Heating Oil Explained

An oil heating system is a straightforward machine once you understand the parts. Knowing roughly how it works makes everything else — service, faults, running out — far less mysterious.

Oil heat is mechanically simpler than most people assume. Fuel is stored in a tank, drawn to a burner, atomised into a mist, ignited, and the heat moved through the house by air or water. Everything else is refinement.

What No. 2 fuel oil is

No. 2 fuel oil is a middle distillate of crude petroleum, in the same fraction as diesel. The number is a grade: lower is lighter and more volatile, higher is heavier and more viscous. No. 2 sits in a practical middle — energy-dense, storable, and liquid enough to pump through a modest fuel line at ordinary temperatures.

Its important properties for a homeowner:

• It is energy-dense. A gallon of heating oil contains a large amount of energy relative to its volume — more per gallon than propane, which matters for the storage comparison on the fuel comparison page.
• It is not readily ignitable in bulk. Its flash point is well above room temperature — a match dropped in a tank of heating oil goes out. It burns only when atomised and heated. The flip side is that it is a contaminant: spilled oil is a serious matter, see tank care.
• It gels in extreme cold. Waxes crystallise at low temperatures and restrict flow, which is why outdoor tanks in Minnesota need consideration a Virginia tank does not. It also degrades over long storage, particularly with water present, forming sludge — which matters for seasonal properties.

Blends containing biodiesel — labelled with a B number for the percentage — are increasingly common, and Minnesota has had biodiesel requirements for diesel fuel for years. Blends are generally compatible with existing equipment at modest percentages, though cold-weather and solvent characteristics differ. What your system is rated for is a question for its documentation or your technician.

How an oil furnace or boiler works

The distinction is what carries the heat: a furnace heats air and blows it through ducts; a boiler heats water and circulates it through baseboards, radiators, or in-floor tubing. Many older Minnesota homes have boilers, since hydronic heat was standard for decades. The combustion side is the same in both.

The sequence, when the thermostat calls for heat:

• The fuel pump draws oil from the tank and raises it to high pressure, through filters that remove particulates — typically one at the tank and a smaller one at the burner.
• The nozzle is the heart of the system — a precisely machined orifice that atomises the oil into a fine mist at a specific flow rate and spray pattern. Oil cannot burn as a liquid; only as a mist with air mixed in.
• A blower supplies combustion air, and the ignition transformer sparks across two electrodes to light the mixture. In normal running the flame sustains itself.
• The heat exchanger transfers combustion heat to the air or water without letting exhaust gases mix with it, venting them out the flue. Its integrity is a safety matter — a crack can put combustion products into living space.
• The cad cell is a light-sensitive safety device watching for flame. If the burner runs without one appearing, it shuts the system down rather than pumping unburned oil into the chamber — which is why a system that fails to light "locks out" instead of retrying.

Tank types

Indoor tanks, usually in a basement, are the most common arrangement in cold climates and have real advantages: protected from weather, fuel at basement temperature so gelling is a non-issue, and leaks visible before they become environmental problems. A common size is around 275 gallons, in the oval shape that fits through a doorway.

Outdoor above-ground tanks free up basement space and simplify delivery, but they take the weather. In Minnesota that means cold-flow treatment is a genuine consideration, and exposure to precipitation is a corrosion issue.

Underground tanks require the most care, for one dominant reason: you cannot see them. A buried steel tank corrodes from both sides, and a leak may be invisible for years while contaminating soil and potentially groundwater. Remediation costs can be very large, and the liability generally attaches to the property owner. Many have been removed or professionally abandoned in place. If a property has one, understand it fully — see tank care and safety.

Tanks are steel or, increasingly, polyethylene, sometimes with an outer containment jacket — which addresses exactly the failure mode that makes single-wall tanks a liability.

Consumption, and why cold snaps matter

The practical implication of all this: the nozzle and filter are consumables, and combustion quality depends on them being right — which is why annual service is not upselling. See winter preparation.

Household consumption varies enormously — with size and insulation, equipment age, thermostat setting, and whether oil also makes hot water. Any published average is meaningless applied to a specific home. Your own delivery history is the only figure that describes your house.

What is worth internalising is the shape of consumption, because it is not linear. Fuel use tracks the difference between indoor and outdoor temperature — the colder outside, the faster the house loses heat. This is what heating degree days measure, and what dealers use to estimate when you need a delivery.

The consequence every oil-heated Minnesota household should understand: a cold snap does not increase consumption a little. It increases it dramatically. A tank that would last weeks of a normal January can be drawn down in a fraction of that during a severe stretch — precisely when roads are worst, every dealer is at capacity, and an emergency delivery is hardest to get.

This is the argument for filling ahead of the season rather than reacting to the gauge. See winter preparation, and the main guide for how delivery arrangements handle it.