Heating oil is made by refining crude petroleum through a process called fractional distillation, which separates the raw material into products based on their boiling points. The fraction that becomes heating oil, known in the industry as No. 2 fuel oil, comes off the distillation column at roughly the same temperature range as diesel fuel. What follows distillation is a series of cleaning and treatment steps that reduce sulfur and other impurities before the finished product is shipped to homes and businesses. The process is straightforward in concept but has some interesting wrinkles, from seasonal shifts in what refineries prioritize to the growing role of renewable blends.
What Crude Oil Actually Is
Crude oil is a mixture of thousands of different hydrocarbon molecules, ranging from very light gases like methane and propane to extremely heavy, tar-like compounds. These molecules differ mainly in how many carbon atoms are chained together. Lighter molecules have fewer carbons and boil at lower temperatures, while heavier ones need more heat to vaporize. The whole point of refining is to sort this soup into useful groups. Gasoline comes from the lighter end. Heating oil and diesel sit in the middle. Asphalt and heavy industrial fuels sit at the bottom.
Not all crude oil is the same. “Light sweet” crudes contain more of the middle-range molecules that become heating oil and diesel, along with less sulfur (the “sweet” part). “Heavy sour” crudes require more processing to yield the same products. Refineries choose their crude feedstock based on what they want to produce and what’s economically available, and many are configured to handle a specific range of crude types.
Fractional Distillation Separates the Mix
The heart of every refinery is the atmospheric distillation unit, a tall column that can reach 60 meters or more. Crude oil is heated in a furnace to around 350–400 °C, which vaporizes most of its components. The hot vapor enters the column near the bottom, and as it rises, it cools. Different hydrocarbons condense back into liquid at different heights. Trays or structured packing inside the column catch these condensing liquids and channel them into side draws.
The lightest fractions, like propane and butane, exit as vapor from the top. Gasoline-range hydrocarbons condense a bit lower. Kerosene and jet fuel come off in the middle. Heating oil and diesel are drawn from a zone where temperatures sit roughly between 250 and 350 °C. Everything too heavy to vaporize in this first column collects at the bottom and gets sent to a vacuum distillation unit for further separation under reduced pressure.
The fraction that becomes No. 2 heating oil consists mostly of hydrocarbons with carbon chains between about 12 and 20 atoms long. This gives it a boiling range and energy density well suited for residential heating. One gallon contains roughly 138,500 BTUs of energy, enough to warm a typical home noticeably on a cold day.
Secondary Processing and Cracking
Distillation alone doesn’t produce enough of the products people actually want. Demand for gasoline and middle distillates (the category that includes heating oil and diesel) typically exceeds what simple distillation yields from a barrel of crude. Refineries solve this with secondary processes that chemically rearrange heavier molecules into lighter, more valuable ones.
Catalytic cracking is the workhorse here. It uses heat and a catalyst to break large, heavy hydrocarbon molecules into smaller ones. A related process, hydrocracking, does the same thing but under high hydrogen pressure, which also helps remove sulfur and nitrogen. Research into combining mild hydrocracking with fluid catalytic cracking has shown that these processes can achieve conversion rates around 85% when optimizing for lighter products like gasoline.1Fuel. Combined mild hydrocracking and fluid catalytic cracking process for efficient conversion of light cycle oil into high-quality gasoline While that particular work focused on gasoline output, the same family of cracking technologies feeds the middle-distillate pool that heating oil comes from. By adjusting temperatures, catalysts, and pressures, a refinery can shift its output toward heavier or lighter products depending on market demand.
Removing Sulfur and Other Impurities
Raw distillate straight from the column contains sulfur compounds, nitrogen, and trace metals that would cause pollution and equipment problems if left in. Hydrotreating, sometimes called hydrodesulfurization, passes the distillate over a catalyst bed in the presence of hydrogen gas. The sulfur bonds to hydrogen and exits as hydrogen sulfide gas, which the refinery captures and converts into elemental sulfur for sale. Nitrogen and some metals are removed the same way.
