Reading a Burner Nozzle Chart: Which Size Fits?

The Snapshot

• A #6 hollow element rated 1.50 gph at 100 psi delivers 5.68 L/h of oil.
• A #4 to #6 element step multiplies flow by 1.50 at any reference pressure.
• 1.20 gph at 140,000 Btu per gallon is 168 MBH, which needs 10.5 to 21 ft³ of firebox.
• A pump that holds 144 psi turns a 1.35 gph chart row into 1.62 gph, a 20 percent error.

The Number on the Ticket Is Not a Delivered Number

A tech meets low output by installing the next size up. The boiler then fires rich, because the chart row assumed 100 psi. The pump actually holds 144 psi, and sqrt(144/100) = 1.20, so a 1.35 gph element delivers 1.62 gph. That is 405 excess gallons in one 1,500 hour season. At a 3 to 4 dollar band, the misread row costs 1,200 to 1,600 dollars.

A burner nozzle chart is a physics statement, not a shopping list. Each column of an oil nozzle chart encodes one controllable variable. Every number stays conditional on a stated reference state. Reading it well starts with the columns and ends with a measured firing rate.

What Each Chart Column Actually Encodes

An oil burner nozzle size is not one number. It is orifice area, swirl volume, reference pressure and reference viscosity, printed as one line. Decode the columns one by one.

Column	Typical entry	What it encodes	What to verify
Element code	BS, B, A, 3, 4, 5, 6, 7, 8	Orifice and swirl size class	Step ratios, next section
Flow rating	1.35 gph at 100 psi	Volume per hour at that reference	Gauge reading, timed catch
Spray angle	60 degrees	Nominal included cone angle	Static pattern test
Pattern type	hollow, solid, semi-hollow, sight feed	Radial spread of the same flow	Firebox diameter and tile
Pressure reference	100 psi, sometimes 70 psi	The one state where gph is true	Pump curve at the nozzle tee
Viscosity reference	No. 2 oil near 20 Celsius	Assumed fluid for the flow row	Oil temperature at the nozzle

Treat rating tolerance as a published band of about 0.10 gph either side at reference. Confirm by measurement, not by label. The column most readers skip is the pressure reference. That row is a promise, not a delivered value.

NFPA 31 governs the supply piping and controls around the burner. The chart is the nozzle datasheet inside that system.

The pattern letter deserves its own look, because two rows with identical gph burn differently. A hollow element puts most liquid at the cone edge and leaves a light core. A solid element spreads mass toward the centre. Semi-hollow splits the difference.

A sight feed element adds a small pilot jet through the same housing. That jet lets a tech confirm atomization through a port, without opening the door. None of these labels changes the volume column. They change where the same volume lands. Pattern and angle belong to the firebox, not to the fuel pump.

The Arithmetic of the Element Size Ladder

Each element size is a fixed area step, so flow ratios are constants, not opinions. The published rating ladder at 100 psi reference runs as follows.

Element	gph at 100 psi	Same element at 70 psi	Ratio to the rung below
4	1.00	0.84	-
5	1.25	1.05	1.25
6	1.50	1.26	1.20
7	2.00	1.67	1.33

The #4 to #6 step is 1.50 divided by 1.00, so flow rises 1.50 times. Adjacent rungs sit between 1.20 and 1.33 times. Both rows scale with the square root of pressure, so the ratios survive any reference. At 70 psi the same step reads 1.26 divided by 0.84, which is 1.50.

Pressure changes flow, not the ratio between rungs. Some charts drop odd rungs, so the ladder you see is a subset. A pump curve that ends between printed rows means input sits between rungs. Choose the element from that ratio, never from the old ticket.

This is where an oil burner nozzle size debate becomes plain arithmetic. Two suppliers can print 1.25 and 1.35 gph for nominally similar elements. The honest comparison is the ratio to the rung below, not the two-digit label.

Firing Angle Spray Pattern and Cone Geometry

Pattern type decides where fuel lands, not how much lands. A hollow cone from a spin chamber puts liquid on a ring with a light core. A solid cone fills the disc. A semi-hollow sits between the two.

A sight feed element adds a pilot stream, so a tech sees atomization through a port. The firing angle spray pattern choice is a fit check against firebox diameter.

