Table of Contents
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.
The three numbers are not independent options you can mix freely. They are one system: GPH sets how much fuel enters, the angle sets where the cone lands, and the pattern sets how the combustion air can reach every droplet. Change any one of them and the other two stop meaning what the nameplate says. This guide walks each of the three in the order they fail, then shows how to read the whole triplet against the firebox drawing instead of against the parts bin.
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 BTU or kW 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 number is easy to convert because fuel oil carries a nearly fixed amount of heat per gallon. A US gallon of No. 2 fuel oil releases roughly 140,000 BTU when burned completely, so the design GPH falls straight out of the furnace rating:
GPH = furnace input (BTU/hr) ÷ 140,000
That produces the familiar catalog sizes: a 420,000 BTU/hr furnace wants 3.0 GPH, a 280,000 BTU/hr furnace wants 2.0 GPH, and a 700,000 BTU/hr unit wants 5.0 GPH.
| Furnace input (BTU/hr) | Design GPH at 100 psi | Typical duty |
|---|---|---|
| 140,000 | 1.00 | Small space heaters, residential boilers |
| 210,000 | 1.50 | Small commercial boilers |
| 280,000 | 2.00 | Medium boilers and furnaces |
| 350,000 | 2.50 | Commercial heat |
| 420,000 | 3.00 | Larger boilers |
| 560,000 | 4.00 | Industrial process heat |
| 700,000 | 5.00 | Larger industrial burners |
Typical values for No. 2 fuel oil. Heavier grades carry slightly more heat per gallon and lighter grades slightly less, so convert with the fuel’s actual heating value when the duty is critical.
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.
The geometry is worth doing once with a calculator, because it converts a catalog number into a physical footprint. The cone the oil occupies at a distance d from the cap has a diameter of:
Cone width ≈ 2 × d × tan(θ/2)
At 12 inches from the cap, a 60° cone is about 13.9 inches wide, a 90° cone is 24 inches wide, and a 30° cone is only 6.4 inches wide. At 18 inches the same nozzles spread to 20.8, 36 and 9.6 inches. The angle alone moves the flame’s footprint by a factor of three or more with no change in fuel rate.
| Spray angle | Cone width at 12 in | Cone width at 18 in | Firebox it fits |
|---|---|---|---|
| 30° | 6.4 in | 9.6 in | Long, narrow chambers; flame must travel |
| 45° | 9.9 in | 14.9 in | Long chambers, moderate diameter |
| 60° | 13.9 in | 20.8 in | Balanced boxes, mid-length |
| 80° | 20.1 in | 30.2 in | Short, wide chambers |
| 90° | 24.0 in | 36.0 in | Very wide, shallow fireboxes |
Pick the angle so the cone fills the firebox cross-section in the first third of the chamber, with a margin of a few inches from the walls. 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.
Hollow, Solid or Semi-Solid: Match the Pattern to the 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 center. It needs a swirling air head that can wrap the ring and burn it from both sides. It atomizes finely and lights fast, which is why it suits many pressure-jet burners with a strong tangential air register.
- Solid cone fills the center, 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.
| Pattern | Droplet distribution | Air head it matches | Where it earns its keep |
|---|---|---|---|
| Hollow cone | Ring of fine droplets, empty center | Swirling / tangential register | Fast light-off, fine atomization, short flames |
| Solid cone | Dense fill across the whole cone | Axial airflow into the load | Deep chambers, flame must reach the load |
| Semi-solid | Filled core, softer edge | Mixed swirl plus axial | The common compromise on packaged burners |
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. The air register geometry is part of the burner’s design, not something a cap change can compensate for.
What the Wrong Angle Does to the Firebox
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.
| Symptom | Likely cause | Fix |
|---|---|---|
| Black wet streaks on side walls | Angle too wide; cone washes the walls | Narrow the angle one family |
| Local hot spot, spalled tile | Angle too narrow; cone impinges the back | Widen the angle |
| Long, lazy, sooty flame | Cone too narrow for the chamber length | Widen angle; check GPH |
| Clean walls but smoke | Pattern wrong for the air head | Change pattern, not angle |
The angle and pattern symptoms look alike from the control panel, which is why the diagnosis belongs on the firebox drawing: measure the chamber, draw the cone at the actual firing distance, and the wrong number shows itself before you buy the next cap.
