to remove impurities, particularly sulfur. The finished product is then blended to meet industry specifications set by ASTM International, which define acceptable ranges for viscosity, flash point, and sulfur content.
Heating Oil vs. Diesel Fuel
Heating oil and diesel fuel are chemically very similar. Both are No. 2 distillate fuels with overlapping carbon chain lengths. The practical differences come down to regulation rather than chemistry. Heating oil is dyed red by law so regulators can distinguish it from road diesel, which carries highway fuel taxes. Heating oil also tends to have a lower sulfur content than some diesel grades, though both fuels have seen sulfur limits tighten significantly in recent decades.
You could technically burn diesel in a home oil furnace, and vice versa, but using red-dyed heating oil in a vehicle is illegal because road fuel taxes haven’t been paid on it.
Sulfur Content and Cleaner Formulas
Sulfur is the most significant impurity in heating oil. When the fuel burns, sulfur converts to sulfur dioxide, a gas that contributes to acid rain and respiratory irritation. Historically, No. 2 heating oil could contain up to 0.5 percent sulfur by weight (5,000 parts per million). Many states, particularly in the Northeast, have moved to ultra-low sulfur heating oil with limits of 15 ppm, a reduction of more than 99 percent.
Lower sulfur content doesn’t just reduce air pollution. It also means less corrosive residue building up inside your furnace and flue, which can extend equipment life and reduce maintenance.
Bioheat: The Biodiesel Blend
A growing share of home heating oil now contains biodiesel, a renewable fuel made from plant oils or animal fats (technically, fatty acid methyl esters). These blends are marketed as Bioheat fuel. The naming convention is straightforward: B5 means 5 percent biodiesel mixed with 95 percent petroleum heating oil, B20 means 20 percent biodiesel, and so on.
Standard No. 1 and No. 2 heating oil grades already cover blends of up to 5 percent biodiesel without requiring any specification changes. ASTM has also released separate performance standards for B6 through B20 blends, which allow a slightly higher distillation temperature to account for the biodiesel component. Testing at Brookhaven National Laboratory found that customers using biodiesel blends reported similar or better performance compared to conventional heating oil, and some dealers have sold B20 successfully for over a decade.
Several Northeastern states now mandate minimum biodiesel percentages in heating oil, with blend requirements gradually increasing over time.
What Gets Added Before Delivery
The base fuel often receives a package of chemical additives before it reaches your storage tank. These serve several purposes:
- Stabilizers prevent the oil from oxidizing and breaking down during storage, which is important since a tank of heating oil may sit for months between deliveries.
- Demulsifiers help separate water from the oil. Water inevitably enters tanks through condensation, and if it mixes into the fuel, it can cause burner problems and corrosion.
- Biocides kill bacteria and fungi that can grow at the boundary where water meets oil inside a tank. Left unchecked, microbial growth creates sludge that clogs filters and fuel lines.
Some fuel dealers include premium additive packages as a standard part of their deliveries, while others offer them as an upgrade. The additives are present in very small concentrations and don’t change the fundamental chemistry of the oil.
What Happens When It Burns
When heating oil combusts in your furnace, the hydrocarbons react with oxygen to produce heat, carbon dioxide, and water vapor. That is the ideal reaction. In practice, combustion is never perfectly complete, so small amounts of other byproducts form as well.
The main emissions from a properly tuned oil burner include nitrogen oxides, formed when the extreme heat of the flame causes nitrogen and oxygen in the air to combine. With distillate oils like No. 2 heating oil, this thermal reaction is the primary source of nitrogen oxide emissions because the fuel itself contains very little nitrogen. Small amounts of carbon monoxide also form when combustion is incomplete, which is why annual furnace maintenance matters for both efficiency and safety.
Particulate matter from oil burners is primarily soot, tiny carbon particles left over from incomplete combustion rather than from mineral ash in the fuel. A well-maintained burner with a clean nozzle and proper air-to-fuel ratio minimizes these particles significantly. Trace amounts of volatile organic compounds, including benzene and toluene, also appear in flue gases, though at far lower concentrations than you would see from burning solid fuels like wood or coal.