Viscosity is the fuel property that decides whether a liquid fuel burner can make a spray at all. It rises steeply as the oil cools, and everything between the tank and the nozzle heats the oil for that reason. When the temperature at the nozzle moves off target, the flame changes before anything else does.
Why viscosity controls the spray
A nozzle atomises oil by forcing it through a small orifice at pressure, which shears the stream into droplets. Shearing gets harder as the oil thickens, because a thicker fluid resists breaking up. The result at a nozzle that is too cold is a coarser spray: larger droplets, a longer flame, more unburned oil in the flue and carbon monoxide where there should be none.
The relationship is not linear, which is why a small temperature difference matters. A viscosity curve falls steeply through the range a burner operates in, so the oil can be too thick for good atomisation and then too thin to pump within a short distance of temperature. That is why the target is a specific figure rather than simply as warm as possible.
Heating the oil, and knowing when it has arrived
The heater is usually a tank unit and an in-line unit, and the in-line one is what matters at the nozzle. A tank heater keeps the bulk of the oil above its pour point so the pump can draw without cavitating; the in-line heater delivers the atomising temperature. A system with only a tank heater starts reliably, then fails to produce a clean spray once the line is long or the ambient cold.
Control matters as much as capacity. A heater cycling hard overshoots and cools between cycles, and oil arriving at the nozzle in pulses produces a flame that pulses with it. The symptoms look like a combustion problem, because carbon monoxide rises and falls, when the cause is upstream.
What the oil carries with it
Viscosity is not the only property that changes the spray. Water in the oil will not atomise with it; it sits at the bottom of the tank until it reaches the nozzle, or passes through as droplets, and both cases disturb the flame. Solid contaminants do the damage twice: once by disturbing the spray and once by eroding the nozzle orifice, which then makes the spray worse.
This is why a viscosity problem and a contamination problem are often diagnosed as the same fault. The nozzle selection notes cover what wear does to a spray, and a fuel test settles both questions where a visual inspection cannot.
Choosing an oil for the climate
Fuels are supplied against a specification, and the one that matters covers the temperature range the site actually experiences. A heated tank in a mild climate is an easier problem than an unheated tank in a cold one, and the same burner can be appropriate for one and marginal for the other. Moving a burner between sites, or changing supplier, changes the answer without changing the appliance.
Where the ambient range is wide, the practical approach is to set the oil circuit for the worst case rather than the average. In-line heating sized for the coldest period runs with a smaller margin through the rest of the year, and that margin usually costs less than one combustion fault in production. Our light oil burner range covers the appliance types where sizing differs.
Reading a viscosity figure correctly
A viscosity figure on a data sheet is meaningful only at a stated temperature. Two fuels can share a nominal grade and differ substantially at the temperature the burner sees, which is why comparing a data sheet against a reading from a cold line produces a wrong conclusion more often than not.
Readings are best taken where the decision is made, at the nozzle inlet. A reading from the tank tells you about the tank, not the line, the heater or the ambient losses along the way. Where a site is unsure whether its heating is adequate, a reading at the nozzle settles it directly.
Frequently asked questions
What happens to a burner when the fuel oil is too cold?
Atomisation gets worse, the droplets get larger, and the flame lengthens and becomes less complete. The usual symptoms are rising carbon monoxide, a visible change in flame shape, and a flame that will not hold at the lower end of the range. Nothing about the air path or controls changes, so the fault is diagnosed on the fuel side though it presents as a combustion one.
Does a tank heater alone make a liquid fuel burner reliable?
It makes the oil pumpable, which differs from making it atomisable. A tank heater keeps the bulk above its pour point; an in-line unit brings it to the atomising temperature at the nozzle. On a long line or in a cold climate, a system with only a tank heater starts and then fails to hold a clean spray.
How can I tell whether water in the oil is causing a flame problem?
Drain a sample from the bottom of the tank, where water settles, and look at it first. Water reaching the nozzle does not atomise with the oil, so it disturbs the spray and shows as a flame that changes with no change in settings. A clean sample rules that out and points the investigation at viscosity or contamination.
Why does the flame change even though the oil specification has not changed?
Because the specification describes the fuel and the flame responds to its temperature. A cold ambient, a longer run or a heater not keeping up all change the temperature at the nozzle while the oil in the tank is the same product. Reading the temperature at the nozzle, rather than trusting the supply specification, identifies the cause.
Should a burner be chosen for the climate it will run in?
Yes, and more precisely for the oil supply than for the average temperature. What matters is the range at the nozzle, which combines ambient conditions, line length and tank arrangement. Sizing the heating for the worst case leaves a margin all year, and that margin normally costs less than one combustion fault in production.
How do I verify the oil circuit is delivering the right temperature?
Measure at the nozzle inlet rather than at the tank, with the burner running, since a static reading says little about what happens once flow starts. Record the reading alongside the fuel in use and the ambient conditions. A changed fuel oil pump or a modified oil line alters the heat loss and invalidates a previous reading.
Request a quote
Zhongli Weiye supplies liquid fuel burners, the pumps, nozzles and in-line heating components that set the oil up for a duty, and matches replacement parts to the appliance and oil in use. Send the burner model, the oil fired and the ambient range, and the recommendation comes back with the parts that fit and what to confirm about the oil circuit.
Related products
The parts and product families referenced in this guide:
- in line oil heating and pumping
- atomisation and oil temperature
- nozzle wear and orifice erosion
- oil pump and filter arrangement
- oil fired appliance selection
This guide covers Burner Fuel Oil Viscosity: The Number Behind a Clean Spray. For the exact part that fits your duty, send us the model plate photo and we confirm availability, lead time and a matched quotation.