Pressure and Viscosity: The Two Numbers That Move the Cap
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 the angle varying only a little with pressure). Flow through the orifice follows the square-root law:
Q₂ = Q₁ × √(P₂ / P₁)
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. A 3.0 GPH cap at 100 psi delivers only 2.60 GPH at 75 psi and 2.12 GPH at 50 psi. Conversely, the same cap at 150 psi flows 3.67 GPH, 22 percent over its stamp without anyone changing the part.
| Supply pressure | Flow factor (√P) | A 3.0 GPH stamp delivers |
|---|---|---|
| 50 psi | 0.71 | 2.12 GPH |
| 75 psi | 0.87 | 2.60 GPH |
| 100 psi | 1.00 | 3.00 GPH |
| 125 psi | 1.12 | 3.35 GPH |
| 150 psi | 1.22 | 3.67 GPH |
Viscosity is the second mover. Pressure atomizers work on the fluid’s ability to shear into droplets, and they hold their spec inside a viscosity band: roughly the low hundreds of Saybolt seconds at the nozzle for most furnace caps. Heavy grades (No. 4 and No. 6 oil) must be preheated until they sit inside that band, or the same cap coarsens, widens its droplet size, and soots regardless of how well you matched it. If the fuel changes grade or the preheater drifts, re-check the triplet. The cap has not moved, but the fluid it is atomizing has.
When a Pressure-Jet Cap Is Not the Right Tool
The GPH / angle / pattern triplet assumes a pressure-jet burner: the oil is pushed through the cap’s swirl chamber at line pressure and atomizes by pressure alone. That covers most packaged oil burners, but not all of them. When the fuel is too viscous to atomize at practical pressures, or the duty needs a finer, more controllable spray than a fixed cap can give, the burner moves to air-atomizing (or air-assisted) atomization, where compressed air or steam does the shearing instead of pressure alone.
The boundary is practical, not theoretical. A pressure cap is a fixed-geometry device: one GPH, one angle, one pattern, one pressure rating. An air-atomizing burner turns down and up over a wider range, handles heavier oil with less preheat, and changes droplet size with the air-to-liquid ratio. But it costs more to run, compressed air is real money, and it needs a compressor and controls the fixed cap does not. If the furnace runs on No. 2 oil at a steady load, the fixed cap is the cheaper, simpler answer. If the load swings, the fuel is heavy, or the flame must be shaped precisely, that is the air atomization side of the nozzle family, a different selection, with the same rule: match the atomizer to the air and the firebox, not to habit.
Putting GPH, Angle and Pattern Together
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.
| Triplet | Fits when | Fails when |
|---|---|---|
| High GPH + wide angle | Short wide chambers at high load | The chamber is deep; the walls wash |
| Low GPH + narrow angle | Long narrow chambers | Load shortfall; flame never reaches the load |
| Hollow cone + swirling air | Fast light-off and fine spray | Axial air heads; wall wetting |
| Solid cone + axial air | Penetrating flame into the load | Swirling heads; soot |
A practical field 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.
A worked example
A furnace is rated 420,000 BTU/hr, so the design GPH at 100 psi is 3.0. The firebox is 24 inches in diameter and 48 inches long, short relative to its width, so the flame must fill the cross-section quickly rather than travel. From the width table, an 80° cone reaches 20.1 inches wide at 12 inches from the cap, filling the 24-inch box with a safe margin; a 90° cone is 24 inches wide, touching the walls. The burner’s air register is a mixed swirl-and-axial design, so the pattern is semi-solid. The correct triplet: 3.0 GPH / 80° / semi-solid. It 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 9.9 inches wide at 12 inches, less than half the firebox, so the flame stretches the full 48 inches hunting for air 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 / 80° / semi-solid, not a stronger igniter.
GPH to Burner Model: What the Appliance Plate Tells You
A pressure-jet cap is rated in gallons per hour at a reference pressure, but the burner it bolts onto has a published capacity window. Read the appliance nameplate or the burner model spec first, then pick a cap whose GPH lands inside that window with margin, never the other way around. The commercial burner families below (R.W. Beckett CF series, published capacity data) show how wide those windows are:
| Burner model | Capacity window | Typical service | Cap GPH to start from |
|---|---|---|---|
| Beckett CF375 | 1.65 – 3.75 GPH | Light commercial / process heat | 1.65–3.00 |
| Beckett CF1000 | 4.00 – 10.00 GPH | Commercial boilers / furnaces | 4.00–8.00 |
| Beckett CF1400 | 4.00 – 13.60 GPH | Larger process burners | 6.00–12.00 |
| Beckett CF2300 | 17.00 – 19.90 GPH | High-output commercial | 17.00–19.00 |
| Beckett CF3500 | 17.00 – 35.00 GPH | Light industrial, dual-nozzle | 20.00–30.00 |
Three things the table does not tell you, and that matter more than the model number:
- The pump pressure moves the window. Beckett commercial fuel units run 100–300 psig. A 4.0 GPH cap at 150 psi delivers only about 2.4 GPH effective at 100 psi (flow ∝ √P), so a CF1000 at low pump pressure needs a larger cap than the nameplate math suggests. Always size from the actual pump pressure, not from the printed window.
