How to Estimate Oil Use From Nozzle Size and Burner Run Time

Why this matters

"We filled the tank a month ago and it's nearly empty. Is something wrong?" An oil customer asking that is really asking one of three questions: is oil leaving the tank some way other than through the burner, is the burner firing more than it should, or is the house simply needing more heat. You cannot tell those apart from the tank gauge. You can tell them apart with two numbers that already exist: how much oil the burner sprays per hour, and how many hours it ran. If gallons per hour times hours matches the delivery tickets, the oil went through the burner, and the conversation moves to why the house needed it. If it does not match, the size and direction of the gap tell you where to look.

Before anything: the leak and the licensed work

• Look and smell first. Walk the tank, the filter, the supply line and the burner. Fresh staining, a wet line, oil on the floor or a strong oil odor means stop: do not run the burner, keep flame and sparks away, put absorbent down where oil is pooling, and call the fuel dealer. An outdoor or buried tank that may be leaking into soil is also an environmental reporting matter, and who must be told and how fast varies by state; the dealer usually knows the local rule.
• Do not open the burner to read the nozzle or the pump. Pulling the drawer assembly or putting a gauge on the fuel pump is oil-burner service, and some states and cities license it. Everything on this card works from the service records, the burner's labels and an hour meter. If a number has to be measured, the oil technician measures it.
• Hour meters and thermostats are read, not wired, unless you are the person qualified to install one on that burner.

The formula

Gallons = nozzle flow (gallons per hour) x hours the oil valve was open

Cost = gallons x the price per gallon on the delivery ticket

Nozzles are rated in gallons per hour (gph) at a standard pump pressure of 100 psi, and the rating is stamped on the nozzle body with the spray angle and pattern. The service tag or combustion report from the last tune-up usually records the nozzle and the pump pressure the technician left it at. If neither is on file, the burner label lists the nozzle and pressure the manufacturer specified for that appliance, which is a starting point, not proof of what is installed.

Step 1: Correct the nozzle rating for the actual pump pressure

Flow through a nozzle rises with the square root of the pressure across it:

Actual gph = rated gph x square root of (actual psi / 100)

Many current flame-retention burners are set up well above 100 psi, often with a smaller nozzle chosen to give the right firing rate at that pressure. A 0.85 gph nozzle at 140 psi flows 0.85 x the square root of 1.40 = 0.85 x 1.183 = 1.006 gph, about 18 percent more than its stamp.

What breaks if you skip it: the estimate comes in low by exactly that factor, and a tech who reads 18 percent "missing" oil starts hunting for a leak that is not there.

The square-root relationship holds for the same nozzle and the same oil. Cold, thick oil, a worn nozzle and a partly clogged one all shift real flow off the calculated figure, which is one reason the answer is an estimate and the delivery tickets are the check.

Step 2: Get burner hours, and know what the counter is counting

An hour meter on the burner is the best source. Note which circuit it is on. A meter on the burner motor counts the motor's whole run, including prepurge (the motor runs before the oil valve opens, to clear the chamber) and post-purge (it runs after the valve closes). A meter on the oil valve counts only spraying time. If yours is on the motor, take the purge out: read the purge times from the control's settings or documentation, count or estimate cycles, and subtract.

Thermostat runtime history is weaker. On a warm-air furnace, the burner mostly fires when there is a call, but a limit can hold it off during a call. On a boiler, the call and the burner are loosely connected: the boiler's own control fires on water temperature, a hot boiler may answer a call without firing, and a tankless domestic coil fires the burner with no thermostat call at all.

Timing cycles works when nothing else is available: time several burner-on periods and the off periods between them in the weather you are trying to explain, and scale. It is the least reliable of the three because the cycle changes with outdoor temperature.

Step 3: Pick the interval between two fills

The comparison only works over a period that starts and ends with a fill to the same level. Many oil tanks are filled until the vent alarm stops whistling, which is close to the same level each time; a delivery of a fixed number of gallons is not. If one end is a partial delivery, extend the interval to the next full fill. How to Estimate Delivered Fuel Use From a Run of Tickets covers lining up a whole season of tickets.

Step 4: Multiply, and set the result beside the ticket

Read the gap as a direction, not just a size:

• Tickets well above the estimate: oil leaving by some other route (a leak, theft, a second appliance on the same tank), hours undercounted, or a nozzle larger than the records say.
• Estimate well above the tickets: the interval was not fill-to-fill, purge time was not removed, or the nozzle or pressure is lower than recorded.
• Close agreement: the oil went through the burner. The next question is how much heat the house needed, which is a weather question.

"Close" here means within a few percent. Tank fill level, nozzle tolerance and the purge estimate each move the answer a little, and none of them is worth arguing below about 5 percent.

