Picking nozzles for fuel oil furnace duty is treated like a parts-bin decision: read the gallon number, grab the cap, fit it. But the gallon number is only the first of three things the firebox cares about. The spray angle decides where the flame goes, and the atomization pattern decides how the air head can shape it. A nozzle that is perfect on GPH but wrong on angle will scour the furnace wall; wrong on pattern it will soot no matter how clean the oil is. This guide covers how to match all three to the furnace instead of to the catalogue.
GPH is heat load, not a part number
The GPH stamped on a fuel oil burner spray nozzle is the heat the burner must deliver, expressed as fuel. It is set by the furnace duty — the kWh or BTU the chamber is designed to put into the load — not by the operator’s preference. Size GPH to that load at the pressure the pump actually holds, and you have the first half of a correct nozzle.
The mistake is sizing GPH to “what the last nozzle was” or “one bigger because it was weak,” which is how furnaces end up over-fired or starved. Start from the firing rate the furnace was designed for, confirm the supply pressure at the nozzle while hot, and let GPH fall out of those two numbers. The oil burner nozzle size guide covers why the pressure matters as much as the stamp.
Spray angle decides where the flame goes
Spray angle is the cone the oil fills as it leaves the cap, and it runs roughly 30° to 90° on furnace nozzles. The angle has to match the firebox diameter and length. Too narrow and the cone stays a tight pencil that drives straight into the back wall or the burner tile; too wide and it fans out and washes the side walls long before the oil has burned. Neither is a combustion problem you can tune out with air — it is geometry, fixed the moment you chose the cap.
A 30° to 45° narrow cone belongs in a long, narrow chamber where the flame should travel. A 60° to 80° wide cone belongs in a short, fat chamber where the flame must fill the cross-section quickly. The 90° end is for very wide, shallow fireboxes. Pick against the chamber drawing, not against habit.
When the angle is wrong: wall impingement and a long lazy flame
Wrong-angle failures are unmistakable once you know them. An angle that is too wide lays a black, wet streak of unburned oil on the side walls — the oil hits cold refractory, soaks in, and cokes. An angle that is too narrow throws a hard pencil that impinges the back wall or the burner tile, burns a local hot spot, and can spall the refractory.
The other signature is a flame that is simply too long. A narrow cone in a short chamber means the flame stretches down the chamber hunting for air, soots, and heats the wrong zone. Operators often “fix” this by opening the air register, but the air was never the problem — the cone was aimed at the wrong geometry. Changing the angle shortens and tidies the flame without touching the air.
Hollow, solid and semi-solid: which pattern for which air head
The pattern is the shape of the liquid cone, and it has to match the burner’s air head — the way the combustion air is delivered around the nozzle. Three families cover furnace duty:
- Hollow cone throws a ring of fine droplets with an empty centre. It needs a swirling air head that can wrap the ring and burn it from both sides. It atomises finely and lights fast, which is why it suits many pressure-jet burners with a strong tangential air register.
- Solid cone fills the centre, giving a dense, penetrating cone. It suits fireboxes where the flame must reach into the load rather than spread wide, and where the air head feeds more axially.
- Semi-solid (or part-full) cone sits between the two — a filled core with a softer edge — and is the common compromise for burners whose air head is neither purely swirling nor purely axial.
The trap is fitting a hollow-cone cap to an axial air head, or a solid cone to a swirling one. The pattern and the air stop cooperating, droplets fall out of the airstream unburned, and you get soot and coke despite a correct GPH and angle. Match the pattern to the air head the burner was built with.
Putting GPH and angle together for the firebox
The three numbers are a system. GPH sets how much oil; angle sets where the cone lands; pattern sets how the air head can burn it. The correct choice is the triplet that fills the firebox completely, reaches the load, and leaves no oil hitting a cold surface. A practical check:
- Too much soot on the walls → angle too wide or pattern not matching the air head.
- Local hot spot or spalled tile → angle too narrow, impinging.
- Flame too long and lazy → angle too narrow for a short chamber, or GPH too high for the air.
- Clean walls, tidy flame, stable light-offs → the triplet is right.
Pressure, viscosity and the nameplate
Even a perfectly matched nozzle only behaves to spec if the fluid and pressure cooperate. The GPH, angle and pattern on the cap are all quoted at the rating pressure (commonly 100 psi for the GPH, with angle varying only a little with pressure). If your supply sags, GPH falls with the square root of pressure and the cone tightens slightly, so a wide-angle nozzle starts behaving narrower than stamped. Preheat heavy oil to the viscosity band the burner wants, or the same cap coarsens and soots regardless of how well you matched it.
A worked sizing example
Say a furnace is designed for 3.0 GPH at the burner and the pump holds 100 psi at the nozzle, hot. The chamber is short and wide, so a 60° semi-solid cone fits the geometry and the burner’s mixed air head. That triplet — 3.0 GPH, 60°, semi-solid — fills the box, lights clean, and leaves the walls dry.
Now suppose someone fits a 4.5 GPH, 45° hollow instead, “for more output.” The GPH is 50 percent high for the air, the 45° cone is too narrow for the short chamber so it stretches and soots, and the hollow pattern fights the mixed air head. The result is black smoke, a long flame and coke — three failures from one wrong triplet. The fix is the original 3.0 / 60° / semi-solid, not a stronger igniter.
The full range of GPH, angle and pattern options for nozzles for fuel oil furnace duty is on the BoreJet oil burner nozzles page. Send us the furnace drawing and the pump pressure at the nozzle and our application team will return the triplet that fills your firebox instead of scouring it.
Frequently asked questions
How do I choose the spray angle for my furnace? Match it to the firebox. Narrow 30–45° cones suit long chambers where the flame should travel; wide 60–90° cones suit short, fat chambers that need the flame to fill the cross-section fast. Too wide washes the walls, too narrow impinges them.
Hollow, solid or semi-solid cone — which is right? It depends on the burner air head. Hollow cones need a swirling air register; solid cones suit axial airflow into the load; semi-solid is the compromise for mixed air heads. Match the pattern to the air delivery, not just the GPH.
My flame is too long and soots — is that the nozzle? Often yes. A cone too narrow for a short chamber stretches the flame down the box hunting for air. Widening the angle shortens and tidies the flame without opening the air register.
Does supply pressure change the angle? GPH falls with the square root of pressure when the supply sags, and the cone tightens slightly, so a wide-angle nozzle behaves narrower than stamped. Hold rating pressure at the nozzle, hot, for the cap to match the firebox.
Can the right GPH still soot if the pattern is wrong? Yes. A hollow cone on an axial air head, or a solid cone on a swirling one, drops droplets out of the airstream unburned. The pattern has to match the air head or you coke regardless of GPH.