The oil pump on a burner is the difference between a spray that atomises and one that does not. It takes fuel from the tank at ambient conditions and delivers it at the pressure and flow the nozzle needs, held steady while the burner modulates. When that pressure moves, the spray pattern changes and combustion degrades even though the nozzle itself has not changed.
What the pump is actually holding
A gear pump does not produce a fixed pressure on its own. It produces a roughly fixed flow, and the pressure it sees is whatever the downstream circuit imposes. That distinction matters in a modulating burner, where the nozzle has a fixed orifice and the required flow falls with burner output. On a fixed speed pump, lowering the required flow raises the pressure, and the excess is dumped through the bypass or the modulating return rather than through the nozzle.
So the useful checks are at the pump discharge and at the nozzle, and the two should not be assumed to agree. A gauge at the pump can look correct while the nozzle sees something different because of a restriction or a long line. Since the pump also supplies the pressure that opens the oil solenoid and drives the return, the same wear that destabilises nozzle pressure shows up in the way the burner sequences.
Viscosity is the variable that governs everything
Gear pumps are viscosity-dependent by design. Within their working range, higher viscosity means higher internal losses, lower actual flow at the same speed, and more heat generated in the pump itself. Outside that range the behaviour changes qualitatively: at low viscosity the pump can lose its ability to fill its own casing and start losing suction, and at high viscosity the internal friction becomes large enough that flow drops away and the casing can crack.
That is why fuel temperature belongs in the enquiry. It is not a secondary detail alongside viscosity, it is the reason viscosity is what it is on the day of the enquiry. Fuel oil viscosity and temperature covers the relationship in full, and getting it wrong shows up as a hard start in the morning and an easy start in the afternoon on the same burner with the same fuel.
What pump wear looks like from the control room
Drift before failure is the normal pattern. Hard starting when the burner has been off, a flame that turns sooty as it comes up to output, and oil pressure that sags as the burner load rises rather than holding flat. A noise that rises with pressure suggests cavitation on the suction side, which is a supply problem rather than a wear problem.
A bypass that will not stay fully closed is the opposite signature and is easy to overlook, because the burner still fires. Excess return flow at part load keeps pressure up into the nozzle range, so the burner runs rich at low output, and the flue gas analysis shows it as higher carbon monoxide at low fire than at full. That pattern is worth looking at before the nozzle gets blamed.
Rebuild or replace
Most industrial oil burner pumps are rebuildable, and the parts involved are a seal or gasket kit and sometimes a gear set. A pump with a cracked body, a scored shaft or a worn housing has usually gone past the point where a rebuild saves anything, and a seized pump should not be forced. The decision is largely about whether the case is intact, because the internal wear components are the ones that get replaced.
Whenever a pump is rebuilt, the in-line strainer should be renewed at the same time, and the housing filled and bled properly before it runs. Running a rebuilt pump on the old strainer reintroduces the debris that caused the wear. Pump families across the common burner makes are covered in the oil burner pump range, and the burner itself in the fuel burner category.
Getting the answer from the burner
Pressure at the pump is necessary but not sufficient. The nozzle has to be matched to the pump output at the burner, so that the flow the pump makes at full fire is what the nozzle passes at its rated pressure, and the pair has to be matched again at low fire through the modulating range. Nozzle selection covers that matching, and it is worth doing as a pair rather than replacing a nozzle on its own.
After any pump work the burner needs re-commissioning rather than a restart. Pressure has to be confirmed across the modulating range, and the flue gas readings at both ends of the load are what confirm that combustion recovered. That last step is the one that gets skipped, which is how a pump that is delivering pressure but the wrong amount of it keeps running.
Frequently asked questions
What pressure should an oil burner pump deliver?
Whatever the nozzle requires for the rated flow, which is a figure set by the nozzle and the fuel rather than by the pump. The pump has to deliver that pressure across the whole modulating range, and the useful measurements are at both the pump discharge and the nozzle rather than at one gauge alone.
Why does an oil burner pump have a bypass?
Because a gear pump produces flow rather than pressure. When the burner load falls and the nozzle passes less, the excess has to go somewhere, and the bypass is where it goes. A bypass that will not close fully keeps pressure up at low fire and makes the burner run rich there.
Can an oil burner pump be rebuilt?
Usually, since the wear components are a seal or gasket kit and sometimes a gear set. A cracked body, a scored shaft or a worn housing has usually gone past the point of rebuild. The in-line strainer should be renewed whenever the pump is opened.
What does cavitation in an oil pump indicate?
A supply side problem rather than pump wear. Not enough inlet pressure for the viscosity and speed in use, a suction line too long or restricted, or a strainer that has blocked. The noise rises with pump pressure, which is what distinguishes it from the steady hum of normal operation.
Why does an oil burner start hard in the morning?
Fuel viscosity rises when the fuel is cold, and the pump loses flow and pressure outside its working range at higher viscosity. It is a temperature problem rather than a pump defect, which is why the same burner often starts easily later in the day with fuel that has been circulating.
How do you know a pump is worn rather than a nozzle blocked?
By measuring. Pressure at the pump while the burner runs tells you which side of the system is losing it, and a stuck bypass shows up as pressure that stays high at low fire. Replacing a nozzle on assumption wastes the part and leaves the fault in place.
Request a quote
Send the burner model, the nozzle in use and the fuel you run, and we can match the pump and confirm what has to travel with it. If the pump on your burner is behaving oddly, adding the oil temperature and whether the trouble is worst at start-up or at low fire usually identifies the side of the circuit that is at fault before anything is opened.
Related products
The parts and product families referenced in this guide:
This guide covers Oil Burner Pump: Pressure, Flow and When to Rebuild. For the exact part that fits your duty, send us the model plate photo and we confirm availability, lead time and a matched quotation.