Worked example: the January tank

OIL USE CHECK  (warm-air furnace, electric water heater)
Interval                  Jan 8 fill -> Feb 9 fill
Days                      32
Nozzle (service tag)      0.85 gph at 100 psi rating
Pump pressure (tag)       140 psi
Corrected flow            0.85 x sqrt(1.40) = 1.006 gph
Hour meter (motor)        1,842.6 -> 2,102.6 = 260.0 h
Cycles (12 min each)      260.0 x 60 / 12 = 1,300
Purge per cycle           15 s pre + 15 s post = 30 s
Purge hours               1,300 x 30 / 3,600 = 10.8 h
Oil-valve hours           260.0 - 10.8 = 249.2 h
Estimated gallons         249.2 x 1.006 = 250.7
Feb 9 ticket (to whistle) 255.0
Gap                       4.3 gal, ticket 1.7% higher

The call. A customer with a 275 gallon basement tank says it was filled January 8 and was down near a quarter by early February; in past winters a fill lasted well over a month. The house has an oil warm-air furnace and an electric water heater, so every gallon on the tickets went to space heat. The dealer filled again on February 9.

Physical check. No staining, no odor, no wet fittings at the tank, filter or line. Nothing else draws from the tank.

The nozzle and the pressure. The last tune-up tag reads 0.85 gph and 140 psi. The burner's own label calls for 0.85 gph at 100 psi for this furnace. So the burner is spraying about 1.006 gph, roughly 18 percent over the firing rate the appliance was built for. Hold that thought; it matters, but not to the gallons.

The hours. The dealer had fitted an hour meter on the burner motor. The control's documentation gives a 15-second prepurge and a 15-second post-purge (illustrative; read the actual control's settings). The tech timed a few January cycles at about 12 minutes of burner run. Removing the purge time takes 10.8 hours off the meter's 260.0.

The verdict on where the oil went. 250.7 estimated against 255.0 delivered is 1.7 percent apart, inside the noise of a whistle fill. There is no hidden loss. The oil went through the nozzle.

Had the tech used the stamp instead of the corrected flow, the estimate would have been 249.2 x 0.85 = 211.8 gallons, 43.2 short of the ticket, and the 17 percent "missing" oil would have sent the call down the leak path.

Why the house needed 255 gallons. The 32 days had 1,250 heating degree days at the nearest weather station; the same dates last year had 1,020. Last year's matching fill-to-fill delivery was 205 gallons. Per degree day that is 255 / 1,250 = 0.204 gallons this year against 205 / 1,020 = 0.201 last year, within 1.5 percent of each other. With no domestic hot water on the oil, there is no base use to take out first. A January about 23 percent colder (1,250 / 1,020 = 1.23) explains the faster tank. See related: How to Weather-Normalize Two Winters of Bills.

The finding that is not about the bill. A furnace firing about 18 percent over its labeled rate is a combustion and heat-exchanger question, not a fuel-use one: a higher firing rate shortens the burner's run, it does not by itself multiply the gallons, which are set by the heat the house needs and the efficiency it is delivered at. It still goes to the oil technician for a combustion test and a nozzle-and-pressure setup that matches the label. Do not tell the customer the overfire is why the tank emptied; the numbers above say it is not.

At the delivered price, the interval cost 255 times the price per gallon on the ticket, and an hour of burner run costs 1.006 times that price.

What changes the answer

• A boiler with a tankless coil, or an oil-fired water heater on the same tank. The tickets now carry a year-round base use. Estimate it from a summer interval (gallons per day with no heating) and remove it before any per-degree-day comparison.
• No hour meter. The estimate rests on timed cycles or thermostat history and carries a wider band. Agreement within about 10 percent is the most it can show; a 30 percent gap is still a finding.
• A partial fill at either end. Without a fill-to-fill interval the delivery is not use, and the comparison has to wait for the next full fill or lean on gauge readings, which on a basement tank are coarse.

How to know you got it right

• The nozzle and pressure come from a record of what is installed (a tag, a report), and the record is named in your note.
• The flow figure shows the square-root correction as its own line.
• The hour figure says what the meter is wired to and shows the purge subtraction, or says why none applies.
• The comparison runs between two fills to the same level.
• The conclusion matches the direction of the gap, and a close agreement sends you to the weather, not to the tank.

References

• See related: Propane and Heating Oil Bills: When the Fuel Arrives by Truck
• See related: How to Estimate Delivered Fuel Use From a Run of Tickets
• See related: How to Weather-Normalize Two Winters of Bills
• See related: Duty Cycle: Why Run Hours, Not Clock Hours, Set the Cost
• Burner manufacturer's label and installation instructions, and the nozzle manufacturer's flow-versus-pressure data, which give the rated gph at 100 psi