A dual fuel burner exists because the price and availability of gas and liquid fuel move independently. The appliance is not two burners in one box; it is one firing head fed by two fuel trains, with the flame geometry and the air it needs changing with the train that is open.
Two trains, one firing head
The gas side brings fuel to a manifold through a regulated train with shut-off valves and a proving device. The liquid side brings it through a pump, a filter and an atomising nozzle. Both deliver to the same burner body, but in incompatible ways, and the design problem sits in the transition. A gas flame anchors on a port or target and needs a stable root. A liquid flame is a spray cone held by swirl and air, with no equivalent anchor.
That difference shows up in the air path. A gas burner of a given input can sometimes be adapted from an oil burner, but not the reverse, because the gas version needs a swirler the oil version has no use for. A converted burner is therefore not interchangeable with a gas burner of the same size, even where the mounting matches. Our dual fuel burner range covers appliances built for the duty rather than converted to it.
What a changeover actually involves
Switching fuel on a running appliance is not a valve change. The sequence closes the active train, purges the shared volume with air, then admits the other fuel. Skipping the purge leaves the previous fuel in the firing head, and on a gas-to-liquid change that puts an unburned gas volume in the path of an oil nozzle, a different order of hazard from the reverse.
The purge must cover the volume that is actually shared, which is not the chamber alone. It includes the firing head, the transition between the fuel paths and any common volume between where the trains join and where the flame forms. Appliances differ here, so the purge time belongs to the documentation rather than to a rule of thumb. The start-up sequence notes cover how that volume is defined.
The control side has to arbitrate
Two trains mean the program controller holds more state than a single-fuel one, and the logic must prevent both trains being open at once. That is a hard interlock, arranged so a controller fault cannot leave both energised. The changeover also has to define what happens mid-sequence if a limit opens or the flame is lost, because an incomplete changeover is what most often reaches the interlock chain unexpectedly.
Where the appliance must switch without interrupting the process, the sequence has to finish inside a window the process accepts. That constrains the purge time, and it decides whether an appliance suits the duty: one needing a long purge is fine for a plant changing fuel once a shift, unsuitable for one changing on price signals every few hours. The fuel selection notes set out that judgement.
Where the economics stop working
Dual fuel earns its place where the two fuels genuinely diverge in price, which is not every site. Where gas is on a stable contract and the alternative is only marginally cheaper, the appliance has cost a premium on the burner, the control platform and the training, and returns that only through the fuel it displaces. The arithmetic is straightforward once the switching frequency is known, and that is usually set by price.
Commissioning both sides separately
Each fuel is commissioned as if it were the only one, because that is the only way to tell whether a fault belongs to a train or the firing head. The gas side is proved for pressure, tightness and proving cycle with the oil train blocked; the oil side for pump pressure, filter condition and nozzle delivery with the gas train isolated. Only then is the changeover exercised and the combined behaviour walked across the range.
A burner clean on gas and poor on oil is usually an oil train problem the gas side was masking, because gas lights on a leaner mixture than oil tolerates. The same burner clean on oil and poor on gas points to the gas train or air path. Recording both sides at commissioning makes that distinction possible later from a fault report.
Frequently asked questions
Can any gas burner be converted to run on liquid fuel as well?
No. The firing head geometry is the constraint, not the fuel train. A gas burner anchors its flame on a port or spreader; an oil burner needs a spray cone held by atomisation and directed air. Head and air path would both change, at which point it is a dual fuel burner built for the duty.
How long does a fuel changeover take?
Long enough to close the active train, purge the shared volume with air and admit the other fuel, and that volume is not the chamber alone. It includes the firing head and the transition between the fuel paths. Use the appliance's own figure, and confirm it by timing a changeover on site at temperature, since a purge adequate cold is not adequate hot.
What happens if the flame is lost during a changeover?
The sequence must return to a defined safe state, meaning the burner goes to lockout and stays there until reset. What matters is whether the changeover was interrupted with the purge incomplete, because then the appliance must not restart into the sequence part-way through. That is a controller configuration question, worth confirming at commissioning.
Does a dual fuel burner need more maintenance than a single fuel one?
It carries two fuel systems, so there is more to inspect and more that can fail, and a site needs parts for both. That is a real cost, but not disproportionate if switching is regular. What raises the burden is not the second fuel but incomplete changeover logic, since an interrupted sequence leaves components in an unexpected state.
Can a dual fuel burner modulate across its whole range on both fuels?
Turndown on gas is usually better than on liquid fuel, because atomisation quality falls as flow drops and a poor spray at low flow produces carbon monoxide before a visible flame fault. Appliances are normally specified with a turndown figure for each fuel. The modulation notes cover why fuel-side faults appear at the low end first.
How is a dual fuel burner selected where the gas supply is unstable?
Select it for the duty it will actually run, which usually means the oil side carries the base load and gas covers the periods when it is available and cheap. Confirm the oil train holds the required input continuously, since that determines whether the appliance covers a gas interruption. Our oil pump range covers that liquid side.
Request a quote
Zhongli Weiye supplies dual fuel appliances, the control platforms that run their changeover logic, and the gas train and oil circuit components for either side. Send the burner model, the two fuels and the application, and the recommendation comes back with the parts that fit and anything worth confirming about the sequence.
Related products
The parts and product families referenced in this guide:
- gas and oil burner families
- fuel changeover cost arithmetic
- changeover sequence in the controller
- fuel choice for an unstable gas supply
This guide covers Dual Fuel Burner: Switching Between Gas and Oil Without a Shutdown. For the exact part that fits your duty, send us the model plate photo and we confirm availability, lead time and a matched quotation.