Oil Burner Nozzles

A fuel oil burner nozzle does one job: deliver a precise mass of oil as a controlled spray of the right shape, at the right angle, so the burner air can burn it completely. Get the nozzle wrong and you get soot, unburnt oil on the tubes, flame impingement on the furnace wall and wasted fuel, all from a component the size of a thumbnail.

Unlike a process spray nozzle, an oil burner nozzle is rated by firing rate in gallons per hour at a reference pressure (almost always 100 psi), not by free flow at a chosen bar. That reference-based rating is the first thing to understand.

Firing Rate and Pressure

The GPH rating is fixed at 100 psi. Change the supply pressure and the actual firing rate shifts as the square root of the pressure ratio, exactly like any other pressure-fed nozzle:

actual GPH = rated GPH × sqrt(supply pressure / 100)

Raise the pump pressure and you fire more oil per hour; drop it and you fire less. This is why firing rate is partly a nozzle property and partly a pump property, and why chasing it by cranking pump pressure degrades atomization.

Ratings assume a standard test oil, roughly 3.4 cSt at 80 °F, around 830 kg/m³. Heavier, colder oil flows differently and atomizes coarser, which is why fuel conditioning is part of the spec.

Hollow, Solid and Semi-Solid Patterns

Hollow cone places the oil in a ring and is the common choice for most pressure atomizing oil burners, balancing flame stability with air entrainment. In practice it dominates the small end of the range, up to about 2–3 GPH.

Solid cone concentrates fuel toward the centre, used where a more compact flame is needed.

Semi-solid sits between the two and suits specific burner designs, and from roughly 10 GPH upward it becomes the industry-standard pattern, keeping air and fuel matched across a large spray.

The pattern must match the air register: in most burner types the air is what actually atomizes and mixes the fuel, the nozzle supplies the oil sheet, the air does the rest.

Spray Angle Versus Furnace Geometry

Spray angle determines how far the flame reaches and where the oil sheet meets the combustion air. Common angles run from about 30° (long, narrow flame) to 90° (wide, short flame); the mainstream supply covers 30° to 80°.

Too wide and the spray impinges on the furnace wall or tube bank, cooking oil onto the surface. Too narrow and the flame is long and lazy, with poor mixing and a soft, smoky burn. As a working rule, round and square chambers take the wider angles, slender deep chambers the narrower ones. The correct angle is set by the furnace diameter and the burner throat geometry, not by preference.

Why Oversizing Fails

The instinct is to fit a larger nozzle “for more capacity.” But the burner air is sized for a given firing rate and pattern; a larger nozzle simply dumps oil the air cannot burn. The visible symptoms, black smoke, sooted tubes, oily residue, are the furnace telling you the air side and the fuel side are no longer matched. The right fix is to match nozzle, pump pressure and burner air as a system.

Reference Specifications

The table above is the working reference: six firing rates from 0.5 to 10 GPH, all rated at 100 psi on the standard 9/16“-24 UNEF thread. Atomization type and angle are the independent selection variables.

Range Walkthrough

OB-0.5: 0.5 GPH, 60°, hollow cone. The light end of the range: the smallest residential boilers, workshop and space heaters burning light oil. At 100 psi it delivers roughly 1.9 L/h, about 19 kW of heat, from a tiny orifice that is easy to block. Keep the fuel clean; never clear it with wire.

OB-1.0: 1.0 GPH, 60°, hollow cone. The most common domestic size, the reference point most burner technicians stock against: a single-family boiler or furnace, roughly 3.8 L/h, about 38 kW. When a residential spec calls for the standard nozzle, this is usually it.

OB-2.0: 2.0 GPH, 80°, hollow cone. The top of the hollow-cone class: large domestic systems and light commercial heat, roughly 7.6 L/h, about 76 kW. The wider 80° angle gives a shorter, rounder flame for shallower chambers, hence the upper end of the light-oil bracket.

OB-3.5: 3.5 GPH, 80°, semi-solid. The crossover into boiler duty on medium oil, roughly 13.2 L/h, about 133 kW. Some makers rate this band solid, so confirm the pattern against the burner air register instead of assuming.

OB-6.0: 6.0 GPH, 80°, solid cone. Heavy oil, mid-to-large furnaces, roughly 22.7 L/h, about 228 kW. The solid cone concentrates fuel toward the core for a denser, more compact flame where the air register is designed for it.