Coverage width across a cone at distance r follows one formula:

width = 2 * r * tan(theta / 2)

Substituted at r = 24 in, with theta as the included nozzle angle:

Nozzle angle	Width at 24 in	Fit note
30 degrees	12.9 in	Narrow flame, long throw boxes
45 degrees	19.9 in	Mid-range boxes
60 degrees	27.7 in	Common residential boiler nozzle field
90 degrees	48.0 in	Wide boxes, tile soot risk if too wide

Width scales with distance, which the same formula shows. At 18 in, a 60 degree cone spans 2 * 18 * tan(30) = 20.8 in. At 36 in it spans 41.6 in, exactly double.

Take a 36 in diameter box: the wall sits 18 in out, so a 60 degree cone covers 20.8 in and fills it. Take a 48 in box: the wall sits 24 in out, needing 48 in of width. A 90 degree cone gives 48.0 in there, so 60 degrees would leave the outer gas volume unswept. Chart angles are nominal values, so confirm with a paper test before trusting geometry.

Droplet Size Reads Through the Pressure Column

Charts list gph, yet atomization quality follows the same reference pressure. For pressure atomizers, the published relation is d32 proportional to P^-0.35 to P^-0.5. Take a nominal 60 micron Sauter mean diameter at 100 psi.

Pressure	d32 at exponent -0.5	d32 at exponent -0.35
100 psi	60 micron	60 micron
200 psi	42 micron	47 micron
300 psi	35 micron	41 micron

Doubling pressure cuts the mean droplet size by 21 to 29 percent, a published range. Finer mist burns faster, shortens the flame and raises entrainment. That shift explains why a rich flame at high pressure can still look short and bright. It also warns against judging a chart row by flame color alone. The same 60 micron row can read blue in one box and yellow in another, because residence time changed, not droplets.

Viscosity and Temperature Move the Delivered Flow

A chart row is a contract with one fluid state. No. 2 distillate sits near 3 to 6 centistokes at 20 Celsius, a published range. Viscosity climbs as oil cools.

Oil condition	Approx. viscosity (cSt)	Delivered gph versus a 1.35 gph row
Stored oil at 5 Celsius	8 to 12	about 5 percent low, near 1.28 gph
Reference state, 20 Celsius	3 to 6	1.35 gph, the printed row
Warm line return, 40 Celsius	3 to 4	up to 1 percent high
No. 4 to No. 6 grades, cold	well above range	chart invalid without preheat

Two corrections follow. Discharge coefficient drops as viscosity rises, and published field bands put winter loss for light oil at 3 to 7 percent. The table uses the 5 percent midpoint. Droplet size also coarsens with viscosity. Doubling centistokes raises d32 by 1.15 to 1.23 times, a published exponent band of 0.2 to 0.3.

So a cold morning flame runs wetter even at correct gph. Heavier grades need preheat to about 15 to 20 cSt at the burner tip. No printed row survives cold heavy oil.

Firebox Check Before You Trust Any Row

Convert the row to heat, then test the box. MBH = GPH times heating value per gallon. For No. 2 oil the published heating value range is 138,000 to 146,000 Btu per gallon. Use the midpoint, 140,000:

1.20 gph * 140,000 Btu/gal = 168,000 Btu/h = 168 MBH

At 0.293 kW per MBH, 168 MBH is 49.2 kW. The published firing rate band for atmospheric oil-fired boilers runs about 8,000 to 16,000 Btu per cubic foot per hour. State it as a published range, then test three boxes.

Firebox	Volume (ft³)	MBH per ft³	Verdict
1.25 by 1.25 by 5.0 ft	7.8	21.5	Over the band, soot and coke risk
2.0 by 2.0 by 4.5 ft	18.0	9.3	Inside the band, row is credible
4.0 ft dia by 6.0 ft	75.4	2.2	Under the band, weak flame risk

Undersizing is the mirror trap. A 168,000 Btu/h input needs 168,000 / 140,000 = 1.2 gph. A 0.75 gph element on that nameplate starves the box and trips low-fire limits. Flame length matters beside volume.

A long, soft flame in a short box impinges the tube sheet. The chart row that produced it can still read correct on paper. Volume math sizes heat release. The pattern and angle rows decide whether the flame fits the shape.

Cross-Check the Chart Against the Pump Curve

The chart promises delivery; the pump curve decides it. Measure at the nozzle tee while firing, per NFPA 31 piping practice. UL 296 covers the listed burner assembly. EN 12016-3 covers spray pattern and droplet characterization for verification.

A timed catch turns rows into evidence. Take 1.35 gph: 1.35 * 3.785 = 5.11 L/h, which is 0.0852 L/min. In 3 minutes a graduated cylinder should catch 0.26 L at the stated pressure.