- The air register and chamber decide the pattern. The CF-series manuals call for 45°–70° solid cones as the standard oil-nozzle specification. If the chamber is short and wide, the 70° end of that range fills the box; if it is long and narrow, the 45° end keeps the flame off the walls. Pattern selection still comes last, but it comes after the GPH window is fixed.
- A dual-nozzle burner splits the load. The CF3500 is rated for two nozzles at 250–300 psi. Two caps at 60% each give turndown that one big cap cannot: read each cap’s GPH at the actual supply pressure, not the sum of the ratings at different pressures.
The correct sequence in practice: nameplate heat load → GPH at the actual pump pressure → burner model window check → angle from the firebox geometry → pattern from the air head. Jumping straight to a part number on the shelf skips the two checks that prevent the smoke test failures described above.
Reading the Cap Like a Buyer
For a plant that buys furnace nozzles in batches, the purchase spec is the triplet plus the pressure and fuel it was matched at:
- GPH at rating pressure: the stamp assumes 100 psi unless stated otherwise.
- Spray angle: the one number most buyers skip and most fireboxes punish.
- Pattern: hollow, solid or semi-solid, matched to the air head.
- Rated pressure and fuel grade: what the triplet was quoted at.
- Body and orifice material: steel and stainless caps for standard duty; check the spec for corrosive or abrasive fuels.
Keep the triplet recorded with the furnace, not just in the storeroom. When a cap is replaced, replace it with the same three numbers: a “2.75 instead of 3.00 because the box was empty” swap is exactly how a clean burner starts smoking. Inspect pulled caps: an eroded orifice delivers more flow than the stamp says, and a worn cap behaves like an oversized one. That is the one maintenance item that silently changes the triplet all on its own.
| Inspection finding | What it means | Action |
|---|---|---|
| Orifice visibly enlarged or ragged | Erosion from firing hours | Replace; record the GPH shift |
| Carbon ring on the cap face | Flame impingement or a rich run | Check angle, pattern and air |
| Internal passage plugged | Fuel deposits or filter carry-over | Clean or replace; check fuel filters |
| Cap-to-cap flow spread | Tolerance or wear differences | Batch-match; replace odd caps |
A Quick Selection Sequence
Work the decision in the order that eliminates failures fastest:
- Heat load first: furnace input BTU/hr ÷ 140,000 gives the design GPH at 100 psi.
- Pressure second: measure at the nozzle while firing, hot; correct the GPH with the square-root law.
- Angle third: match the cone width to the firebox cross-section using 2·d·tan(θ/2); leave a margin from the walls.
- Pattern fourth: match the cone shape to the air head: hollow for swirl, solid for axial, semi-solid for mixed.
- Fuel fifth: hold viscosity inside the atomizer’s band; preheat heavy grades.
- Record the triplet: GPH / angle / pattern with the furnace so every future replacement matches.
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. Compute the cone width at the firing distance with 2·d·tan(θ/2) and leave a few inches of margin from the walls.
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.
What if the fuel grade changes mid-season? Re-check the triplet. A heavier grade needs preheat to hold the atomizer’s viscosity band; a lighter grade can coarsen differently. The cap has not moved, but the fluid it atomizes has.
Does an eroded nozzle count as oversizing? Effectively yes. Orifice erosion raises flow above the stamp, shifting the triplet toward the rich side. Inspect pulled caps and replace worn ones with the recorded triplet.
How do I measure the firebox for the angle? Diameter and length are enough. Compute the cone width at a distance equal to the first third of the chamber length, and choose the angle that fills the diameter with margin. That is the whole geometry.
The Triplet Is the Spec
A fuel oil furnace nozzle is three numbers, not one: GPH for the heat load, angle for the firebox geometry, pattern for the air head. Match all three at the pressure the pump actually holds and the burner fires clean; change one and the others stop meaning what the nameplate says. The companion guide on oversized oil burner nozzles covers what happens when the size half of the triplet is wrong. For a furnace drawing, pump pressure at the nozzle and fuel grade, the BoreJet oil burner nozzles range lists the GPH, angle and pattern options together, or send the details through the enquiry form and the match can be confirmed before you buy.
Next Step
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Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.