How much sulfur gets removed depends on the intended product specification. Historically, No. 2 heating oil was allowed to contain much more sulfur than on-road diesel. That gap has been closing. New York State, for example, lowered the allowable sulfur content of No. 2 heating oil from 2,000 parts per million to just 15 ppm starting in July 2012. That single change cut sulfur dioxide and fine particulate emissions from heating oil combustion by over 95%.2ACS Publications. The Public Health Benefits of Reducing Fine Particulate Matter through Conversion to Cleaner Heating Fuels in New York City Several other northeastern states have followed with similar ultralow-sulfur mandates, effectively making today’s heating oil chemically very close to on-road diesel.
The push to ultralow sulfur has real consequences for the refining process. Achieving 15 ppm sulfur requires more aggressive hydrotreating conditions, higher hydrogen consumption, and more expensive catalysts than the old 2,000 ppm standard did. These costs get passed along in the price of heating oil, but the air quality benefits, particularly in dense urban areas, have been dramatic.
How Heating Oil Differs from Diesel
People often hear that heating oil and diesel are “basically the same thing,” and chemically that’s close to true. Both come from the same distillation cut and have similar carbon-chain lengths, energy content, and viscosity. The main differences are regulatory and practical rather than molecular.
On-road diesel in the United States must meet a 15 ppm sulfur limit nationwide, is subject to federal road taxes, and is dyed clear or slightly green. Heating oil is dyed red so tax inspectors can tell it apart from taxed road fuel. In states that haven’t yet adopted ultralow-sulfur mandates for heating oil, the sulfur content can still be somewhat higher than on-road diesel, though this is increasingly uncommon in the Northeast where most heating oil is consumed.
There are also minor additive differences. Diesel typically includes cetane improvers and lubricity agents optimized for injection engines. Heating oil may include stabilizers to prevent degradation during months of storage, pour-point depressants to keep it flowing in cold weather, and sometimes a biocide to prevent microbial growth in tanks. But swap the dyes and additives, and the base fuel is essentially interchangeable. In an emergency, you can burn diesel in a home furnace or use heating oil in a diesel engine (though using untaxed red-dyed fuel on the road is illegal).
How the Fuel Burns in Your Furnace
Once heating oil reaches your basement, a pump draws it from the storage tank through a filter and sends it to the burner at high pressure. A nozzle atomizes the oil into a fine mist of tiny droplets. This atomization step is critical. The way the fuel spray mixes with combustion air determines whether combustion is clean and efficient or smoky and wasteful.3ScienceDirect. Energy and Combustion Science A spark ignites the mist, and the resulting flame heats a heat exchanger. Air or water circulated through the exchanger carries warmth into the living space.
Modern oil burners are remarkably efficient, with annual fuel utilization efficiencies in the mid-80s to low-90s percent range. High-efficiency condensing models can push above 95% by recovering heat from the exhaust gases. The quality of atomization and the fuel-to-air ratio are the two biggest factors in combustion efficiency, which is why annual burner tune-ups make a meaningful difference in fuel consumption. A dirty or misadjusted nozzle creates larger droplets, incomplete combustion, soot buildup, and wasted fuel.
Storage and What Happens Over Time
Most homes with oil heat have a tank holding 275 gallons, either in the basement or buried underground. Heating oil is stable enough to sit for months between deliveries, but it’s not inert. Over time, exposure to oxygen, heat, and any water that accumulates in the tank causes chemical changes. Heavier molecules polymerize into sludge that can clog filters and nozzles.
Water is the bigger problem. It enters tanks through condensation on interior walls, especially in tanks that aren’t kept full during warm, humid months. That water settles to the bottom and becomes a breeding ground for bacteria and fungi. Research simulating contaminated storage tanks has shown that microbial growth starts in the water phase at the tank bottom and then spreads throughout the system via tiny water-in-oil droplets called microemulsions.4Engineering in Life Sciences. Microbial challenges for domestic heating oil storage tanks The microbes feed on the hydrocarbons, producing acidic byproducts that corrode the tank from the inside and generate slimy biomass that clogs fuel lines.
Practical prevention comes down to a few steps: keep the tank as full as practical during off-season months to minimize condensation space, have the tank inspected periodically for water accumulation, and use a fuel stabilizer if you know the oil will sit for an extended period. Underground steel tanks are especially vulnerable to corrosion from the outside as well, which is why many states now require periodic inspections or recommend upgrading to fiberglass or double-walled tanks.