Orifice theory supports the method with water. Q(L/min) = 0.65 * d(mm)² * sqrt(P(bar)), with Cd near 0.95 folded into the constant. Take d = 0.64 mm at 100 psi, since 100 * 0.06895 = 6.895 bar:

0.65 * 0.64² * sqrt(6.895) = 0.65 * 0.4096 * 2.626 = 0.70 L/min

That water case is 42 L/h, or 11.1 gph of water. Oil rows are far smaller because burner orifices are tiny and swirl volume dominates. Use the equation for method checks, not for guessing an oil row from diameter.

Then read combustion. A tuned pressure-atomized oil flame shows a published CO2 band of 13.5 to 15.5 percent, with 14 to 15 percent typical after balancing. If the catch reads 1.62 gph while the chart said 1.35, the mixture is rich before the flue moves. Expect CO2 to sag and the smoke number to climb.

Firebox static should sit near zero. Barometric draft belongs in a published 0.02 to 0.04 in w.c. band. Draft tells the second half of the story.

A weak draft under-fires air, so CO2 drops even when gph matches the chart. Rule out draft before blaming the row.

The logic tree is short:

• Measured gph above chart: check pump pressure and seat wear.
• Measured gph below chart: check cold oil, screens and pump wear.
• Measured gph equal to chart, bad flame: the nozzle angle or pattern is the wrong firebox fit.

Scale Traps When the Chart Uses 70 psi, Metric or a Different Family

Charts are not written to one house style. North American residential rows usually sit at 100 psi. Older European light-oil charts often print at 70 psi. Metric charts list L/h at bar.

A boiler nozzle datasheet from a heating-oil unit may quote input rating in MBH instead of gph. Every one of those is a different unit of the same promise. Decide one reference, then convert all rows into it before comparing anything.

Convert with GPM2 = GPM1 * sqrt(P2 / P1). The factors are sqrt(70/100) = 0.837 and sqrt(100/70) = 1.195. A 1.50 gph row printed at 100 psi is equivalent to 1.50 * 0.837 = 1.26 gph on a 70 psi scale.

Now invert the trap. Order the 1.26 gph element rated at 70 psi, run it at 100 psi, and it delivers 1.26 * 1.195 = 1.51 gph, which is the row you meant. Order the 1.50 gph row read off a 70 psi chart and run it at 100 psi instead, and it delivers 1.50 * 1.195 = 1.79 gph, about 19 percent over the intended firing rate. Convert every row to one reference before ordering.

Metric trap: 5.0 L/h at 20 bar equals 5.0 / 3.785 = 1.32 gph of water at 20 bar. Scale it to 100 psi, which is 6.895 bar: 1.32 * sqrt(6.895/20) = 1.32 * 0.587 = 0.77 gph. A metric row is never a gph row in disguise.

Three label traps remain. Some tables say gal/hr, others L/h, and 1 gph = 3.785 L/h. Sight feed elements often carry no rating row, so demand vendor data. Pattern letter codes differ between families, the same problem any spray tip size chart comparison brings.

Oil burner nozzles suppliers should state the reference pressure on every line. Where they do not, treat that row as unusable.

Selection Workflow, Step by Step

Step	Action	Numeric rule
1	Read nameplate input	168,000 / 140,000 = 1.2 gph target
2	Measure pump pressure at the tee	gauge psi while firing
3	Read the chart at that reference	only matching rows, or convert
4	Firebox diameter check	width = 2 * r * tan(theta/2)
5	Firebox volume check	8 to 16 MBH per ft³, published
6	Element code from the ratio	two rungs near 1.33 to 1.50x
7	Fire and catch	5.11 L/h for a 1.35 gph row
8	Balance combustion	CO2 14 to 15 percent, draft 0.02 to 0.04 in w.c.

Work the steps in order and the chart reads itself. Burner family context sits in the oil burner nozzle overview guide. That page frames where pressure-atomized rows live among steam and mechanical atomizing designs.

Materials, Wear Life and Filter Screens

A hollow element carries the metering orifice and the spin slots. Those two features define the chart row. Elements and bodies ship in brass, with stainless variants. Carbide handles the seat and stem interface in serviceable stems. A worn seat changes flow, not pattern.

Filter screens matter as much. Pump strainers and nozzle inlet screens, in a published 100 to 200 mesh band, catch the solids that foul spin slots. A clogged screen mimics a lower chart row and distorts the cone.

Wear has published arithmetic. A hole diameter increase of x percent raises flow about (1 + x/100)² - 1. A 5 percent worn orifice flows 1.05² - 1 = 10.25 percent high. So a 1.35 gph row becomes 1.49 gph. Over a 2,000 hour season that drift wastes about 280 gallons.

