Fuel oil for heating is a petroleum-based liquid burned in a furnace or boiler to warm your home. The most common type in the United States is No. 2 heating oil, a middle-weight distillate refined from crude oil that contains about 138,500 BTUs of energy per gallon. It’s widely used in the Northeast, where millions of homes rely on oil-fired systems instead of natural gas.
What Heating Oil Is Made Of
No. 2 heating oil is a mixture of hydrocarbons with carbon chains ranging from about 9 to 20 carbon atoms long. In simpler terms, it sits between lighter fuels like gasoline (shorter chains, more volatile) and heavier industrial fuels like bunker oil (longer chains, thicker). Roughly 70% to 79% of the mixture consists of paraffin-type hydrocarbons, which are the primary energy carriers. The remaining 19% to 25% is made up of aromatic hydrocarbons, compounds with ring-shaped molecular structures.
During refining, crude oil is heated in a distillation column, and the fraction that boils between about 163°C and 357°C (325°F to 675°F) is collected as No. 2 distillate. This same cut of petroleum is nearly identical to diesel fuel. The key difference is regulatory: diesel sold for vehicles must meet emissions and road-tax requirements, while heating oil is dyed red to mark it as tax-exempt and intended only for stationary use.
How Fuel Oil Grades Differ
Heating oil comes in several numbered grades, each progressively heavier:
- No. 1 (kerosene): The lightest grade, at about 135,000 BTUs per gallon. It flows well in extreme cold and is sometimes blended with No. 2 to prevent gelling in frigid weather.
- No. 2: The standard residential grade, delivering roughly 138,500 BTUs per gallon. Nearly all home oil furnaces and boilers are designed for this fuel.
- No. 4: A heavier blend (about 145,000 BTUs per gallon) used in larger commercial buildings with equipment built to handle thicker fuel.
- No. 6: The heaviest common grade at around 153,000 BTUs per gallon, sometimes called bunker fuel. It requires preheating before it can flow and is used in industrial plants and large ships, never in homes.
For residential purposes, No. 2 is the default. When someone says “heating oil” without specifying a grade, they mean No. 2.
How an Oil Heating System Works
Most home oil heating systems use a gun-type burner. When your thermostat calls for heat, a pump draws oil from your storage tank and forces it through a precision nozzle at about 100 psi. The nozzle breaks the liquid into an extremely fine mist, a process called atomization. At the same time, a blower pushes air through a tube surrounding the nozzle, and an electric spark ignites the oil-air mixture inside the combustion chamber.
Because the fuel enters as a mist rather than a stream, its surface area increases dramatically. That massive surface exposure allows the oil to vaporize and burn almost instantly. Mixing, vaporization, and combustion all happen in the same space, within a fraction of a second. The heat produced either warms air directly (in a furnace, which blows it through ducts) or heats water (in a boiler, which circulates it through radiators or baseboard units).
Efficiency of Oil Heating Systems
How much of each gallon actually becomes usable warmth depends on your equipment’s efficiency rating, measured as Annual Fuel Utilization Efficiency (AFUE). Older oil systems from the 1970s and 1980s typically run at 56% to 70% AFUE, meaning 30% to 44% of the fuel’s energy escapes up the chimney as waste heat. Mid-efficiency models reach 80% to 83%, and the highest-efficiency modern systems can convert up to 98.5% of the fuel into heat for your home.
If your system is decades old, upgrading to a modern unit can cut your oil consumption by a third or more while heating the same space. Even smaller improvements, like replacing the burner nozzle or adding a flame-retention head, can push an aging system’s efficiency up by several percentage points.
Storage and Shelf Life
Heating oil is stored on-site, typically in a 275-gallon steel tank in your basement, garage, or outside your home. Unlike natural gas, which arrives continuously through a pipe, oil must be delivered by truck and stored until you use it. This means you need to monitor your tank level and schedule refills, or sign up for automatic delivery with your supplier.
No. 2 heating oil stays usable for up to two years under good conditions. Two main factors shorten that lifespan. Bacteria and microbes can colonize the fuel, especially where water collects at the bottom of the tank, breaking the oil down into sludge that clogs filters and nozzles. Corrosion inside the tank also degrades fuel quality over time and can eventually cause leaks. Keeping your tank full during the off-season reduces condensation inside, which limits both microbial growth and rust.
Safety Profile
Heating oil is classified as a combustible liquid, not a flammable one. The distinction matters: flammable liquids like gasoline ignite below 100°F, while combustible liquids require significantly higher temperatures. No. 2 heating oil has a flash point around 126°F to 145°F, meaning it won’t ignite from a dropped match at room temperature. You could actually extinguish a match in a bucket of heating oil. It needs to be atomized into a fine mist and hit with a sustained spark before it will burn, which is exactly what the burner nozzle is designed to do under controlled conditions.
This makes home storage considerably safer than storing gasoline or propane. Leaks are still a concern, not because of fire risk, but because oil can contaminate soil and groundwater. If you notice stains around your tank, a persistent oil smell, or unexpectedly high fuel use, have your tank inspected.
Sulfur Content and Cleaner Blends
Heating oil historically contained significant amounts of sulfur, which produces sulfur dioxide when burned. That contributes to acid rain and corrodes heating equipment from the inside. Starting with New York in 2012, northeastern states began requiring ultra-low sulfur heating oil (ULSD), which contains less than 15 parts per million of sulfur. Connecticut, Maine, Massachusetts, New Jersey, and Vermont followed with similar mandates. This is the same sulfur limit applied to highway diesel fuel.
The shift to ULSD has also made it easier to blend heating oil with biodiesel, a renewable fuel made from vegetable oils or animal fats. These blends, marketed as Bioheat, combine standard No. 2 heating oil meeting ASTM D-396 specifications with biodiesel meeting ASTM D6751 standards. Common blends range from B5 (5% biodiesel) to B20 (20% biodiesel). Bioheat works in existing oil heating equipment without modifications at lower blend levels and reduces net carbon emissions proportionally to the percentage of biodiesel in the mix. Several northeastern states now mandate minimum biodiesel blending percentages, with those requirements increasing over time.
Cost and Regional Use
About 5.3 million U.S. households heat with oil, and the vast majority are in the Northeast, where natural gas infrastructure historically didn’t reach many suburban and rural areas. Heating oil prices fluctuate with crude oil markets and seasonal demand, typically peaking in January and February. The cost to heat a home for a full winter varies widely based on climate, home size, insulation quality, and system efficiency, but a typical household in the Northeast uses between 500 and 1,200 gallons per heating season.
Because oil is bought in bulk rather than metered continuously like gas or electricity, price-conscious homeowners often lock in a per-gallon rate before winter through prepay or fixed-price contracts with their fuel supplier. Others join buying cooperatives to negotiate lower group rates. Monitoring spot prices and filling your tank in late summer, when demand is lowest, can also reduce costs meaningfully.