Oil Fired Appliances: Types, Construction and Operation
Appliance families
- Pressure-jet (gun-type) burners - the dominant domestic type; oil is atomised through a nozzle at pressure, mixed with air, and ignited by electrodes.
- Vaporising (pot) burners - oil vaporises in a heated pot; common on older cookers and some room heaters; sensitive to fuel quality and draught.
- Rotary-cup and air-atomising burners - mainly commercial.
- Appliances: boilers (conventional, system, combination), warm-air units, and room heaters.
Construction and operation
- Heat exchanger - cast iron, steel or stainless; the flue passages must be clean for efficiency.
- Combustion chamber, baffles and turbulators - affect heat transfer and resistance.
- Controls: control thermostat, limit/high-limit stat, overheat cut-out, and the burner control box sequence.
- Sequence of operation: call for heat → burner motor and fan → purge → ignition → oil valve opens → flame established → flame detected → run → satisfied → post-purge/shutdown.
- Efficiency concepts: sensible heat loss up the flue, excess air, and the relationship between CO₂, O₂ and flue temperature.
Traps in questions
- Confusing the control thermostat (adjustable, operating) with the limit thermostat (fixed, safety).
- Forgetting that purge must happen before ignition.
- Assuming a condensing oil boiler behaves exactly like a condensing gas boiler - oil appliances need particular flue and drainage arrangements.
Worked example
A boiler short-cycles on temperature. Rule of thumb: check the control stat and the limit stat first, then heat exchanger fouling. A fouled exchanger passes heat poorly, so the flow temperature rises fast and the burner cycles, while the flue temperature also rises.