Replace the element when the catch test drifts beyond 5 percent of rating. Seasonal change is documented service practice.

Ask why wear moves flow before replacing parts blindly. Erosion opens the metering orifice and rounds the spin-slot edges. One effect raises gph through the area law. The other widens the cone and coarsens droplets at the same pressure. A worn element can therefore match the catch weight and still fail the pattern test, so both checks belong in the record.

Common Chart Reading Errors, With the Numbers

Error	Arithmetic	Consequence
Reading a 100 psi row on a 144 psi pump	1.35 * 1.20 = 1.62 gph	20 percent rich input
Ordering a 70 psi row and running 100 psi	1.06 * 1.195 = 1.27 gph	15 percent under-delivery
Confusing gph with gpm	1.2 gpm = 72 gal/h	60 times intended firing rate
Copying the previous element code	#4 to #6 is 1.50x flow	repeated rich operation
Ignoring 5 percent seat wear	1.05² - 1 = 10.25 percent	about 280 wasted gallons per season
Angle swap without the box check	60 degrees gives 41.6 in at 36 in	flame impingement in a 30 in box
Cold No. 2 oil at 5 Celsius	about 5 percent low flow	CO2 sag and weak flame

Every row above is a reading error, not a hardware defect. That is why verification is the cheapest fix available. If you inherited a plant full of misread rows, see the soot and coke side in the oversized oil burner nozzle consequences catalogue.

Verification and Acceptance Criteria

Write acceptance numbers into the work order before the element goes in. A defensible list:

• Firing rate within 5 percent of the chart row, by timed catch in L/h or gal/h.
• Pump pressure within 5 psi of the reference used to read the burner nozzle chart.
• Firebox intensity inside the published 8 to 16 MBH per ft³ range, volume in ft³.
• Static pattern test at 18 in, cone edge within 10 percent of nominal angle, per the EN 12016-3 method family.
• Combustion: CO2 at least 14 percent, soot-free visual against a smoke comparison block, draft 0.02 to 0.04 in w.c.
• Recorded oil temperature at the nozzle, against the chart assumption near 20 Celsius.

Soot or a short flame despite correct catch weight is an angle or pattern mismatch, not a flow fault. Close the loop with the firing record, not the box label. A boiler nozzle that passes all six lines needs no further argument.

Frequently Asked Questions

Are burner nozzle charts rated at 70 or 100 psi?

Both appear. Residential rows usually print at 100 psi. Some older and European light-oil charts print at 70 psi. Convert with GPM2 = GPM1 * sqrt(P2/P1): a 1.35 gph row at 100 psi equals 1.35 * 0.837 = 1.13 gph on the 70 psi scale.

How much flow does one element size change?

Adjacent rungs on the published ladder run 1.20 to 1.33 times. A #4 to #6 step is 1.00 gph to 1.50 gph, so 1.50. The ratio holds at every reference pressure.

How do I choose the nozzle angle from the firebox?

Use width = 2 * r * tan(theta/2). A 36 in box puts the wall at r = 18 in. Then 2 * 18 * tan(30) = 20.8 in, which is the 60 degree cone width there.

Does pump pressure change the spray angle?

No. The square root law changes gph, not the included angle. A 60 degree element stays a 60 degree cone. It simply carries more or less liquid per hour.

My CO2 dropped but the nozzle is new. What did I read wrong?

Usually the pressure reference. If the pump moved to 144 psi, a new 1.35 gph element delivers 1.62 gph. The flame runs rich regardless of the label.

Will a cold morning change the chart flow?

Slightly. No. 2 oil at 5 Celsius carries higher viscosity. Published field bands put the loss near 5 percent, so expect about 1.28 gph from a 1.35 gph row.

Can oil burner nozzles suppliers from different brands swap chart rows?

Only convert rows sharing the same pressure reference and pattern definition. Pattern naming differs between chart families. The cross-brand spray tip chart guide shows the method for wider catalogs. The high-pressure washer nozzle chart guide covers the GPM world that fuel charts avoid.

The Bottom Line

A chart is a rated promise at one pressure and one fluid state. The check is always a firing test, never the label.

One unverified row moves input by double-digit percents, and the combustion system rides on it. Size from measured pressure, firebox volume and a timed catch, then let stack readings sign off.

Need a second pass on a burner nozzle chart decision? Send the element code, reference pressure and firebox dimensions. The BoreJet contact desk will review the row against your pump curve. Elements for the common rungs sit in the oil burner nozzle line. Start the check through contact, and keep the catch record with the work order.