OB-10: 10 GPH, 80°, semi-solid. The industrial end, roughly 37.9 L/h, about 380 kW , typically the main stage of a large industrial boiler or multi-nozzle burner.

Application Matrix

Application	Typical firing rate	Recommended model	Why
Residential boiler / furnace	0.5–1.0 GPH	OB-0.5 / OB-1.0	Hollow cone, 60°: the classic domestic bracket
Light commercial heating	1.0–2.0 GPH	OB-1.0 / OB-2.0	Top of the hollow-cone class
Commercial boiler, medium oil	3.5 GPH	OB-3.5	Semi-solid, 80°
Industrial furnace / oven / dryer	6.0 GPH	OB-6.0	Solid cone, 80°
Large industrial boiler, staged	10 GPH	OB-10	Semi-solid, 80°
Marine auxiliary boiler	1.0–3.5 GPH	OB-1.0 / OB-3.5	Compact shipboard chambers; angle and fuel viscosity matter most
Waste-oil / bio-blend burners	Varies	Stainless material options	Heavy or aggressive fuel drives material and maintenance, not the firing rate
Multi-nozzle staged burners	Per stage	Several models	Each stage carries its own GPH, angle and pattern

Three-Step Selection

If the full checklist feels heavy, the decision collapses to three questions, in order:

1. The fuel. What oil reaches the nozzle, and at what temperature? Standard #2 oil runs the ratings as published. Heavy or waste oil needs a larger orifice and a stainless body; bio-blends attack brass over time; cold heavy oil is too viscous to atomize and soots. The fuel sets the material and the maintenance schedule first.
2. The firing rate (GPH). Read it from the burner spec or the old nozzle’s markings: the load the burner must fire, rated at 100 psi. Confirm the pump holds 100 psi at that load before ordering.
3. The spray angle. Let the furnace geometry answer, round and square chambers take wider angles, slender deep chambers narrower ones. Then confirm the pattern against the burner’s air register.

If any step is unknown, the old nozzle is the baseline: its markings answer all three.

How to Select an Oil Burner Nozzle

Work through these in order and the choice usually narrows to one or two models.

• Firing rate required: sets the rated GPH at 100 psi; confirm the pump actually holds 100 psi at that load.
• Spray angle: fixed by furnace diameter and burner geometry, not by the operator.
• Atomization pattern: hollow, solid or semi-solid, matched to the air register.
• Oil grade and temperature: viscosity at the nozzle sets atomization quality; cold heavy oil atomizes poorly and soots.
• Whether the air side is matched: a nozzle change that is not matched to burner air just relocates the problem.
• Availability of the rating: work from the rated GPH at 100 psi, never the nominal thread size.

Frequently Asked Questions

My burner smokes after fitting a bigger nozzle: why? The new nozzle fires more oil than the burner air can burn. Match the nozzle firing rate to the burner, not just to the pump’s output.

Does raising pump pressure increase firing rate? Yes, by the square root of the pressure ratio, but it also degrades atomization past a point: droplets coarsen and the pattern distorts.

What spray angle should I use? The one the furnace geometry demands. Round and square chambers take wider angles, slender deep chambers narrower ones. Too wide impinges on the walls; too narrow gives a long, soft, smoky flame. It is not adjustable by feel.

Why does cold oil soot up the furnace? High viscosity at the nozzle gives poor atomization, so droplets are too large to burn cleanly. Heat the oil to its design viscosity before the nozzle.

What does 9/16“-24 UNEF mean? It is the thread of the nozzle body itself, the standard across the industry. Adapters step it to 1/8“ or 1/4“ NPT for the burner’s fuel line. Order by the nozzle thread, not the adapter size.

How often should an oil burner nozzle be replaced? Treat it as a consumable: yearly for residential service, on inspection for industrial. The orifice erodes in service, flow creeps up and the flame soots. Replace on schedule, not on failure.

What is 1 GPH in litres? One US gallon per hour is about 3.8 L/h, so at 100 psi this range runs from roughly 1.9 L/h (0.5 GPH) to 38 L/h (10 GPH). As a heat guide, each GPH is roughly 38 kW on standard #2 oil.

My nozzle is blocked: can I clear it with a wire? No. The orifice is precision, factory-measured; any metal probe enlarges or scores it and the atomization is gone. Filter the fuel and fit a new nozzle.

Need a nozzle matched to your burner? The oil burner nozzle overview walks the full selection, or send your firing rate and chamber details via the enquiry form.