Bioheat and Renewable Blends
Straight petroleum heating oil is gradually being supplemented, and in some states eventually replaced, with blends that include biodiesel. The industry calls these blends “Bioheat.” A B5 blend is 5% biodiesel and 95% petroleum heating oil. B20 is 20% biodiesel. The biodiesel portion is typically made from soybean oil, used cooking oil, or animal fats through a process called transesterification, which converts the fats into fatty acid methyl esters that burn cleanly in standard equipment.
Field trials, including a Vermont project that monitored 26 households running B20 Bioheat in their existing oil heating systems, have shown that the blended fuel works reliably with no adverse effects on equipment or system performance.5Contemporary Agriculture. Bioheat: A Heating Fuel That matters because over 28 million Americans still rely on oil-based heating systems, and replacing all that infrastructure would be enormously expensive. The ability to swap in a cleaner blend without modifying furnaces, tanks, or delivery trucks makes Bioheat a relatively low-friction way to cut carbon emissions from home heating.
Several northeastern states are now mandating minimum biodiesel percentages in heating oil. New York, for instance, has been incrementally increasing its mandate and aims for B50 by 2034. Connecticut, Rhode Island, and Massachusetts have similar trajectories. The long-term vision in these states is to reach something close to a fully renewable liquid fuel, sometimes called B100 or “renewable heating fuel,” that could be a drop-in replacement for petroleum heating oil. Whether supply chains and feedstock availability can keep up with these mandates remains an open question, but the regulatory direction is clear.
How Refineries Adjust Output Seasonally
Refineries don’t produce the same mix of products year-round. In summer, gasoline demand peaks and heating oil demand drops to nearly nothing. In winter, the reverse happens in heating-oil-dependent regions. Refinery planning models account for this by adjusting the operating conditions of distillation and cracking units across seasons.6ScienceDirect. Effective MILP model for oil refinery-wide production planning and better energy utilization Energy consumption at each processing unit changes not just with throughput but with the specific operating mode and the time of year.
This seasonal balancing act is one reason heating oil prices can spike sharply during cold snaps. If a refinery has optimized for maximum gasoline output heading into summer and an unexpectedly cold late-spring week hits, the available heating oil inventory may be thin. The Northeast is particularly sensitive to this because it consumes the vast majority of the nation’s heating oil but has limited refining capacity of its own. Most of the region’s supply comes from refineries along the Gulf Coast and is shipped up by barge and pipeline, adding transit time that makes rapid supply adjustments difficult.
Inventories are tracked weekly by the U.S. Energy Information Administration, and the futures market for heating oil (technically, the NYMEX No. 2 heating oil contract, now called the NY Harbor ULSD contract) is one of the most actively traded energy commodities. When distillate inventories fall below their five-year seasonal average, prices tend to climb well before any consumer actually runs short. For homeowners, this is why locking in a pre-buy contract in summer or early fall often saves money compared to paying the spot price in January.
The Geography of Heating Oil Use
Heating oil consumption in the United States is overwhelmingly concentrated in the Northeast. States from Maine to Pennsylvania account for roughly 80% of all residential heating oil use in the country. This isn’t because oil heat is inherently better suited to the region’s climate; it’s a legacy of infrastructure. When natural gas pipelines expanded across the country in the mid-20th century, they reached the sprawling suburbs and cities of the Midwest, South, and West far more thoroughly than the older, denser housing stock of New England and the Mid-Atlantic. Millions of homes in the Northeast were already built with oil furnaces and storage tanks, and converting to gas requires not just a new furnace but a gas main running to the property, which in rural New England may not exist.
This geographic concentration creates a somewhat fragile market. A single nor’easter or polar vortex event can drive up demand across the entire consuming region simultaneously, with limited ability to redirect supply from elsewhere. It also means that state-level policies in just a handful of northeastern states, like the Bioheat mandates described above, affect a disproportionate share of the national heating oil market.
Outside the U.S., heating oil plays a significant role in parts of the United Kingdom, Ireland, Germany, and Austria, particularly in rural areas where gas networks don’t reach. The trend in those countries mirrors what’s happening in the American Northeast: tightening sulfur standards, growing use of renewable blends, and long-term policy pressure to transition toward heat pumps or fully renewable fuels. The timeline varies by country, but the direction of travel is the same everywhere heating oil is still burned in quantity.